You are reading Part 2 of 4 in this series. What are Quick Facts?

Most American adults report taking vitamins, minerals, or other dietary supplements, but assessments of supplements’ potential benefits and risks are complicated by many scientific and regulatory factors. Although some of these dietary supplements are recommended by doctors for populations at risk of specific nutrient deficiencies, there is little evidence that supplements benefit generally healthy people.

Can supplements prevent or treat disease?

It is well established that vitamins can prevent diseases that are caused directly by vitamin deficiencies, including scurvy (vitamin C), pellagra (niacin), xerophthalmia (vitamin A), rickets (vitamin D) and beriberi (thiamin). Insufficient maternal folic acid intake has also been shown to increase the risk of some brain and spinal cord defects in newborns, a finding that in 1992 prompted the U.S. Public Health Service to recommend that pregnant women and women intending to become pregnant take folic acid supplements. In recent decades, researchers have begun to study whether long-term use of vitamins and other supplements can help prevent other illnesses as well. For many supplements, there isn’t sufficient evidence to draw conclusions. The vast majority of higher-quality trials—costly endeavors because of the need for long-term follow-up—have failed to show consistent benefits, though there are many points of controversy. Examples include:

  • Vitamin D and calcium: Vitamin D has been described in some research as a possible preventive for heart disease and cancer, leading some to seek testing for vitamin D deficiency and subsequent supplementation. Some medical guidelines have also recommended vitamin D and calcium, separately or in combination, to keep bones strong. However, the evidence is more complex and often lacking.
    • A 2014 systematic review of hundreds of studies, conducted by the Agency for Healthcare Research and Quality (AHRQ), found neither a protective nor harmful relationship between vitamin D supplementation and overall death rates. Vitamin D had no consistent effects on autoimmune disorders or infectious diseases, although some individual studies did find evidence suggesting that higher levels of vitamin D are tied to a lower risk of death from cardiovascular disease, more muscle strength, less risk of falls, and fewer fractures. Effects on cancer were described as either complicated or nonexistent.
    • The AHRQ’s report also investigated research on the effects of vitamin D taken with calcium, and in most cases, found that there was insufficient evidence to draw conclusions—or no high-quality evidence at all. The review observed that taking vitamin D and calcium reduced the risk of fractures, but this was only true for some populations and some kinds of fractures.
  • Multivitamins: Although some research has shown that harm is unlikely, several common claims about health benefits of multivitamins are unsupported by scientific evidence.
    • A review of research by the U.S. Preventive Service Task Force (USPSTF) found no effect of multivitamins on death from all causes or on the incidence of cardiovascular disease events, such as stroke or heart failure.
    • A meta-analysis of 18 studies showed there is no heart-health benefit from taking multivitamins.
    • According to the USPSTF review, there is some evidence that multivitamins reduce cancer rates in men.
  • Omega 3 fatty acids (fish oil): Fish oil contains two omega-3 fatty acids—docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA)—that are known to be essential to health and which can’t be made by the body in sufficient amounts, so must be ingested. As far back as the 1940s, scientists observed that people in Greenland and other populations that rely on fish as a major part of their diets suffer from relatively little heart disease. But the evidence around these supplements has not consistently shown their benefits.
    • A 2017 science advisory from the American Heart Association concluded that omega-3 supplements are reasonable for certain patients who have had coronary heart disease or heart failure, although the evidence lacked consensus. It did not recommend these supplements for the general population. However, recent research suggests that omega-3 supplements do not prevent cardiac events in patients who have survived heart attacks.
    • The National Institutes of Health (NIH) have reported that, based on scientific evidence, omega-3 supplements reduce triglycerides, which at high levels have been linked to heart disease , but the supplements did not reduce the risk of actual heart disease. For most other conditions that have been studied, the evidence is either inconclusive or does not indicate a benefit, though there is evidence that these supplements help relieve symptoms of rheumatoid arthritis.
  • Vitamin C: That vitamin C plays a role in preventing scurvy is widely known and corroborated by multiple studies, but its effectiveness in some other contexts is not clear.
    • The USPSTF reviewed the research, and saw no statistically significant effects of vitamin C on cardiovascular disease, cancer, or death from all causes.
    • Vitamin C is featured in many popular products, such as drink powders, that are marketed as helpful in the prevention and treatment of colds or other viral upper respiratory infections. There have been mixed results: One review of studies found no effects of vitamin C on colds. Another observed that vitamin C did not reduce the incidence of colds for the general population, but it did cut down the duration and severity of colds somewhat—although this wasn’t found consistently.
  • Echinacea: There have been mixed results on echinacea’s effects on colds. A 2015 meta-analysis found only weak and inconsistent evidence that herbal extracts of echinacea either prevent or shorten the length of the common cold. However, a 2019 review found evidence that echinacea reduces the incidence of upper respiratory tract infections such as colds, though infection duration was unaffected.
  • Probiotics: Probiotics are live microorganisms that are consumed as supplements and intended to help with bodily functions such as digestion, vitamin production, and the suppression of disease-causing microorganisms. While they are naturally present in some foods such as kimchi and yogurt, they are also sold as supplements in pill and other formats.
    • The NIH reports that three reviews of studies have shown probiotics to protect against diarrhea associated with antibiotics. No effects of probiotics were found on some other conditions, such as urinary tract infections, asthma, or inflammatory bowel disease.

Can supplements be harmful to health?

  • Studies have linked some dosages of particular supplements to adverse health effects. The Food and Drug Administration cautions that taking too much of some supplements, such as vitamins A and D, can harm your health.
  • According to a USPSTF review of evidence on how supplements affect cancer and cardiovascular disease, harms from supplements were uncommon and generally not serious—although this evidence did not focus on excessive intake. Results from the few individual studies that reported negative health consequences include:
    • Higher rates of hip fractures linked to multivitamins and vitamin A supplements
    • Prostate cancer tied to folic acid
    • A slightly higher risk of kidney stones for those taking vitamin D and calcium
    • A heightened risk of lung cancer among smokers and workers exposed to asbestos associated with the combination of vitamin A and beta-carotene

Creative Commons LicenseThe text and video on this page are licensed as Creative Commons CC BY-SA 4.0. Journalists are free to use any text or video on this page with or without attribution to SciLine.

Basics:


Creative Commons LicenseThe text and video on this page are licensed as Creative Commons CC BY-SA 4.0. Journalists are free to use any text or video on this page with or without attribution to SciLine.