A new US review concluded that there is only limited evidence that vitamin and mineral supplementation can prevent the development of cancer or cardiovascular disease in healthy people without known nutritional deficiencies. The researchers acknowledge, however, that the data analysis has weaknesses.
The review analyzed data from 24 randomized controlled trials and two cohort studies that examined the potential benefit and harm of using vitamin (A, C, D, E and beta-carotene) and mineral (calcium) supple- ments for primary prevention of cardiovascular disease (CVD), cancer, or all-cause mortality in a total of 352,311 healthy participants without known nutritional deficiencies (1). The analysis of the clinical trials showed no clear evidence of either the benefit or harm regarding the intake of the single or combined micro- nutrient intakes. Only two large trials reported lower cancer incidence in men taking a multivitamin for more than ten years.
The researchers commented that the data analysis has limitations as it included only primary prevention studies in older adults without known nutritional deficiencies. Thus, the results are not generalizable to the potential efficacy of supplements in people with insufficient or deficient micronutrient intakes or higher-risk individuals. Future studies should clearly identify and separate individuals with already adequate nutrient supply and those with insufficient intakes before they enter the trial. In addition, the studies investigated various supplements and doses for less than ten years. The used doses may have been too low and duration may have been too short to show any preventive effect.
Moreover, the scientists conceded that the review mainly included randomized controlled trials (RCTs), a study design used primarily to evaluate drug therapy, which might not be ideally suited to evaluating nutrient effects. While a control (i.e. placebo) group in an RCT investigating medications is not exposed to the drug, the control group is exposed to a certain extent in a nutrient supplementation study. To conduct a study on the efficacy of nutrients, one must know both the participants’ usual and optimal level of exposure to the nutrients. In practice, however, exposure to the nutrients in the control group is often not known or may change over the course of a trial, which biases trial results. Women in one of the study’s control groups, for example, had twice the average calcium intake of the intake anticipated when the trial was designed, and the vitamin D dose was lower than many now think is necessary to achieve optimal blood levels.
Experts added that micronutrients work in synergy with other nutrients, and also likely in combination with other lifestyle choices, such as exercise. Filling gaps of dietary micronutrient intake with supplements should be considered as one part of a healthy lifestyle, not as a magic solution for preventing serious multifactorial diseases like cancer and cardiovascular disease.