Vitamins
Highlights
Overview
- A diet that is naturally high in vitamins can be the best defense against many diseases. Fresh fruits and vegetables and whole grains are the primary sources of vitamins, carotenoids, and phytochemicals, as well as of fiber and important minerals.
- With the exception of vitamin D, supplements are helpful only for certain people (for example, during pregnancy or for people with deficiencies or certain illnesses), and they may actually be harmful for other people. Still, supplement use is on the rise. More than half of American adults use supplements, most often multivitamins and minerals.
- The Food and Drug Administration (FDA) does not require manufacturers to provide any scientific evidence that their dietary supplements are safe and effective (unlike drugs, which must be proven both safe and effective through clinical trials). If a supplement causes side effects in people once it is sold, the government may place restrictions on the supplement or withdraw it from the market. The FDA may also withdraw products from the market if their labels are misleading or false.
New Standard
The Recommended Dietary Allowance (RDA) for vitamins is gradually being replaced by a new standard called the Dietary Reference Intake (DRI). The DRI is a general term used to describe the types and amounts of nutrients healthy people need.
Vitamins and Health
- Many fresh fruits and vegetables contain chemicals that may fight cancer, including lung, breast, colon, and prostate cancers.
- Evidence shows that beta-carotene supplements can have harmful effects on smokers, increasing their risk for lung cancer and their overall death rates. Beta-carotene from food appears to be safe.
- Studies have reported that a diet high in fruits and vegetables containing beta-carotene, lycopene, and other carotenoids may reduce the risk of a heart attack. Diets low in lycopene (particularly from tomatoes) have been associated with a significantly higher risk for heart disease and stroke. Supplements, however, do not reduce these risks.
- Carotenoids, especially lutein, lycopene, and zeaxanthin protect the eyes, and may help prevent cataracts and macular degeneration.
- Studies are mixed as to whether vitamin supplements protect against colds or other upper respiratory infections. Most evidence suggests there is little or no benefit.
- In 2011, a study reported that vitamin E supplements increase the risk for prostate cancer.
Introduction
Vitamins do not share a common chemistry, but they do share certain characteristics. They are all organic nutrients that are necessary in small amounts for normal body functioning and good health. A well-balanced diet should provide most of the vitamins people need to stay healthy and prevent disease. Vitamin D is the only exception. Supplements can be helpful in some situations, such as pregnancy and certain illnesses. Strict vegetarians may benefit from taking vitamin B12 supplements.
Unlike carbohydrates, fats, and proteins, vitamins are not sources of energy. Instead, vitamins are involved in the body's metabolism, cell production, tissue repair, and other vital processes.
Vitamins are either fat soluble or water soluble.
- Fat-soluble vitamins, which include A, D, E, and K, are stored in the liver and used up by the body very slowly. Because the body stores fat-soluble vitamins, they can be dangerous when taken in large amounts.
- Water-soluble vitamins include vitamin C and the B vitamins. The body uses these vitamins very quickly. Excess amounts are removed in the urine.
Guidelines for Adequate Intake of Vitamins
The Recommended Dietary Allowance (RDA) for vitamins, set by the Food and Nutrition Board of the National Academy of Sciences-National Research Council, has been used for years as a guide for determining the amount of vitamins needed to prevent deficiency diseases. The RDA is an estimate of the average requirements of dietary components such as calories, vitamins, minerals, and proteins that are needed to prevent deficiency. Different groups of people need different amounts of vitamins based on their gender and age.
The RDA is gradually being replaced by a new standard called the Dietary Reference Intake (DRI). The DRI is a general term used to describe the types and amounts of nutrients healthy people need.
The DRI values are based on four categories:
- The recommended dietary allowance (RDA). This is the current rating on most vitamins.
- The estimated average requirement (EAR). This is the amount of the vitamin needed to meet the nutritional requirements for 50% of the population.
- Adequate intake (AI). This amount is used if there are not enough data to calculate the RDA.
- Tolerable upper intake level (UL). This is the maximum dose that is likely to be safe in 98% of the population.
Food and supplement labels now typically list the Daily Value (DV). This is the amount of a nutrient that experts believe people need in their daily diet. On food labels the DV is usually based on one serving size for a person who takes in 2,000 calories a day.
Regulating Quality
Regulation of dietary supplements by the U.S. Food and Drug Administration (FDA) is a complex issue.
Vitamin and other dietary supplement labels now include nutrient information and list all of the ingredients in the supplement, including the parts of plants from which the ingredients are taken. Unlike drug labels, vitamin and supplement labels may not claim to treat any specific disease.
Vitamin and supplement labels include one of the following:
- Health claim -- description of how the substance may reduce the risk of a health-related condition
- Nutrient claim -- description of the amount of the nutrient in the product
- Structure or function claim -- description of how the product may affect organs or systems of the body, without claiming to treat specific diseases
The quality of dietary supplements depends on the manufacturer and is not regulated by the FDA. The U.S. government does not require that supplements be standardized, meaning that the amounts or quality of nutrients may vary depending on the batch. So, more expensive supplements are not necessarily better than less expensive ones. Government regulations are in the process of catching up to the boom in the supplement industry. In the meantime, some companies voluntarily stick to rigorous quality controls, while others do not.
The U.S. Pharmacopeia (USP), an independent organization that sets quality standards for drugs, has also implemented standards for vitamins. Consumers may look for the USP label on products made by companies that follow these standards.
USP verification means the following:
- What is in the bottle matches what is listed on the label.
- There are no harmful levels of contaminants.
- The supplement will be properly absorbed into the body.
- The supplement has been produced according to good manufacturing standards.
The FDA does not require manufacturers to provide any scientific evidence that dietary supplements are safe and effective before a product is sold (unlike drugs, which must be proven both safe and effective through clinical trials). If a supplement causes side effects in people once it is sold, the government may place restrictions on the supplement or withdraw it from the market. The FDA may also withdraw products from the market if their labels are false or misleading.
People Who Should Take Vitamin Supplements
Dietary supplement use is on the rise. More than half of American adults use supplements, most often multivitamins and minerals. In particular, more Americans are taking vitamin D and calcium supplements than in the past.
However, studies have found no difference in mortality rates between people who take vitamin supplements and those who don't take supplements. Most people who eat a healthy diet do not need vitamins, but there are some exceptions.
