Do dietary supplements have beneficial health effects in industrialized nations: what is the evidence?

Abstract

Background: Dietary supplements are regularly used by at least half of the American population, yet the health benefits of these agents are unclear.

Objective: A systematic review to determine the benefits and risks of dietary supplements in Westernized societies.

Data sources: MEDLINE, Embase, Cochrane Register of Controlled Trials and citation review of relevant articles.

Study selection: Randomized, placebo-controlled clinical trials in non-pregnant Westernized adults that evaluated clinical outcomes of nutritional supplements.

Data extraction: Data were abstracted on study design, study size, study setting, patient population, dietary intervention and clinical outcomes. The outcome of each study was classified as non-beneficial, beneficial or harmful according to whether the end-point(s) of interest reached statistical significance.

Data synthesis: Sixty-three studies met the criteria for our systematic review. No benefit was recorded in 45 studies, with 10 of these showing a trend towards harm and with two showing a trend towards benefit. Four studies reported harm with increased cancer deaths (n=2) and increased fractures (n=2). Two studies reported both a harmful as well as a beneficial outcome. A beneficial outcome was reported in 12 studies; 6 which studied vitamin D and three which investigated omega-3 fatty acids. While a benefit was reported in one study each which investigated Vitamin E, folic acid and Ginkgo biloba this benefit was not confirmed by larger and more adequately powered studies.

Conclusions: With the possible exceptions of Vitamin D and omega-3 fatty acids there is no data to support the widespread use of dietary supplements in Westernized populations; indeed, many of these supplements may be harmful.