| Dietary supplementation of vitamins
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| Supplementation of some vitamins provides clear health benefit
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Areas where the benefits of vitamin supplementation are clear include supplementation of folic acid to women who are pregnant or are planning pregnancy, to prevent neural tube defects. Vitamin D provision to people living in areas of low sunlight has also been beneficial.
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| Benefits of vitamin supplementation in cancer and cardiovascular disease are uncertain
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| Because the supplementation of folic acid and vitamin B6 and B12 lowers plasma homocysteine concentration, it has been suggested that it could be beneficial for the prevention of cardiovascular disease. There also were suggestions that supplementation of vitamins A, C and E is protective against cancer. Some observational studies suggested that the supplementation of vitamins C and E could also be useful in the prevention of cardiovascular disease. However, prospective studies of this yielded controversial results. The recommendations of the US Preventive Services Task Force published in 2003 (www. preventiveservices.ahrq.gov) say that 'current evidence is insufficient to recommend for or against the use of supplements of vitamins A, C, or E, multivitamins with folic acid, or antioxidant combinations for the prevention of cancer or cardiovascular disease' (note that these recommendations do not apply to people with nutritional deficiencies, pregnant and lactating women, children, elderly persons and people with chronic illnesses).
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| As mentioned above, high-dose vitamin supplementation may be harmful: the example is the reduction of bone mineral density, hepatotoxicity, and teratogenecity associated with high doses of vitamin A. β-carotene supplementation to smokers was also harmful, resulting in an increase in lung cancer mortality.
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| Fruit and vegetables are the best sources of vitamins
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| In the clinical studies mentioned above the vitamins were supplemented in a pure form, rather than as complete foodstuffs: and it might be that this is why the benefit of supplementation was not evident. Clearly there are benefits of eating diets rich in vegetables and fruit, which are the most important sources of vitamins. However, apart from proven instances of toxicity in excess, there is no reason to discourage people from taking vitamin supplements.
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