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Nutrient Supplementation in the United States

July 5th, 2005

According to recent representative national surveys (partly representing conditions before DSHEA), approximately 40 to 47% of the US population use vitamin or mineral supplements at least occasionally. Slightly more than two thirds of supplement users take only one supplement, generally a multiple vitamin with or without minerals. Current, reliable data are not available to adequately describe the amounts of specific nutrients consumed as supplements. Moreover, supplement use may be commonly underreported or inaccurately reported, especially when determined through self-administered questionnaires. Consistent with earlier reports, recent data indicate greater supplement usage among Caucasians, women, older age groups, those with higher personal incomes, those with more education, and those living in the western United States. Several reports indicate that supplement users (whether they are daily or occasional users) also have higher intakes of nutrients from foods, lower dietary fat, and higher fruit, vegetable, and dietary fiber consumption. Although these differences vary according to gender, age, and ethnicity, the greater nutrient intakes among supplement users, compared with nonusers, persist after adjusting for income, education, employment status, age, sex and ethnicity. Use of supplements has been positively associated with a lower body mass index, with agreement that “eating a variety of foods each day gives you all the vitamins and minerals you need,” with more frequent exercise, with self-described excellent or very good personal health, with not smoking, not drinking heavily, and believing that diet affects disease.

Because many of the lifestyle characteristics of supplement users are health related, unless studies test nutrient and other supplements in comparison with placebos, randomly and blindly assigned to volunteers that are otherwise similar, health benefits observed in such studies cannot be reliably attributed to supplement use.

The Need for Strong Scientific Evidence Based on Controlled Clinical Trials

Epidemiological studies show associations, but are not able to establish cause-and-effect because they do not control for other diet and lifestyle variables. These variables may also influence the results and may not be evident or even suspected. Cause-and-effect relationships can be most conclusively tested with prospective, randomized, double-blind, placebo-controlled supplementation trials. Such trials control for confounding variables by randomly assigning the supplement to participants, who are otherwise identically treated and evaluated by investigators. These experiments are generally better controlled with supplements rather than dietary changes, both because they require fewer and simpler behavioral changes and because the treatment can easily be blinded from both participants and investigators. Such studies must be conducted to provide firm research support for the safety and efficacy of dietary supplements.

As an example, although high fruit and vegetable consumption increases dietary and blood beta-carotene levels, controlled investigations with beta-carotene supplementation were needed to determine whether it accounted for the reduction in cancer risk that is associated with fruit and vegetable intake. Three randomized placebo-controlled trials prospectively investigated whether beta-carotene supplements would reduce cancer incidence. None of these three trials found beneficial effects of beta-carotene in reducing cancer incidence. Unexpectedly, in the two studies that included smokers or workers exposed to asbestos, beta-carotene supplementation resulted in a higher incidence of lung cancer and of total mortality in these participants. The third study, which involved US physicians, found no harm but no benefit from beta-carotene supplementation. The reason for the adverse effects is unclear. One potential explanation is that beta-carotene supplements increase blood levels to a greater degree than an equal amount of beta-carotene in food. Supplements may have different (either higher or lower) bioavailability and may affect body stores differently than foods, and more is not necessarily better. In supplemental amounts, beta-carotene may interfere with the intestinal absorption of other potentially beneficial related compounds, such as canth-axanthin, lutein, and lycopene. Or, especially in the lungs of smokers, large amounts of beta-carotene may be readily oxidized into pro-carcinogenic products. Only randomized, controlled supplementation trials were able to detect the pro-carcinogenic effect of beta-carotene in smokers. Before these studies, beta-carotene was widely considered safe, because there were no harmful short-term adverse effects, even with very high doses.

In addition to giving considerably greater acceptance to the results of randomized controlled trials and recognizing the limitations of descriptive observations, evaluations of the research literature should look for reproducibility of results. Rarely can a single study stand alone as scientific confirmation of a hypothesis. Results obtained from some groups may not be representative of results in other populations (eg, benefits observed in a developing country may not apply to a better-nourished Western population). Results may differ with age and gender. Beneficial effects may require longer studies or more sensitive testing. Conclusions are strengthened when the research measures the true functional or disease endpoints of interest, rather than intermediate biomarkers (eg, fracture incidence vs bone density; or initial myocardial infarction vs platelet aggregation). Similarly, conclusions drawn from experimental work in vitro, in cell-culture or in animal models should not be the basis for using dietary supplements without confirmation in controlled human studies.

In helping consumers with questions about dietary supplement claims, dietetics professionals can readily access an online database of medical research literature (eg, Medline) to determine the amount and kinds of scientific research available and select more in-depth reading as appropriate to the situation. As an example, a recent Medline search for chromium picolinate produced 76 citations. The list was reduced to 16 by searching for chromium picolinate and placebo. Of these, eight described placebo-controlled research on the effects of chromium picolinate on body composition: five found no effect, two found beneficial effects and one found an effect in women but not in men. Although additional reading is necessary to make valid conclusions about the relevance and quality of each study, this quick comparison suggests that the beneficial claims for this supplement have not been fully supported by research evidence. The Federal Trade Commission, which regulates the advertising of dietary supplements, has taken action against unsubstantiated weight loss and health benefit claims for chromium picolinate. Unfortunately, an Internet search also rapidly demonstrates that related claims continue to abound.

Entry Filed under: General Nutrition

Nutrient Intakes from Food, as Estimated from Dietary Surveys Circumstances When Nutrient Supplementation is Indicated