With a few exceptions, regular consumption will probably do you no good

Some 20-30% of the population in developed countries take a daily vitamin supplement. Does it do them any good? Our current estimates of vitamin requirements are based on the amounts needed to prevent deficiency diseases; in most countries deficiency is no longer a major problem. The question is whether higher levels of intake provide health benefits. There are two ways to answer this question: to identify biomarkers of optimum nutritional status, rather than the absence of deficiency; or epidemiological studies to identify nutrients associated with a lower incidence of chronic diseases, followed by intervention studies. Neither approach has yet provided satisfactory answers, and a recent review finds little convincing evidence in favour of supplements.¹

A number of promising suggestions for biomarkers exist, including metabolic markers of damage from radicals, immune responses, and damage to DNA. None is responsive to only a single nutrient, and all are affected by a plethora of non-nutritional factors.^{2 3} To date we do not have any markers that can be used to determine optimum intakes.

The epidemiological approach has prompted a number of intervention trials, most of which have been disappointing. There is clear epidemiological evidence that people with a high plasma concentration of vitamin E are less at risk from cardiovascular disease. The Cambridge heart antioxidant study showed a reduction in non-fatal but not in fatal myocardial infarctions.⁴ While the benefits from reducing non-fatal infarctions are obvious, this is hardly convincing evidence of the benefits of vitamin E supplementation.

Similarly, there is evidence that high intakes of &bgr; carotene are associated with lower incidence of lung, prostate, and other cancers, although &bgr; carotene may simply be a marker of fruit and vegetable consumption. Carotenes are antioxidants and might be expected to reduce the damage from radicals that underlies the development of cancer and cardiovascular disease. However, most compounds that act as antioxidants do so by forming stable radicals that persist long enough to undergo metabolism to non-radical compounds. By definition they therefore form radicals that can penetrate deeper into tissues and plasma lipoproteins, and potentially cause more damage than the oxygen radicals they have replaced. The results of two major intervention studies with &bgr; carotene, one in Finland among smokers and the other in the United States among people who had been exposed to asbestos, yielded unexpected and unwanted results: more people receiving the supposedly protective supplements died from lung (and other) cancer than people receiving placebo.⁵

Vitamin C is an antioxidant, and it also inhibits the formation of carcinogenic nitrosamines from dietary amines and nitrites. It might therefore be expected to have protective action against the development of cancer and cardiovascular disease. The evidence with respect to cardiovascular disease is unconvincing.¹ The epidemiological evidence linking a high intake of vitamin C with reduced cancer incidence is confounded by the fact that the fruits and vegetables that are sources of vitamin C are also rich in a variety of other compounds that may be protective. There is a long held belief that vitamin C (perhaps in very large amounts) is protective against the common cold. A systematic review did not support this but did find some evidence of a modest benefit in reducing the duration of symptoms of colds.⁷

An intake of vitamin D above what can be obtained from normal diets (possibly in combination with supplementary calcium) delays the loss of bone with increasing age, so supplements may be advisable to prevent osteoporosis and osteomalacia.⁸ For most people, increased exposure to sunlight is probably more effective than supplements, although we may have to balance the beneficial effects on bone health against the increased risk of skin cancer.

The benefits of folic acid supplements taken periconceptually in preventing neural tube defect have been shown convincingly.⁹ High intakes of folic acid also reduce plasma homocysteine, a risk factor for cardiovascular disease independent of plasma lipids and other risk factors, and low intakes of folic acid are associated with increased risk of colorectal cancer.¹⁰ This has led to mandatory fortification of cereal products in the United States and elsewhere. However, although folic acid lowers plasma homocysteine, there is no evidence yet from controlled trials whether or not this will reduce cardiovascular disease or cancer. Until the results of intervention trials in progress are available, the benefits of folic acid supplements other than to prevent neural tube defects remain unproved.¹²

The answer to the question of whether we should take a multivitamin tablet every day must be that unless our intake is inadequate as a result of a poor diet then supplements will probably do us no good—apart from folic acid taken periconceptually and, possibly, vitamin D by elderly people.