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For healthy people, supplements may help prevent vitamin and mineral deficiencies when the diet does not provide all necessary nutrients. They can also supply amounts of nutrients larger than the diet can provide. Larger amounts of some nutrients may help to protect against future disease. Many of these nutrients will be briefly discussed here. However, for more information, refer to individual nutrient articles.
People may consume diets that are deficient in one or more nutrients for a variety of reasons. The typical Western diet often supplies less than adequate amounts of several essential vitamins and minerals.1 Recent nutrition surveys in the U.S. have found large numbers of people consume too little calcium, magnesium, iron, zinc, and, possibly, copper and manganese.2, 3
Weight-loss, pure vegetarian, macrobiotic, and several other diets can also place some people at risk of deficiencies that vary with the type of diet. Certain groups of people are at especially high risk of dietary deficiencies. Studies have found that elderly people living in their own homes often have dietary deficiencies of vitamin D, vitamin A, vitamin E, calcium, and zinc,4 and occasionally of vitamin B1 and vitamin B2.5 Premenopausal women have been found often to consume low amounts of calcium, iron, vitamin A, and vitamin C.6
Dietary deficiency of vitamin A is uncommon in healthy people except in older age groups.7 Although vitamin A is important for the function of the immune system, vitamin A supplementation did not help prevent infections in elderly people living in nursing homes, in one study.8 Due to concerns about birth defects9 and bone loss,10 people should not take over 10,000 IU of supplemental vitamin A in the form of retinol without consulting a doctor.
Beta-carotene is a precursor to vitamin A, but may have a separate role in human health. Controlled research has shown that beta-carotene supplements can increase the numbers of some white blood cells and enhance cancer-fighting immune functions in healthy people who take 25,000 to 100,000 IU per day.11, 12 However, some studies of smokers have reported that supplements of synthetic beta-carotene increased the risk of both heart disease13, 14 and lung cancer.15, 16 Other trials found no positive or negative effect of synthetic beta-carotene on the risk of many other diseases, including several types of cancer,17, 18, 19, 20, 21, 22, 23, 24angina pectoris,25diabetes,26 age-related eye disease,27, 28 or intermittent claudication.29 Natural beta-carotene, though similar to synthetic, was found in one preliminary study to reverse pre-cancerous changes, while synthetic beta-carotene did not.30 No other studies have investigated whether natural beta-carotene could be more effective than synthetic in preventing other diseases, but the potential harm from taking synthetic beta-carotene suggests it should be avoided by smokers.
Some of the B-vitamins, including thiamine (vitamin B1), riboflavin (vitamin B2), and niacin (vitamin B3), may be adequately supplied by the typical Western diet, because they are added to white flour products and other foods that have been depleted of those vitamins. Another vitamin, biotin, is produced by intestinal bacteria in amounts that, along with typical dietary biotin intake, provide enough of this vitamin to prevent deficiency in healthy people. Pantothenic acid (vitamin B5), on the other hand, appears to be in short supply in the typical diet. In one study, 49% of a group of male and female adolescents were consuming less-than-adequate amounts of pantothenic acid in their diet.31 No research has investigated whether supplements of these B-vitamins prevent disease.
Vitamin B6 (pyridoxine) deficiency, at least in a mild form, may exist in 10 to 25% of people living in Western societies,32 and may be most common in the elderly.33, 34, 35 The possible role of vitamin B6 in the prevention of heart disease by helping to regulate blood homocysteine levels is discussed below. No other research on preventive effects of vitamin B6 supplementation has been done.
Folic acid deficiency has been considered somewhat common in the U.S. It affects about 11% of healthy people,36 with a higher prevalence among African-Americans and Mexican-Americans.37 Recently, however, the U.S. Food and Drug Administration (FDA) mandated that some grain products provide supplemental folic acid. This appears to be causing a reduction in the prevalence of folic acid deficiency in the general population.38 Nonetheless, some authorities believe the recent increases in folic acid content of the food supply are not enough to optimally prevent diseases such as heart disease and birth defects (see discussions below).39
The requirement for folic acid doubles during pregnancy,40 and insufficient intake of folic acid has been linked to low birth weight and an increased incidence of neural tube defects in newborns. Supplementation with 400 mcg per day of folic acid prior to, and shortly after, conception is effective in preventing neural tube defects.41
Vitamin B12 deficiency is not common among healthy young people,42 except for vegans (vegetarians who also avoid dairy and eggs).43 However, about 12 to 15% of the elderly in the U.S. have been found deficient in this vitamin.44, 45 Vitamin B12 deficiency may also occur in people who take acid-blocking drugs or antacids for long periods of time.46 Although vitamin B12 deficiency in the elderly is often due to age-related declines in absorption of vitamin B12 from food, vitamin B12 supplements can be absorbed sufficiently in members of this age group if they do not have pernicious anemia or other significant gastrointestinal disorders.47 Supplementation with 100 mcg per day of vitamin B12 was adequate to reverse vitamin B12 deficiency in healthy elderly people, according to one recent report.48
The B-vitamins folic acid, B12, and pyridoxine (vitamin B6) are important for the control of homocysteine levels in the blood.49 Elevated homocysteine levels are associated with several diseases, including heart disease,50stroke,51Alzheimer’s disease,52 and osteoporosis,53 and some, though not all, research suggests that homocysteine has a direct role in causing these diseases. Daily supplementation with these B-vitamins, typically at least 400 mcg of folic acid, 10 mg of vitamin B6, and 50 mcg of vitamin B12, lowers elevated homocysteine levels in most people.54, 55, 56 Some studies have shown that supplementing with one or more of these vitamins helps prevent or reverse hardening of the arteries (atherosclerosis) and may also reduce the risk of bone fractures.57, 58
Severe vitamin C deficiency is uncommon in people who consume Western diets, but mild insufficiency is found in 6% of healthy adults59 and larger numbers of college students60 and smokers.61 On the basis of extensive analyses of published vitamin C studies, some authorities have suggested that optimal intake for disease prevention may be at least 90 to 100 mg per day.62, 63 However, supplementation with over 200 mg per day of vitamin C by healthy people does not result in higher vitamin C concentrations in the body,64 and may be no more helpful for preventing disease than smaller amounts.