Pregnant and Breastfeeding Women. Women who are pregnant or breastfeeding generally need additional vitamins. Folic acid and vitamins B6 and B12 are particularly important during pregnancy. Women who are vegetarians need to take enough of vitamin B12. A deficiency in this vitamin can harm their baby. Folic acid reduces the risk for neural tube defects and possibly deformities of the face, such as cleft palate. Studies also link low folate levels during pregnancy with low birth weight, which may increase the risk of heart disease in adulthood. A woman's best approach is to start taking extra folic acid plus multivitamin supplements before she becomes pregnant.
Pregnant women who eat a healthy diet may still have low folate levels and need to take folic acid supplements. Requirements are as follows:
- The recommended dietary allowance (RDA) for folic acid before getting pregnant is 400 mcg.
- During pregnancy the RDA is 600 mcg.
- Women who are breast-feeding should get 500 mcg of folic acid.
Some women have low vitamin A reserves in their liver. However, getting too much vitamin A from food or supplements significantly increases the risk for birth defects. Experts recommend that pregnant women get no more than 3,000 mcg of vitamin A each day.
Infants and Children. Infants who are breast-fed by healthy mothers should receive enough vitamins. However, in some cases, infants may not get enough of vitamins K and D.
- Human milk contains low levels of vitamin K, and the newborn's immature intestinal tract may not produce enough of the baby's own supply. Most babies are given an injection of this vitamin at birth.
- Infants who are breast-fed by malnourished women or who do not get enough sunlight exposure may be deficient in vitamin D. In these cases, supplements of 200 - 300 IU are recommended.
Formulas are required to contain enough vitamins and minerals. After infancy, most American children receive all the vitamins they need from their diet, unless they are severely deprived. However, research suggests that many healthy children ages 1 to 11, especially African-American and Hispanic children, are not getting enough vitamin D.
Smokers. Smoking interferes with the absorption of several vitamins, especially vitamins C and D. Smoking can also interfere with the metabolism of vitamin D, resulting in poor muscle function.
Taking high doses of antioxidant vitamins, especially beta-carotene, is harmful to smokers. Instead of taking supplements, smokers should eat a diet rich in fresh fruits, vegetables, and whole grains. Smoking cessation is the most important intervention.
Click the icon to see an image detailing sources of vitamin C.
Alcoholics. Alcoholics often have several vitamin deficiencies. The most dangerous deficiencies are:
- Vitamin B1 (thiamin)
- Vitamin B2 (riboflavin)
- Vitamin B6 (pyridoxine)
- Folic acid
- Vitamin C
Low levels of vitamin B6 may increase the risk for colorectal cancer in men who drink large amounts of alcohol.
People Who Have Had Gastric Bypass Surgery. Vitamin deficiency is a complication of gastric bypass surgery. Women, African-Americans, and adults who have had laparoscopic Roux-en-Y bypass surgery are at highest risk. The deficiency is treated with water-soluble vitamin supplements.
Strict Vegetarians. Strict vegetarians need to take vitamin B12 supplements, unless they get enough of this vitamin from fortified cereals and other grain products. they may also need to take vitamin D and riboflavin supplements, or a multivitamin, and watch their iron levels. Vegans, who do not eat dairy or eggs as well as meat, may be at risk for vitamin A deficiencies if they do not eat plenty of dark-colored fruits and vegetables.
Vitamin deficiencies may be particularly harmful in vegetarian children. Pregnant and breastfeeding women who are vegetarians must get enough vitamins. Mothers who do not get enough vitamin B12 may cause growth and nervous system problems in their newborns.
Dieters. People who are on weight-reduction diets of fewer than 1,000 calories a day should probably take a multivitamin. They should also check in regularly with a physician to make sure they are getting enough nutrients.
Click the icon to see an image detailing sources of vitamin D.
Older Adults. Almost a third of elderly people do not get enough of certain vitamins and important minerals. Often their dietary habits slip and they fail to regularly eat balanced meals. In addition, older adults are more likely to take medications that prevent the absorption of certain vitamins.
Common vitamin deficiencies in the elderly:
- Elderly people, particularly if they are not exposed to sunlight, may be deficient in vitamin D. Older adults should get at least 800 IU of vitamin D a day. People who are obese, or who have osteoporosis, limited sun exposure, or poor nutrient absorption may need to increase their intake to as much as 2,000 IU per day.
- Seniors also may have low levels of important B vitamins. Older adults with symptoms of dementia should be tested for a B12 deficiency.
Seniors need to use caution when taking vitamin supplements. Because metabolism slows with age, it takes the liver longer to remove vitamins from the body. Therefore, the effect of some vitamin supplements may be intensified in older adults. For example, a dose of vitamin A that might be harmless in a younger adult could be toxic in an elderly patient.
People Who Avoid Sunlight. People who avoid sunlight or who are housebound, and whose diet is low in foods that contain vitamin D should take supplements. People with darker skin are at higher risk for vitamin D deficiencies than those with whiter skin. (Note: Vitamin D is toxic in high doses, and no one should exceed the RDI of vitamin D except under a physician's direction.)
