Dietary supplements: Can they also be harmful?

Photo of dietary supplement tablets
PantherMedia / Vladimir Raus

Antioxidant dietary supplements do not help prevent cancer and other life-threatening conditions. Excessive doses of the antioxidants vitamin A, vitamin E and can even shorten your life expectancy.

Many people take vitamin C or supplements in the hope of improving their health and preventing illness. These products are often claimed to help people live longer by protecting them from cardiovascular (heart and circulation) diseases and cancer.

This assumption is based on the theory of "free radicals." "Free radicals" are constantly being made in the cells of the body wherever oxygen is used to produce energy. Because they can cause damage to the cells, all of the body's cells have ways to protect themselves from the free radicals.

There is some debate as to whether antioxidants can increase this level of protection. Antioxidants are substances that reduce the amount of free radicals in our cells. They include vitamins A, C and E, and , and are mainly found in fruit and vegetables. Many people think that they could boost their health by taking dietary supplements or eating or drinking antioxidant-enriched foods.

Research on the long-term use of antioxidants

Researchers from the wanted to find out whether there are any health benefits to regularly taking dietary supplements, or whether it could even be harmful. They specifically wanted to find out whether dietary supplements can help people live longer. They found 78 studies on this question, involving almost 300,000 adults in total. These studies were a good basis for evaluating the benefits of antioxidants.

The study participants generally took either one or more antioxidants, a placebo (fake medication) or no medication every day – some of them over many years. Most of the participants were healthy when they started the study. One quarter of them already had health problems affecting things like their stomach, bowels, heart, skin or kidneys.

Most of the participants took the antioxidants every day in the following amounts:

  • Vitamin C: 60 to 2,000 milligrams
  • Beta-carotene: 1.2 to 50 milligrams
  • Vitamin E: 10 to 5,000 IU (international units)
  • Vitamin A: 1,333 to 25,000 IU
  • Selenium: 20 to 200 micrograms

These were mostly much higher amounts than you would get as part of a balanced diet.

Most of the participants took high-dose multivitamins. The studies lasted an average of almost three years, with some lasting up to twelve years.

No proof of benefits – harmful effects can't be ruled out

The results disproved the belief that dietary supplements help people live longer. In fact, the analysis of the studies showed that taking vitamins A, E and tended to shorten lives instead. But the studies didn't state the causes of death. The researchers assumed that the main causes of death were most likely to have been cancer or cardiovascular (heart and blood vessel) disease.

The results were the same for those participants who were healthy and those who had an illness at the beginning of the studies. They don't apply to all antioxidants, though: Selenium and vitamin C did not increase mortality. But there was also no proof that and vitamin C prevent earlier death.

Good-quality research found that all dietary supplements containing vitamin A, E and led to an increase in the number of deaths. For example, the results of studies on dietary supplements containing vitamin E showed the following:

  • In the group that took a placebo or nothing, around 10 out of 100 people died during the studies.
  • In the group that took vitamin E, around 12 out of 100 people died during this time.

Antioxidants can also have side effects. Vitamin E, and may cause constipation, diarrhea and gas, for example. Taking very large amounts of vitamin A and C can cause itching.

A balanced diet provides enough protection

The Cochrane review didn’t find any proof that dietary supplements prevent cancer or other life-threatening diseases in healthy people. Excessive doses may even increase the risk. Other studies on dietary supplements came to similar conclusions.

That doesn’t mean that these substances should be avoided, of course: Our bodies need vitamins and minerals. If we don't get enough of them for a long time, then taking supplements can help to make up for it. In Germany and other industrialized countries you can usually get everything that your body needs by eating a balanced diet with enough fruits and vegetables. Excessive amounts from taking too many dietary supplements could be harmful, though. There are currently no binding recommendations for manufacturers of dietary supplements in Germany or other European countries regarding safe upper limits. But there are plans to define upper limits for important minerals. You can find more information on the website of the German Federal Institute for Risk Assessment (BfR).

Al-Khudairy L, Flowers N, Wheelhouse R et al. Vitamin C supplementation for the primary prevention of cardiovascular disease. Cochrane Database Syst Rev 2017; (3): CD011114.

Bjelakovic G, Nikolova D, Gluud LL et al. Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Cochrane Database Syst Rev 2012; (3): CD007176.

Cortés-Jofré M, Rueda JR, Asenjo-Lobos C et al. Drugs for preventing lung cancer in healthy people. Cochrane Database Syst Rev 2020; (3): CD002141.

Das JK, Salam RA, Mahmood SB et al. Food fortification with multiple micronutrients: impact on health outcomes in general population. Cochrane Database Syst Rev 2019; (12): CD011400.

Fortmann SP, Burda BU, Senger CA et al. Vitamin, Mineral, and Multivitamin Supplements for the Primary Prevention of Cardiovascular Disease and Cancer: A Systematic Evidence Review for the U.S. Preventive Services Task Force. 2013.

Imdad A, Mayo-Wilson E, Herzer K et al. Vitamin A supplementation for preventing morbidity and mortality in children from six months to five years of age. Cochrane Database Syst Rev 2017; (3): CD008524.

McCleery J, Abraham RP, Denton DA et al. Vitamin and mineral supplementation for preventing dementia or delaying cognitive decline in people with mild cognitive impairment. Cochrane Database Syst Rev 2018; (11): CD011905.

Rutjes AW, Denton DA, Di Nisio M et al. Vitamin and mineral supplementation for maintaining cognitive function in cognitively healthy people in mid and late life. Cochrane Database Syst Rev 2018; (12): CD011906.

Schwingshackl L, Boeing H, Stelmach-Mardas M et al. Dietary Supplements and Risk of Cause-Specific Death, Cardiovascular Disease, and Cancer: A Systematic Review and Meta-Analysis of Primary Prevention Trials. Adv Nutr 2017; 8(1): 27-39.

Suchdev PS, Jefferds ME, Ota E et al. Home fortification of foods with multiple micronutrient powders for health and nutrition in children under two years of age. Cochrane Database Syst Rev 2020; (2): CD008959.

Vinceti M, Filippini T, Del Giovane C et al. Selenium for preventing cancer. Cochrane Database Syst Rev 2018; (1): CD005195.

IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. informedhealth.org can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.

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Updated on August 24, 2021
Next planned update: 2024

Authors/Publishers:

Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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