Vitamin D can be obtained from the diet or from sunlight exposure, but these sources can be insufficient, especially in older people and vegans during the winter months.65 In healthy adult Western European populations, 6 to 14% percent have been reported to be vitamin D deficient,66 but good data are not available for the US. Vitamin D insufficiency is associated with bone loss and fractures in older people.67, 68 Reduced bone loss from vitamin D supplements has been reported in some,69, 70 though not all,71 studies. In one double-blind study, supplementation with 800 IU per day of vitamin D prevented bone loss more effectively than 200 IU per day in postmenopausal women.72 In addition, vitamin D supplementation has been shown to reduce the risk of falls in older people.73 While vitamin D is known to be toxic in very high amounts, up to 2000 IU per day is considered safe.74
A nationwide study recently reported that 27% of the U.S. population had low blood levels of vitamin E.75 Supplementing with at least 100 IU per day of vitamin E is associated with lowered risk of heart disease,76, 77 and a double-blind study found that 400 to 800 IU of vitamin E per day reduced the risk of nonfatal heart attacks, but not fatal ones.78 However, another double-blind trial found no benefit from 400 IU per day of vitamin E supplementation on the risk of non-fatal heart attacks,79 while another study found that 50 IU per day had no effect on heart attack risk.80 A more recent study found that taking large amounts of vitamin E (400 IU per day or more) may result in a small increase in all-cause mortality,81 while another study found that 400 IU per day increased the risk of heart failure.82 Because of these studies, some doctors are advising people not to take large amounts of vitamin E. On the other hand, circumstantial evidence suggests that “mixed tocopherols,” as opposed to the more widely used alpha-tocopherol, may be safer and more beneficial with respect to heart-disease prevention.83, 84
A reduced risk of prostate cancer in smokers was reported in a double-blind trial with 50 IU per day of vitamin E.85 However, similar studies have not found vitamin E supplements to protect against other cancers.86, 87, 88, 89
Vitamin K deficiency severe enough to cause bleeding problems is rare in healthy people. However, low vitamin K in the blood90, 91 and in the diet92 has been associated with osteoporosis. Preliminary research has suggested that supplements of at least 1 mg per day of vitamin K reduce indicators of bone loss in some women.93, 94, 95, 96
Compared with recent calcium intake recommendations, most people have calcium-deficient diets, and less than 10% of women in the U.S. have adequate dietary intakes.97 Good calcium nutrition throughout life is essential for achieving peak bone mass and preventing deficiency-related bone loss.98 Calcium supplements are effective in increasing bone mass in children99, 100, 101 and slowing bone loss in adults according to most,102, 103, 104, 105, 106, 107 though not all,108 double-blind studies. Calcium supplements have also been shown to reduce the risk of bone fractures in some elderly adults.109, 110, 111, 112 The protective effect of calcium on bone is one of very few health claims permitted by the FDA. In order to achieve the 1,500 mg per day calcium intake deemed optimal by many researchers for postmenopausal women, 800 to 1,000 mg of supplemental calcium are generally added to diets that commonly contain between 500 and 700 mg of calcium per day.