Vitamin A and Provitamin A Carotenoids (Beta-Carotene) | |
Benefits | Essential for:
Beta-carotene is an antioxidant that helps protect the body against the damaging effects on cells that can lead to disease. |
Recommended dietary allowance (RDA), AI, or dietary reference intake (DRI) (mcg = micrograms, mg = milligrams, IU = international units) | Children:
Non-pregnant adolescents and women:
Pregnant women:
Warning: The use of preformed vitamin A, including the skin acne medication tretinoin (a vitamin A derivative), during pregnancy can cause birth defects. Breast-feeding women:
Adolescent boys and men:
Note: Some experts recommend also getting 3 - 6 mg of beta-carotene, which converts to vitamin A. Vitamin A is now measured with a unit called the Retinol Activity Equivalent (RAE). One RAE is equal to:
Retinol is the most active form of vitamin A. |
Foods containing the vitamin |
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Effects of deficiencies |
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People at risk for deficiencies |
Healthy adults usually have a year's store of vitamin A in the liver. A temporary lack of nutrients is unlikely to cause a serious vitamin A deficiency. |
Toxicities | Vitamin A is very toxic when taken in high-dose supplements (more than 25,000 IU a day) for long periods of time. Large amounts of beta-carotene will not make people sick, but they can turn the skin yellow or orange. Excess vitamin A can affect almost every part of the body, including the eyes, bones, blood, skin, central nervous system, liver, and genital and urinary tracts. Symptoms of vitamin A overdose include:
Severe toxicity can cause blindness and may even be life threatening. High doses of vitamin A may also increase the risk for gastric cancer, osteoporosis, and hip fractures. In children, long-term vitamin A overdose can cause fluid on the brain, liver damage, as well as the same complications seen in adults. Pregnant women who take amounts of vitamin A that are not much higher than the RDA increase the risk for birth defects in their children. |
B Vitamins, part 1 | ||
| B Vitamins: General Information | Vitamin B1 (thiamin) | |
Benefits | The B vitamins have a wide and varied range of functions in the human body. Most B vitamins are involved in the process of converting blood sugar into energy. | Vitamin B1 is essential for converting carbohydrates from food into energy. It is also involved in heart, muscle, and nerve function. |
Recommended dietary allowance (RDA), AI, or dietary reference intake (DRI) (mcg = micrograms, mg = milligrams, IU = international units) | Infants:
Children:
Adolescents and adults:
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Foods containing the vitamin | Good sources are fortified cereals and breads, dried milk, nuts, legumes (dried beans), cauliflower, lean meats, and sunflower seeds. People who eat a normal diet and are in good health do not need to take supplements. | |
Effects of deficiencies | Deficiencies are uncommon in the U.S., but when they occur, they usually involve several B vitamins, since many of them come from the same food groups. | Vitamin B1 deficiency can cause:
Severe vitamin B1 deficiency is known as beriberi. |
People at risk for deficiencies |
| See general vitamin B description. |
Toxicities | Because the B vitamins are water-soluble and removed in the urine, toxic reactions from taking most of them by mouth are extremely rare. (Exceptions are niacin and vitamin B6.) *Note: Substances known as B15 (pangamic acid) and B17 (laetrile) are not vitamins or nutrients. Both chemicals are highly dangerous and have no proven nutritional or health value. | No toxic effects have been reported from thiamin. |
B Vitamins, part 2 | |||
| Vitamin B2 (riboflavin) | Vitamin B3 (niacin) also known as nicotinic acid | Vitamin B5 (pantothenic acid) | |
Benefits | Important for growth, red blood cell production, and the conversion of carbohydrates into energy. | Helps break down food for energy. Helps the digestive system, skin, and nerves function. Widens blood vessels and increases blood flow. May be prescribed in very high doses for improving cholesterol levels, but medical supervision is recommended. | Important for helping the body break down fats, carbohydrates, and proteins. Essential for growth, and the production of steroid hormones and cholesterol. |
Recommended dietary allowance (RDA), AI, or dietary reference intake (DRI) (mcg = micrograms, mg = milligrams, IU = international units) | Infants:
Children:
Adolescents and adults:
| Infants:
Children:
Adolescents and adults:
Some people take 1 to 3 grams of niacin per day to treat low HDL ("good") cholesterol and high LDL ("bad") cholesterol and triglycerides. | Infants:
Children:
Adolescents and adults:
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Foods containing the vitamin | Liver, fortified breads and cereals, milk and other dairy products, eggs, nuts, fish, and some dark green leafy vegetables. People who eat a normal diet and are in good health do not need to take supplements. | Fish, chicken, veal, beans, nuts, fortified breads and cereals, dairy products, eggs, pork, salmon, and beef liver. People who eat a normal diet and are in good health do not need to take supplements. | Whole-grain cereals, beans, milk, avocado, broccoli and other vegetables in the cabbage family, milk and liver. People who eat a normal diet and are in good health do not need to take supplements. |
Effects of deficiencies | Effects can include:
| Symptoms can include:
Niacin deficiency is called pellagra. | Deficiency is unlikely to occur, except together with other B vitamin deficiencies. Symptoms of a deficiency include:
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People at risk for deficiencies | See general vitamin B description. | Alcoholics and people who are malnourished. | Alcoholics and people who are malnourished. |
Toxicities | No toxic effects have been reported, even from large doses of riboflavin. | Even mildly high doses of niacin can cause hot flushes of the face and shoulders, headaches, itchiness, and stomach problems. Large doses may trigger erectile dysfunction, ulcers, gout, diabetes, and liver damage. These symptoms are usually reversed when high doses are stopped. | Although no toxic effects have been reported in humans, high doses have caused liver damage in rats. |
B Vitamins, part 3 | ||
| Vitamin B6 (pyridoxine) | Vitamin B12 (cobalamin) | |
Benefits | Vitamin B6 affects over 60 proteins in the body, including proteins that play a role in the nervous system, red and white blood cell production, the immune system, and heart disease. | Vitamin B12 is essential for the production of red blood cells, manufacture of genetic material, and function of the nervous system. |
Recommended dietary allowance (RDA), AI, or dietary reference intake (DRI) (mcg = micrograms, mg = milligrams, IU = international units) | Infants:
Children:
Adolescents and adults:
| Infants:
Chidlren:
Adolescents and adults:
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Foods containing the vitamin | Meats, oily fish, poultry, whole grains, fortified cereals, soybeans, avocados, baked potatoes with skins, watermelon, plantains, bananas, peanuts, and brewer's yeast. | The only natural dietary sources are animal products, including meats, milk and other dairy products, eggs, and fish (clams and oily fish are very high in vitamin B12). Like other B vitamins, B12 is added to fortified cereals and grain products. |
Effects of deficiencies |
In unborn children, a lack of vitamin B6, vitamin B12, and folic acid may cause defects such as cleft lip and palate and spina bifida. Women should take B vitamin supplements before and during their pregnancy. Note: People who have been regularly taking more than 50 mg of vitamin B6 and stop suddenly are at risk for a "rebound deficiency." That's why people should taper off vitamin B6 slowly. |
Children who lack this vitamin may not grow properly. Deficiencies in pregnant and breast-feeding women may cause nervous system problems in their babies. The gene defect that causes vitamin B12 deficiencies is responsible for pernicious anemia, a serious disorder that causes rapid heart rate, shortness of breath, dizziness, weakness, and fatigue. It must be treated with injections of vitamin B12 or very high oral doses to prevent nervous system damage. |
People at risk for deficiencies |
