Dietary Supplements Offer Little to No Benefit and May Be Harmful

A recent Journal of Parenteral and Enteral Nutrition (JPEN) systematic review revealed that, with a few possible exceptions, dietary supplements offer no benefits to well-nourished adults eating a Western diet and, in many cases, may be harmful. The results of this study reinforce Worst Pills, Best Pills News’ longstanding view that there is little evidence that dietary supplements are either safe or effective.

What are dietary supplements, and how are they regulated?

Used regularly by at least half of all Americans, dietary supplements are defined by law (see the paragraph below) as any products intended to supplement the diet that contain a vitamin, a mineral, an herb or other botanical; an amino acid; or “a dietary substance for use by man to supplement the diet by increasing the total dietary intake.” According to the Food and Drug Administration (FDA), dietary substances include enzymes or tissues from animal organs or glands.

The use of dietary supplements has grown steadily since 1994, when Congress passed the Dietary Supplement Health and Education Act (DSHEA), and is now widespread in the America. DSHEA clarified that supplements were to be regulated as foods, not drugs, and thus were exempt from the tougher regulations accorded to drugs, such as the requirement to prove that they are both safe and effective.

No supplement has been demonstrated to be safe and effective under the rigorous standards the FDA applies to drugs. Furthermore, while drug companies have to report any serious or unexpected adverse event they learn about to the FDA, there is no such reporting requirement for manufacturers of dietary supplements.

Still, manufacturers are permitted to aggressively promote dietary supplements. While a supplement manufacturer is prohibited under FDA regulations from making health claims when promoting a supplement, it may make structure or function claims. For example, a supplement manufacturer, without any supporting evidence whatsoever, can assert that its product “promotes prostate health” (a structure or function claim) but is precluded from claiming that it “treats the symptoms of an enlarged prostate” (a health claim).

JPEN study overview

Researchers have conducted numerous studies to assess the safety and effectiveness of some commonly used dietary supplements. The quality and validity of these studies is highly variable.

In the JPEN article, Dr. Paul Marik and his co-author conducted a systematic review of published randomized controlled trials (RCTs) — the gold standard for clinical trial design — that evaluated the benefits and safety of dietary supplements. The researchers limited their analysis to studies involving adults and evaluating objective, clinically relevant outcomes, including heart attack, stroke, death from cardiovascular disease, cancer (new or recurrent), death from cancer, death from any cause, type 2 diabetes, fractures, change in cognitive function, falls and visual acuity.

The authors excluded from their review studies involving undernourished patients, patients with specific nutritional disorders, pediatric patients and pregnant women. They also excluded RCTs enrolling fewer than 200 subjects (because such studies are more prone to statistical error) as well as RCTs lasting less than one year (because there is likely to be a time delay between starting a dietary supplement and any detectable clinical outcome).

The authors searched multiple medical literature databases for studies published between 1966 and 2010 that met the above criteria. Their search found 63 RCTs that had enrolled a total of 428,357 subjects, with an average of 6,693 subjects per trial. The average study duration was 4.7 years.

Findings of JPEN systematic review

The 63 RCTs in the review included evaluations of the following dietary supplements, either alone or in combination: beta-carotene; vitamins A, B6, B12, C, D and E; folic acid; calcium; selenium; zinc; omega-3 fatty acids; ginkgo biloba; glucosamine; saw palmetto; and milk thistle (the table below provides a summary of these studies).

Marik and his co-author reported that 43 RCTs (68 percent) showed no statistically significant benefit for the dietary supplements being evaluated. Of these studies, 10 actually showed a trend toward harm in one or more adverse outcomes, and one showed a trend toward a benefit. But these trends were not statistically significant, so these trials are not counted as showing a definitive benefit or harm.

Five of the RCTs (8 percent) showed statistically significant evidence of harm:

• One clinical trial testing vitamin A and beta-carotene — and another evaluating folic acid, vitamin B6 and vitamin B12 — demonstrated an increased risk of cancer and cancer mortality
• Two studies evaluating vitamin D supplementation in elderly adults revealed an increased risk of fractures, although, as discussed below, several other studies found the opposite outcome.
• One study in elderly people found that vitamin E supplementation was associated with more severe upper tract respiratory infections (colds).

One trial (2 percent) demonstrated both benefits and harms with supplements. The trial evaluated the effect of selenium in preventing cancer in 1,312 patients who previously had skin cancer. Treatment with selenium increased the risk of type 2 diabetes but decreased the risk of developing cancer. Of note, a much larger study included in the JPEN review, involving more than 35,000 subjects, showed no reduction in cancer risk in subjects treated with selenium alone or in combination with vitamin E.

Only 14 RCTs (22 percent) reported a beneficial outcome with dietary supplements:

• Six trials showing benefit involved vitamin D or vitamin D plus calcium, with three showing a reduction in the risk of fractures, two a reduction in the risk of falls and one a reduction in the risk of cancer.
• One trial showed a reduced risk of fractures and colonic polyps in subjects treated with calcium supplements.
• Three trials demonstrating benefit involved omega-3 fatty acid supplements, with each finding a reduction in the risk of adverse cardiovascular events, such as angina, heart attack, stroke and death from cardiovascular causes. (However, intake of this nutrient can be significantly increased simply by eating more fish, especially salmon, herring, mackerel, anchovies, sardines and, to a lesser extent, tuna.)
• One trial testing vitamin E found a reduced risk of adverse cardiovascular events. However, three other much larger trials of vitamin E demonstrated no cardiovascular benefit.
• One study found that vitamin E slowed the progression of cataracts but showed no effect on visual acuity.
• One study of ginkgo biloba in 309 subjects with Alzheimer’s disease showed slightly better cognitive function outcomes. On the other hand, a much larger study of this supplement in elderly adults found no beneficial effects on cognitive function.
• Finally, one study of folic acid supplementation demonstrated improvement in cognitive function outcomes in adults older than age 50. Three other trials of folic acid detected no benefit, although these studies measured outcomes other than cognitive function.

Summary of Randomized Controlled Trials of Dietary Supplements

* CVS: adverse cardiovascular events
** Capitalized “BENEFIT” and “HARM” indicate statistically significant results.

Conclusions

The study authors concluded that with the possible exception of vitamin D in elderly patients and omega-3 fatty acids in patients with a history of cardiovascular disease, no data support the widespread use of dietary supplements in the U.S. and other Western countries. Indeed, the data suggest that certain commonly used dietary supplements, including beta-carotene, vitamin A and vitamin E, may be harmful. We agree.

This article has been revised to reflect the following correction:

Correction: December 14, 2012

An earlier version of this article stated that four clinical trials involving vitamin D or vitamin D plus calcium showed a reduction in the risk of fractures. The text has been corrected to indicate that three of the trials did so.