Dietary supplements: Can they also be harmful?

Researchers from the Cochrane Collaboration 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 antioxidant 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.

The results disproved the belief that antioxidant dietary supplements help people live longer. In fact, the analysis of the studies showed that taking vitamins A, E and beta-carotene 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 selenium and vitamin C prevent earlier death.

Good-quality research found that all dietary supplements containing vitamin A, E and beta-carotene 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, beta-carotene and selenium may cause constipation, diarrhea and gas, for example. Taking very large amounts of vitamin A and C can cause itching.

The Cochrane review didn’t find any proof that antioxidant 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).