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People over age 50, and those who have Crohn's disease, celiac disease, or who have undergone certain gastrointestinal surgeries may have trouble absorbing vitamin B12 and need to take supplements. |
Toxicities | Very high doses can cause nerve damage, which may be permanent in some cases. The babies of pregnant women who take large doses, such as for morning sickness, may have adverse health effects. | There is no evidence of toxic effects from this vitamin. |
B Vitamins, part 4 | |||
| Biotin (a B vitamin) | Choline (a B vitamin) | Folate, or Folic Acid, its synthetic form (a B vitamin) | |
Benefits | Biotin is involved in the production of amino acids, proteins, hormones, and fatty acids. | Choline is essential for a baby's brain development and for learning and memory. | Folate ismportant for many processes in the body. It is used to produce chemical messengers in the brain, break down and produce proteins, and make DNA--the genetic information that controls cell functions. It is very important for pregnant women to get enough folic acid to prevent birth defects in their babies. |
Recommended dietary allowance (RDA), AI, or dietary reference intake (DRI) (mcg = micrograms, mg = milligrams, IU = international units) | Infants:
Children:
Adolescents and adults:
| Infants:
Children:
Adolescents and adults:
| Supplements may be in the form of folate (natural) or folic acid (synthetic). Folic acid is nearly twice as potent as folate. Infants:
Children:
Adolescents and adults age 14 and older:
Women who are planning to become pregnant should take at least 400 mcg of folic acid before conception, during pregnancy, and while nursing. |
Foods containing the vitamin | Dietary sources are cereal, egg yolks, chocolate, milk, legumes, liver, mushrooms, bananas, tomatoes, whole grains, nuts, and brewer's yeast. Also produced by bacteria in the intestines. | Peanuts, eggs, cauliflower, and meats, especially liver. | Avocado, bananas, beets, oranges and orange juice, cereal, asparagus, broccoli, green leafy vegetables, dried beans and peas, and yeast. Breads and cereals are now suppleemnted with folic acid. |
Effects of deficiencies | Deficiencies are almost unheard of. | Low levels during pregnancy increase the risk of birth defects in newborns. | As with vitamins B6 and B12, deficiencies of folate raise levels of homocysteine, an amino acid in the body that may increase the risk for heart disease, and possibly Alzheimer's disease. Folic acid supplements lower homocysteine levels, but they have little or no impact on the risk of heart disease. This suggests that homocysteine may be a marker of heart disease, rather than a cause. However, some evidence suggests that folic acid supplementation in patients with low folic acid levels substantially reduces the risk of a first stroke. Low folic acid levels during pregnancy increase the risk of birth defects in newborns. Folic acid supplementation plays a key role in preventing birth defects. Folic acid deficiencies can also cause depression, anemia, and problems with concentration, memory, and hearing. |
People at risk for deficiencies |
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Toxicities | There are no known toxic effects of biotin. | Excessive doses can cause intestinal problems. There is also some concern that high doses can cause cancer. | There is a possible link between high-dose folic acid supplements and an increased risk for colorectal, prostate, and breast cancers. More research is needed. Researchers have also found a link between high doses of folic acid and central nervous system disorders, zinc deficiency, and seizures in epileptics. This risk appears to be low, but results indicate that people should avoid taking megadoses of folic acid. Large amounts of folic acid may mask symptoms of vitamin B12 deficiency. |
Vitamin C (Ascorbic Acid) | |
Benefits | Vitamin C acts as an antioxidant (it reduces harm from damaging chemical processes in the body) and helps the body absorb iron. Vitamin C is essential for the production of collagen, the basic protein in bones, cartilage, tendons, and ligaments. It also helps with wound healing. Another possible, but still unproven benefit of vitamin C is protection against narrowing of the airways during exercise in people with asthma. Vitamin C may also help boost the immune system. |
Recommended dietary allowance (RDA), AI, or dietary reference intake (DRI) (mcg = micrograms, mg = milligrams, IU = international units) | Infants:
Children:
Adolescents:
Adults:
Pregnant and breast-feeding women:
Smokers need 35 mg/day more vitamin C than non-smokers. |
Foods containing the vitamin | Orange juice is the most important source of vitamin C in the U.S. Other sources include citrus fruits and juices, papayas, hot chili peppers, bell peppers, broccoli, potatoes, dark leafy greens, kale, red cabbage, cauliflower, cantaloupe, sweet potatoes, tomatoes, and Brussels sprouts. The best way to get vitamin C from these foods is by eating them raw. Cooking foods that are rich in vitamin C or storing them for long periods of time can reduce the vitamin C content. |
Effects of deficiencies | Symptoms of vitamin C deficiency include:
Scurvy is the main disease of vitamin C deficiency. It affects most body tissues, especially the bones, teeth, and blood vessels. Symptoms include tiredness, weakness, irritability, weight loss, and muscle wasting. Vitamin C deficiencies may also contribute to gum disease and gallstones. In children, a lack of vitamin C in the diet has been associated with poor lung function. Low vitamin C intake may also increase lead levels in the blood. |
People at risk for deficiencies | True vitamin C deficiency is rare in the U.S. It only occasionally occurs in the elderly, alcoholics, cancer patients, and some people who are on severely limited diets that are low in fresh fruits and vegetables. However, studies now suggest that as many as 16% of middle-aged Americans are low in vitamin C, with the highest risk in smokers and middle-aged men. Taking high doses of aspirin taken over a long period of time can interfere with vitamin C absorption. |
Toxicities | The upper limit of vitamin C is 2,000 mg/day in adults (the limit is lower in children). High doses may cause headaches and diarrhea. Long-term high doses may increase the risk for kidney stones. Vitamin C increases iron absorption, so people with blood disorders such as hemochromatosis, thalassemia, or sideroblastic anemia should avoid taking high doses. Large doses may also thin the blood and interfere with blood thinning medications, blood tests used in diabetes, and stool tests. Rebound scurvy can occur after a person stops taking large doses of vitamin C. This rebound effect may be more significant in infants or pregnant women. |
Vitamin D | |
Benefits | Vitamin D is a single term for several hormones. These hormones are stored mainly in the liver and also in fat and muscle tissue. Vitamin D is essential for the absorption of calcium into bone and for normal bone growth. |
Recommended dietary allowance (RDA), AI, or dietary reference intake (DRI) (mcg = micrograms, mg = milligrams, IU = international units) | Infants:
Safe upper limit for infants is 1,000 to 1,500 IU/day. Children:
Safe upper limit for children is 2,500 to 3,000 IU/day. Older children and adults:
Pregnant and breast-feeding women:
Safe upper limit for older children, adults, and pregnant and breast-feeding women is 4,000 IU/day. Breast-fed infants, as well as people who are obese, have osteoporosis, limited sun exposure, poor nutrient absorption, or dark skin should take vitamin D supplements. |
How the body gets the vitamin | There are two forms of vitamin D. Vitamin D3 is made in the body from a chemical reaction triggered by the ultraviolet radiation in sunlight. People can get enough vitamin D by exposing the skin to 10 to 15 minutes of sunshine three times a week (without sunscreen). Vitamin D2 is found in a few food sources, including vitamin D-fortified milk, cheese, butter, fatty fish, oysters, egg yolks, and liver. |
Effects of deficiencies |
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People at risk for deficiencies |
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Toxicities | Vitamin D is very toxic in high doses. Too much vitamin D can cause the intestines to absorb excess calcium, leading to high blood calcium levels.