Phosphorus is a necessary nutrient, but diets are almost always adequate in this mineral.113 Some authorities have suggested that excess intake of phosphorus is hazardous to normal calcium and bone metabolism.114 However, this idea has been challanged.115 In any case, for most people there does not seem to be a need for phosphorus supplementation. For this reason, many multivitamin-mineral supplements do not contain phosphorus. The only exception is for elderly people, whose diets tend to be lower in phosphorus. Calcium interferes with phosphorus absorption, so older people who are taking a calcium supplement might benefit from taking additional phosphorus.116
Dietary magnesium deficiency may occur in up to 25% of adult women in the U.S. and in even higher numbers of elderly people of both sexes.117 Magnesium supplements of at least 250 mg per day may help prevent bone loss.118, 119
While potassium is lower in modern diets compared with so-called primitive diets, true deficiencies are uncommon.120 Some,121 though not all,122 research suggests that raising potassium intake may help prevent high blood pressure. Other research suggests higher potassium intake may help prevent stroke.123 However, the maximum amount of supplemental potassium allowed in one pill (99 mg) is far below the recommended amounts (at least 2,400 mg per day). Multiple potassium pills should not be taken in an attempt to get a higher amount, since they can irritate the stomach. The best way to get extra potassium is to eat several servings per day of fruits, vegetables, or their juices.
Iron deficiency is not uncommon among some groups of healthy people, including some vegetarians,124 menstruating girls and women,125pregnant women,126 and female and adolescent athletes.127 Nonetheless, many people in these groups are not iron deficient 128 and excessive iron intake has been associated in some studies with heart disease,129 some cancers,130diabetes,131 increased risk of infection,132 and exacerbation of rheumatoid arthritis.133 While none of these links has yet been proven, people should avoid iron supplements unless they have been diagnosed with having, or being at high risk of, iron deficiency.
Iodine deficiency is a concern in many developing countries,134 Until recently it was considered abundant in Western diets, due to the introduction of iodized salt and the iodine added to many foods.135 However, iodine intake has decreased considerably in recent years and may be low in as many as 12% of the total U.S. population, and 15% of women of childbearing age.136 These numbers may be greater in countries where iodized salt is not available. Still, most people have adequate iodine intake unless they avoid iodized salt, seafood, and sea vegetables. People with thyroid diseases should check with their doctor before using iodine supplements.
Zinc deficiency is not common in Western countries, except in people with low incomes.137, 138 Zinc supplements (10 mg per day) have prevented growth impairment in deficient American and Canadian children.139 Supplementation with 25 to 150 mg of zinc per day has been shown to increase immune function in healthy people.140, 141, 142, 143 However, too much zinc has been reported to impair immune function and some healthcare practitioners recommend no more than 30 to 50 mg per day.144 It is unknown whether these immune system changes are sufficient to cause or prevent infections or other diseases in people taking zinc supplements. Regular supplementation with zinc should be accompanied by copper supplements to prevent zinc-induced copper deficiency.
The average dietary copper intake in the U.S. has been found to be below accepted standards.145 However, the significance of this is unclear, since symptomatic copper deficiency is quite rare.146 Supplementation with 3 mg per day of copper may help prevent bone loss.147 Since zinc can interfere with copper absorption, copper should be taken whenever zinc supplements are taken for more than a few weeks.148
Dietary intake of manganese is adequate for most people, according to recent studies in the U.S.149 However, manganese, along with other trace minerals, is often low in refined and processed foods.150, 151 People whose diets consist primarily of these types of foods may have low manganese intake. Manganese deficiency has been associated with osteoporosis in an unpublished study.152 A double-blind trial found that a combination of mineral supplements including manganese prevented bone loss in postmenopausal women.153 No other studies have investigated the health effects of manganese supplementation. Manganese may be especially important to include when iron is supplemented, since iron can reduce manganese absorption and cause lower body levels of manganese.154
Chromium nutrition has been difficult to study because of technical problems in analyzing foods and human body fluids for chromium content. Partly for this reason, there is disagreement about the extent of chromium deficiency in Western societies. Many studies have found sub-optimal levels of chromium in the diet, compared to published recommendations.155, 156, 157 However, some authorities question the validity of the recommended minimum requirements.158 Chromium deficiency has been associated with blood sugar and cholesterol abnormalities.159 Also, chromium levels in the body decline as people get older, which is when these problems often appear.160 Therefore, while chromium supplements have not been tested for their ability to prevent diabetes or heart disease, many healthcare practitioners recommend chromium supplements as a reasonable precaution. A few single case reports have described possible serious side effects in people taking large amounts of chromium, from 600 to 2400 mcg per day, 161, 162, 163 although it is not clear whether chromium was responsible for these reactions.
Dietary intake of selenium appears to be adequate in most people. This is according to recent studies in the U.S. based on the Recommended Dietary Allowance of 70 mcg per day of selenium.164 However, a double-blind study found that people given a supplement of 200 mcg of yeast-based selenium per day for 4.5 years had a 50% drop in the cancer death rate over seven years compared with the placebo group.165 Higher amounts of selenium than are available in the diet may be necessary for this protective effect. The upper end of safe long-term selenium intake has been estimated to be 350 to 400 mcg per day.166
Molybdenum is an essential trace element with low potential for toxicity.167 Since little is known about human needs and deficiencies are quite rare, estimated requirements are based on what people typically receive in their diets.168Cancer and cardiovascular disease prevention studies in China found no benefit from a supplement containing molybdenum and vitamin C.169, 170 No other research has investigated disease prevention with molybdenum supplements.