Harvard Heart Letter

Dietary supplements: Sorting out the science

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The widespread belief that most dietary supplements are effective and safe simply isn't true.

A few supplements show limited, possible benefits for people with heart disease. But some popular ones don't—and others may be dangerous.

Every day, about half of all American adults swallow a dietary supplement—a vitamin, mineral, herb, amino acid, or other substance. Most say they simply hope to improve or maintain their health, although many seek to stave off heart disease. As a nation, we shell out more than $32 billion a year on about 85,000 different products.

"A lot of people want to add something natural and alternative to the conventional medications they're taking, and they assume that dietary supplements might help and can't hurt," says Dr. Pieter Cohen, an assistant professor of medicine at Harvard Medical School. But that's not the case.

No benefit from popular pills

Unlike pharmaceuticals, which undergo extensive testing to prove they're effective and safe before they can be sold, dietary supplements can be sold without proof of effectiveness or safety. Moreover, supplement makers can claim their products enhance health, despite a dearth of evidence in most cases. No wonder people are confused.

For instance, one of the most popular dietary supplements taken by people trying to prevent heart disease—a daily multivitamin—does not lower the risk of heart disease. This evidence comes from large, "gold standard" trials considered to be the most reliable and trustworthy.

What about research on other dietary supplements? Information on some is available free at the government's MedlinePlus website, www.nlm.nih.gov/medlineplus/druginfo/herb_All.html . But note that most have only limited evidence of any benefit. According to the website, a handful of supplements appear to be "possibly effective" for treating high blood pressure and high cholesterol, two major risk factors for heart disease. For example, some small studies show that in people with high blood pressure, garlic supplements can lower blood pressure by up to 8%. Just like conventional medicines, however, garlic supplements may interact with a variety of different drugs—including those taken by people with heart disease, such as warfarin (Coumadin). But you won't typically see that information on a supplement label.

Similarly, red yeast rice supplements were rated "possibly effective" for lowering cholesterol—no surprise, since some of these products contain chemicals similar to statins, the prescription drugs used to lower cholesterol. But studies suggest the amount of the active ingredient in different products varies widely, from very low to very high amounts. What's more, in one study, one-third of the products were contaminated with a kidney toxin called citrinin.

Such contamination isn't rare. As Dr. Cohen noted in a perspective in The New England Journal of Medicine earlier this year, the FDA has found more than 500 supplements adulterated with pharmaceuticals or closely related compounds. The offenders include stimulants, bodybuilding steroids, antidepressants, weight-loss medications, and supplements aimed at treating erectile dysfunction. All can cause unwanted side effects and may be especially risky when taken with heart drugs or other prescription medications.

For example, a man using nitrates to treat his chest pain should not take an erectile dysfunction drug such as sildenafil (Viagra). Doing so can cause dangerously low blood pressure. "So he takes an 'all-natural' herbal supplement instead. But he's actually getting the Viagra his doctor told him not to take," says Dr. Cohen, noting that many such products contain compounds similar or identical to Viagra, sometimes in doses higher than the prescription version.

Because of the potential risks and unclear benefits of these and similar herbal supplements, most doctors advise people to avoid them. However, doctors often recommend specific vitamin and mineral supplements to their patients, such as calcium and vitamin D to prevent osteoporosis and iron for people with iron deficiency.

But for most people, "a well-balanced diet rich in whole foods including fruits, vegetables, fish, olive oil, and nuts negates the need for any supplements," says Dr. Cohen. If you choose to try a supplement, follow these tips:

  • Consider only single-ingredient supplements. With a multi-ingredient supplement, it's impossible to tease out which substance is having an effect—either good or bad. Also, combination products are more likely to be adulterated with banned drugs.

  • Talk to your doctor. Tell your doctor about any supplement you take, so he or she can double-check if a particular ingredient interacts with any of the medicines you're on.

  • Look for the USP or NSF stamp. The United States Pharmacopeia (USP) and NSF International are independent, nongovernmental organizations that test dietary supplements. USP verifies the identity, quality, strength, and purity of supplements; NSF confirms that the supplement contains the listed ingredients and nothing else. Look for one of these stamps on the label, but keep in mind that neither indicates anything about the effectiveness of the product.