Eating a low-calcium diet and stopping the vitamin can usually reverse these side effects, except for kidney failure. |
Other Vitamins | ||
| Vitamin E (Tocopherol or Tocotrienol) | Vitamin K | |
Benefits | This vitamin helps protect cells from damage that can lead to disease and premature aging. Vitamin E also helps with the production of red blood cells and prevents clots from forming inside blood vessels. Researchers once thought that vitamin E might protect against heart disease, but this theory has been disproven. It is clear that vitamin E does not prevent prostate cancer, and one study found that it actually increases the risk. | The most important function of vitamin K is its role in blood clotting and bleeding prevention. It also helps maintain healthy bones and heal fractures. Vitamin K is widely used in Japan to treat osteoporosis, and studies suggest it may be effective in treating rheumatoid arthritis. |
Recommended dietary allowance (RDA), AI, or dietary reference intake (DRI) (mcg = micrograms, mg = milligrams, IU = international units) | Infants:
Children:
Upper limits:
Adolescents and adults:
Upper limits:
Vitamin E is composed of eight compounds (four tocopherols and four tocotrienols). It is most often available as dl alpha tocopherol (a synthetic form) supplements. People should take vitamin E supplements with some oil or fat to help their body absorb this vitamin most efficiently. | Infants:
Children:
Adolescents and adults:
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Foods containing the vitamin | Vegetable oils (particularly wheat germ oil), sweet potatoes, turnip greens, mangoes, avocados, spinach, broccoli, nuts (almonds, peanuts, hazelnuts), sunflower seeds, fortified breakfast cereals, and soybeans. Tocotrienol (a possibly beneficial form) is found in natural tropical oils. Palm oil sold in the US is refined and does not contain tocotrienol. | The best dietary sources are canola oil, green leafy vegetables such as spinach and kale, Brussels sprouts, broccoli, and soybean oil. Other good sources are beef liver, fish, bran, and olive oil. Vitamin K is also produced by bacteria in the intestines. |
Effects of deficiencies | Deficiencies have not been established. |
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People at risk for deficiencies |
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Some evidence suggests that more young people may be deficient than was previously believed. |
Toxicities | Although vitamin E is one of the best-studied vitamins, research has yielded conflicting results. Definitive conclusions about the benefits and risks of vitamin E have not yet been determined. Although vitamin E from foods is not dangerous, large doses in supplement form may cause bleeding problems, particularly in people taking anti-clotting medications. Some research now indicates that vitamin E, like other antioxidants, may have damaging effects. Studies of people who took large amounts of vitamin E supplements indicated a higher risk of heart failure and death. | Allergic responses, including rash and itching, to high doses have been reported. People who are taking Coumadin, an anticoagulant, should not take vitamin K without consulting a physician. |
Carotenoids
Carotenoids are a group of more than 700 fat-soluble nutrients that produce the colors in foods such as carrots, pumpkins, sweet potatoes, tomatoes, and other deep green, yellow, orange, and red fruits and vegetables. Many carotenoids are proving to be very important for health. Beta-carotene is the most widely studied carotenoid, but others are also of great interest. Some carotenoids, including beta-carotene, are known as provitamin A because they convert to vitamin A in the body.
Carotenoids are categorized as either xanthophylls or carotenes according to their chemical composition. Xanthophylls will be covered under the phytochemicals section of this report.
Carotenes
Most carotenes are found in yellow, orange, and red vegetables. They include beta- and alpha-carotene and lycopene.
- Beta-Carotene and other Provitamin A Carotenoids. Beta-carotene, alpha-carotene, and beta-cryptoxanthin are carotenes that are converted into vitamin A or retinol (the active form of vitamin A) in the body. They are found in many yellow fruits and vegetables. Beta-carotene is the most widely studied carotenoid. Evidence now strongly suggests that when taken as a separate supplement, it can have harmful effects in smokers.
- Lycopene. Lycopene is responsible for the red color in fruits and vegetables, including tomatoes, red grapes, watermelon, and pink grapefruit. It is also found in papayas and apricots. It does not convert to vitamin A, but it may have important cancer fighting properties and other health benefits.
The beneficial actions of most carotenes, such as those in tomatoes, corn, and carrots, appear to be enhanced by cooking them, especially in oil (preferably olive, canola, or another monounsaturated oil). (Note: Cooking can also destroy certain nutrients, such as vitamin C, in these vegetables.)
Phytochemicals
The word phytochemical means plant chemical. Phytochemicals are plant-based nutrients, rather than vitamins. Researchers are studying hundreds of phytochemicals. Many are believed to have a major positive impact on human health. Some contribute to the bright and vivid colors found in fruits and vegetables.
The results of studies on certain phytochemicals may not necessarily apply to the vegetables or fruits that contain only small amounts of these chemicals. The health benefits of vegetables and fruits are probably due to some balance of phytochemicals, carotenoids, vitamins, fibers, and minerals rather than to any single substance.
