There is still insufficient evidence to determine whether vitamins A, C and E or multivitamins can reduce the risk for cardiovascular disease or cancer, concludes a report by the US Preventive Services Task Force (USPSTF), a government agency that provides evidence and research on health care outcomes.
"The available evidence from randomised trials is either inadequate or conflicting, and the influence of confounding variables on observed outcomes in observational studies cannot be determined. As a result, the USPSTF could not determine the balance of benefits and harms of routine use of supplements of vitamins A, C, or E, multivitamins with folic acid, or antioxidant combinations for the prevention of cancer or cardiovascular disease," wrote the authors in the 1 July issue of the Annals of Internal Medicine.
The USPSTF also recommends against the use of beta-carotene supplements to protect against cancer or cardiovascular disease, saying that it had found good evidence that beta-carotene supplementation provides no benefit in the prevention of cancer or cardiovascular disease in middle-aged and older adults.
The agency said that the vitamin was unlikely to provide important benefits and might cause harm in some groups (referring to two trials on heavy smokers where beta-carotene supplementation was associated with higher incidence of lung cancer and higher all-cause mortality), although it added that there is no evidence to suggest that beta-carotene occurring naturally in foods is harmful to smokers.
The group did stress that vitamin supplements could be important for people with nutritional deficiencies, and that "there is little reason to discourage people from taking vitamin supplements" for other health benefits.
"The value of taking vitamin supplements for other purposes, such as folic acid supplementation by women capable of pregnancy to prevent the birth of babies with neural tube defects, has stronger scientific support," wrote the authors. However they also said that there is more consistent evidence that a diet high in fruit, vegetables, and legumes has important benefits, suggesting that other constituents besides vitamins may account for the benefits of such diets.
The report, which refers both to positive results of multivitamins and the inconsistencies of certain trials, comes after a series of studies and reports questioning the benefits of vitamins. There are enough positive findings to prompt further research however, such as studies on the primary prevention of cardiovascular disease.
Last week doctors writing in the British Medical Journal proposed a 'polypill' composed of several components that could prevent 80 or 90 per cent of heart disease and stroke. The pill included folic acid, showing the value stored by certain scientists in the B vitamin's role in reducing homocysteine levels and therefore reducing the risk of cardiovascular disease.