The benefits of individual phytochemical supplements are unproven. Furthermore, these supplements are not regulated. High concentrations of some phytochemicals may act like drugs and be toxic, possibly even contributing to cancer cell growth.
Xanthophylls
Xanthophylls include lutein and zeaxanthin, which are both stored in the retina of the eye. Neither of these converts to vitamin A. Both lutein and zeaxanthin are powerful antioxidants that may be very important for healthy eyes.
Most xanthophylls are found in green vegetables such as broccoli, cabbage, and kale. They are also in yellow fruits and vegetables. Cooking may reduce the antioxidant activity of some xanthophylls in foods, although probably not to any significant degree.
Polyphenols and Flavonoids
Polyphenols are important phytochemicals. Flavonoids (or catechins) are members of the polyphenol family that may have significant health benefits. Laboratory (but not human) studies have shown that specific flavonoids suppress tumor growth, interfere with sex hormones, prevent blood clots, and have anti-inflammatory properties. Flavonoids are found in celery, cranberries, onions, kale, dark chocolate, broccoli, apples, cherries, berries, tea, red wine or purple grape juice, parsley, soybeans, tomatoes, eggplant, and thyme. Most common berries contain flavonoids and are rich in potent antioxidants.
Resveratrol, quercetin, and catechin are three important flavonoids. Evidence suggests that resveratrol (found in red wine, grapes, and olive oil) may be extremely potent. In laboratory studies, resveratrol increased cell survival and lengthened the lifespan of worms and fruit flies. Catechins are the primary flavonoids in tea, and may be responsible for its healthful effects. Flavonoids in dark chocolate may also have health benefits.
Isoflavones (Phytoestrogens)
Isoflavones, commonly known as phytoestrogens, have actions that are similar to the female hormone estrogen. However, no evidence to date indicates that soy-rich foods or phytoestrogen supplements help with hot flashes or other menopause symptoms.
Lignan is another phytoestrogen. It is found in whole grains, berries, some seeds, some vegetables, and a few fruits. In laboratory studies, it seems to have anti-cancer properties.
Isothiocyanates
Isothiocyanates are responsible for the sharp taste in cruciferous vegetables such as broccoli, cabbage, Brussels sprouts, cauliflower, collards, kale, kohlrabi, mustard greens, rutabaga, turnips, and bok choy. Isothiocyanates stimulate enzymes that convert estrogen to a more harmless form and may block the steroid hormones that promote breast and prostate cancers. (Cruciferous vegetables are also high in fiber, vitamin C, and selenium.)
Monoterpenes
Monoterpenes contain two important phytochemicals, perillyl alcohol and limonene. They block proteins that stimulate cell growth and reproduction, and are being tested for their ability to fight cancer. Limonene is found in the peels of citrus fruits.
Organosulfur Compounds
Organosulfurs are found in garlic, leeks, onions, chives, scallions, and shallots. They are part of the allium family of phytochemicals. Organosulfur compounds such as allicin may benefit the immune system, help the liver make cancer-causing substances harmless, and reduce the production of cholesterol in the liver.
Capsaicin
Capsaicin is found in hot red peppers. It seems to reduce levels of substance P, a compound that contributes to inflammation and the delivery of pain impulses from the central nervous system. Research suggests that it may inhibit cancer-causing substances.
Sterols
Sterols, which include sitosterol, stigmasterol, campesterol, and squalene, are found in vegetable oils. Sitosterol is the best-studied sterol, and it appears to have cholesterol-lowering effects.
Beta-sitosterols may help improve urine flow and urinary symptoms in men with an enlarged prostate gland (benign prostatic hyperplasia, or BPH). Beta-sitosterols come from South African star grass (Hypoxis rooperi) or the Pinus and Picea tree species.
Healthy Foods
Evidence increasingly suggests that it's not individual food chemicals, but a varied diet that is essential for good health and longevity. These types of diets are rich in fresh fruits and vegetables and whole grains, and low in saturated fats.
Some Examples of Healthy Foods | |||
Foods | Phytochemicals and Carotenoids | Vitamins and other valuable food components | Claimed Benefits |
Apples | Flavonoids | Thought to work against certain cancers (lung), but this is not proven. Also may help maintain healthy cholesterol levels. | |
Beans | Flavonoids | Folate, iron, potassium, and zinc | Some experts believe beans are the perfect food, because they are high in protein and fiber, and they have beneficial effects on the digestive tract and heart. |
Berries, dark-colored | Ellagic Acid | Vitamin C, minerals | The anthocyanins in berries such as blueberries, cranberries, and elderberries have many health properties, including antioxidant effects. Blueberries may help protect the aging brain. |
Broccoli (also kale, Brussels sprouts, and cauliflower) | Flavonoids, isothiocyanates, lutein, beta- and alpha-carotene. Note: Young sprouts of broccoli and cauliflower contain much higher levels of isothiocyanates than their mature forms. | Vitamin C, folate, fiber, and selenium | The selenium in these vegetables may have anti-cancer properties, but this is not proven. |
Carrots and other bright yellow vegetables | Lutein, beta-carotene, and other provitamin A carotenoids | Vitamin A (converted from carotenoids), vitamin C | Protects eyes, lungs. (Cooking carrots may increase the potency of their nutrients.) |
Chocolate, dark. Note: Milk chocolate does not have health benefits. | Flavonoids | Heart protective (may help prevent blood clotting). Claimed to have protective properties against lung cancer (not other cancers), but this is not proven. | |
Eggs | Lutein | Many B vitamins, vitamin A, vitamin D | Although egg yolks are high in cholesterol, eggs do not appear to be harmful for people with normal cholesterol levels. (People with diabetes or those with high cholesterol should limit eggs, however.) |
Fish, oily (mackerel, salmon, sardines) | Vitamins B3, B12. Essential fatty acids, selenium | Protects the heart and brain. | |
Garlic | Allium (organosulfurs) | Claimed to protect against certain cancers, heart disease, and infection, but this is not proven. Heating garlic can reduce its benefits. Allowing crushed fresh garlic to stand for 10 minutes before heating, however, may preserve beneficial chemicals while cooking. | |
Ginger | Zingiberaceae | Claimed to have cancer-fighting properties. | |
Grains (whole) | Lignans (phytoestrogens) | Vitamin B, selenium (important antioxidant mineral), fiber, folate | Claimed to reduce the ability of cancer cells to invade healthy tissue, but this is not proven. |
Grapes, including purple grape juice, and red wine | Flavonoids, (resveratrol, quercetin, and catechin) | Claimed to fight heart disease and cancer, and to lower the risk for asthma, but these effects have not been proven. | |
Nuts | Vitamin E, vitamin B1, essential fatty acids, folate | Heart-healthy benefits include lowering LDL cholesterol and reducing the risk of developing blood clots that can lead to a heart attack. | |
Onions | Flavonoids, allium (organosulfurs) | Claimed to have activity against certain cancers (lung). | |
Oranges | Monoterpenes | Vitamin C, folate, potassium. | Many health benefits, including increasing "good" HDL cholesterol levels. |
Potatoes (sweet) | Vitamin C, vitamin E, vitamin A | Many health benefits, including antioxidant and anti-inflammatory properties. | |
Soy (the best sources are tofu, soy milk, or whole soy protein) | Isoflavones (phytoestrogens), flavonoids, phytosterol, phytate, saponins | May have effects similar to estrogen, including maintaining bone calcium. May also help protect against prostate cancer and possibly other cancers, but this is poorly proven. More studies are needed. The American Cancer Society recommends that women with breast cancer eat only moderate amounts of soy foods and avoid taking dietary supplements that are high in isoflavones. Isoflavones are a type of phytoestrogen (estrogen-like plant chemical). There have been concerns that high intakes of soy may increase the risk of estrogen-responsive cancers such as breast cancer. | |
Spinach and other dark green leafy vegetables | Zeaxanthin, beta-carotene | Vitamin C, folate, vitamin A (converted from carotenoids) | May protect the lungs and brain. |
Tea (Both black and green tea are beneficial, but green tea appears to have the greatest effectiveness.) | Flavonoids (primarily catechins) | Cancer-fighting properties, particularly in green tea, which may be especially beneficial for smokers. Both black and green tea may protect against heart disease and stroke, although studies are mixed. Tea drinking also may help control weight and prevent osteoporosis. | |
Tomatoes | Lycopene, flavonoids | Vitamin C, biotin, minerals | Lycopene, which is found in tomatoes, has been a target of research. However, the evidence that it protects against prostate cancer is not conclusive. Lycopene may help fight infections. |
Note on Organic versus Inorganic Products. There is some evidence that organic produce has higher levels of antioxidants and that some agricultural chemicals may destroy flavonoids. Nevertheless, organic produce is expensive. Fruits and vegetables, no matter how they are grown, are still filled with healthful nutrients. | |||
Dietary Health Benefits
The best way to get healthy levels of important nutrients is by eating healthy foods.
However, the benefit of any individual food or nutrient is very difficult to prove. To date, there is little evidence that most dietary supplements protect against major diseases in otherwise healthy people with normal eating habits. A diet that is naturally high in vitamins and minerals can be the best defense against many diseases. Fresh fruits and vegetables and whole grains are the primary sources of vitamins, carotenoids, and vitamins, as well as of fiber and important minerals.
Antioxidants: Pros and Cons
Free Radicals (Oxidants). Currently, the most important benefit claimed for vitamins A, C, E, and many of the carotenoids and phytochemicals is their role as antioxidants, which are scavengers of particles known as free radicals (also sometimes called oxidants). These chemically active particles are byproducts of many of the body's normal chemical processes.
Warnings on High-Dose Antioxidants and Other Supplements. Antioxidant vitamins (A, C, and E), carotenoids, and many phytochemicals can neutralize harmful free radicals. Although it is clear that small amounts of these vitamins prevent deficiency diseases, high-dose vitamin C, vitamin E, and beta-carotene supplements may also have pro-oxidant effects, which can be harmful in patients with cancer. In these people, high doses of antioxidant vitamins may actually protect cancer cells just as they do healthy cells. Because there is no strong evidence that antioxidants decrease the risk of cancer, people should talk with their physician before taking higher doses of these vitamins or supplements.
Protection against Heart Disease
A low dietary intake of vitamins A, C, E, and beta-carotene has been linked to heart disease. Deficiencies in the B vitamins folate (also known as folic acid) and B12 have been associated with high blood levels of the amino acid homocysteine. Higher homocysteine levels may increase the risk for heart disease, stroke, and heart failure. However, supplements of these vitamins, alone or in combination, have not been shown to protect against heart disease.
Calcium, which is often taken along with vitamin D to protect bones, may actually increase the risk for heart disease. Research suggests that calcium supplements, when taken alone or with vitamin D, increase the likelihood of heart problems, especially heart attack. This risk may prompt older adults to reevaluate their use of calcium supplements for bone protection. More research on the subject is needed.
Phytochemicals and Heart Protection. Foods containing phytochemicals have been shown to protect the heart. However, the reduced risk of heart disease is more likely due to a generally health diet that is low in harmful fats and other unhealthy foods than to any one fruit or vegetable.
Phytochemicals that may benefit the heart:
- Flavonoids. Certain flavonoids, which are found in both black and green teas, dark chocolate, onions, red wine or red grape juice, and apples, may protect the heart. Flavonoids may help prevent cholesterol damage and blood clots, but further studies are needed.
- Organosulfurs. Organosulfurs, found in onions and garlic, have been under investigation for their possible cholesterol-lowering effects.
- Isoflavones. Soy protein, found in foods like tofu and soy milk, is the best-studied source of isoflavones (known as phytoestrogens, or plant estrogens), which may help lower LDL cholesterol and triglycerides.
- Sterols. Plant sterols, including sitosterol, are also proving to be potent cholesterol fighters. They work by blocking the absorption of cholesterol in the intestine.
Protection against Stroke
A healthy diet that is rich in fruits and vegetables and low in salt and saturated fat may significantly lower the risk for a first stroke, perhaps by protecting against high blood pressure -- a major stroke risk factor.
Vitamins and Stroke Protection. The effects of antioxidant vitamins and carotenoids (vitamins C or E, or beta-carotene) on stroke have been studied extensively. Most studies have found that these vitamins do not protect against stroke. Although B vitamin supplements help lower homocysteine levels, they have no effect on heart disease outcomes. More research is under way to evaluate the effect of vitamins on reducing stroke risk.
Protection against Cancer
Studies have found that diets rich in fresh fruits and vegetables may decrease the risk for many cancers, including cancers of the lung, breast, colon, and prostate. Examples of possible cancer-fighting foods include cruciferous vegetables (such as cabbage, Brussels sprouts, and broccoli), tomatoes (which contain lycopene), and carrots (which contain alpha-carotene).
Because many cancers are thought to be caused by the effects of oxygen-free radicals on DNA, the antioxidants A, C, and E and beta-carotene have been intensively studied for cancer prevention.
For the most part, vitamin or phytochemical supplements have not shown any benefit against cancer. Any protective effects of vitamins or phytochemicals against cancer may depend on the combined effect of these nutrients in foods. In 2006, a National Institutes of Health study reviewed randomized trials that evaluated the effectiveness and safety of multivitamin and mineral supplements for preventing cancer and chronic disease. The reviewers concluded that current evidence is not sufficient to determine whether multivitamin and mineral supplements prevent cancer or chronic disease.
Certain supplements may actually encourage tumor growth. Research finds that beta-carotene supplements increase lung cancer risk in smokers and people who have been exposed to asbestos.
Vitamins and breast cancer prevention:
- Eating foods that are rich in calcium and vitamin D (such as low-fat or non-fat yogurt and milk) may modestly reduce the risk of breast cancer in premenopausal women. It is not clear whether fruits and vegetables can prevent breast cancer from developing or returning.
- The American Cancer Society recommends that women with breast cancer eat only moderate amounts of soy foods and avoid taking dietary supplements that contain high amounts of isoflavones. Isoflavones are a type of phytoestrogen (estrogen-like plant chemical). There have been concerns that high intakes of soy may increase the risk of estrogen-responsive cancers such as breast cancer.
Dietary supplements and other cancers:
- Taking folic acid supplements does NOT lower the risk of developing colorectal cancer. In fact, supplements appear to increase the risk for repeated colorectal polyps. Eating a diet rich in fruits and vegeteables may provide some protection against colorectal cancers, however.
- There is NOT enough evidence to support the idea that antioxidant supplements (including vitamins C, E, A, folate, and beta-carotene) decrease the risk for lung cancer. Evidence now suggests that vitamin C, vitamin E, and beta-carotene supplements may actually increase lung cancer risk in smokers.
- Evidence has clearly shown that vitamin E does NOT prevent prostate cancer, and it may increase the risk. While eating soy foods may decrease the risk for prostate cancer, soy or isoflavone supplements have not yet been proven to reduce that risk.
- Selenium supplements do NOT reduce the risk of bladder or prostate cancer.
- A combination of selenium, beta-carotene, and alpha-tocopherol may decrease the risk of death from gastric cancer.
- There is some evidence that, in women who are already at high risk for skin cancer, vitamin D plus calcium may offer some protection against melanoma.
Studies on the health benefits of vitamins and minerals have some important limitations. Some are held to rigorous standards, while others are not. In most cases, the results of existing research are complex, because they can be complicated by factors such as diet, exercise, healthy or unhealthy lifestyle behaviors, as well as environmental and genetic factors.
Evidence of Protection against Other Diseases from Vitamins, Carotenoids, and Phytochemicals | |||
Disease or Condition | Vitamins | Carotenoids, Phytochemicals, and Healthy Foods | |
Alzheimer's Disease | Vitamin E. There is some evidence that foods high in vitamin E might reduce the risks of dementia and Alzheimer's disease. B Vitamins. Some studies suggest that deficiencies of vitamins B6, B12, and folate (folic acid) may be a risk factor for Alzheimer's disease. However, there is no evidence from randomized, controlled trials that these supplements prevent Alzheimer's disease. Vitamin D. There is some evidence that older adults with low vitamin D levels are more likely to experience cognitive decline, including problems with thinking, learning, and memory. Whether supplementing with vitamin D can help reverse these cognitive changes is unclear. | ||
Infectious Disease | Studies are mixed as to whether vitamin supplements protect against upper respiratory infections. The evidence suggests there is little or no benefit. It is possible that vitamin C or multivitamin supplements may be helpful in specific people, such as those who are vitamin deficient or who have medical problems that impair their immune system. | ||
Eye Disorder | Cataracts and Macular Degeneration. Oxygen-free radicals play a role in cataract formation and age-related macular degeneration, the most common cause of irreversible blindness in the elderly. A low level of vitamin C in the lens of the eye has been a particularly strong predictor of cataracts. People with cataracts are often deficient in vitamin A, the carotenes, lutein, and zeaxanthin. Studies have not demonstrated that antioxidant vitamin supplements (such as vitamins C and E) prevent cataracts, however. A combination of zinc and antioxidants, including vitamins C and E, may slow the progression of macular degeneration. However, these supplements will not prevent macular degeneration from developing in the first place. | Several studies have associated antioxidant-rich foods with a decreased risk for cataracts. Carotenoids, especially lutein, lycopene, and zeaxanthin, are especially eye-protective. However, there is not enough evidence to suggest that taking supplements containing carotenoids lowers the risk for cataracts. | |
Osteoporosis | Vitamin D. A combination of calcium and vitamin D can reduce the risk of osteoporosis. To keep bones strong, the National Osteoporosis Foundation (NOF) recommends:
| ||
Menstrual Disorders | Vitamin B6. Limited evidence suggests that vitamin B6 may help reduce premenstrual symptoms, including depression. Typically, women take 100 mg of vitamin B6 per day, although one study suggested that a lower dose (50 mg) may have the same effect. | ||
Resources
- http://fnic.nal.usda.gov -- The Food and Nutrition Information Center
- http://dietary-supplements.info.nih.gov -- Office of Dietary Supplements, National Institutes of Health
- www.ars.usda.gov/ba/bhnrc/ndl -- Nutrient Data Laboratory
- www.fda.gov -- Food and Drug Administration
- www.eatright.org -- The American Dietetic Association
- www.acsh.org -- American Council on Science and Health
- www.aicr.org -- American Institute for Cancer Research
- www.nutritiondata.com -- Information on vitamins and nutrients in foods
- www.consumerlab.com -- Independent testing of nutritional supplements' contents and quality
- www.usp.org -- US Pharmacopeia
- www.herbs.org -- Herb Research Foundation
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Review Date:
10/18/2011 Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc. |
