Supplements in diet may do more bad than good

About two years ago I announced to my wife at breakfast my intention to start taking a regular omega-3 fatty acid dietary supplement, but added cynically – "and I will continue until we hear that it causes cancer". Sitting again at breakfast on July 10th, 2013, I heard radio news announce that a new study had shown that omega-3 fatty acids may cause cancer. Such an experience is common. We are regularly advised that supplementing our diet with biochemical X will yield health benefits, only to have these hopes dashed soon afterwards when new research shows contrary results.

Many people take supplements of vitamins A, C and E, thinking these antioxidants will help to ward off cancer. The hypothesis is that antioxidant supplements neutralise chemicals in cells called "free radicals", which arise naturally and can damage DNA. However, recent Swedish research suggests that antioxidant dietary supplements may actually increase cancer risk in certain people.

Decades ago it was observed that people who eat lots of fruits and vegetables seem to get less cancer. This was “explained” by the hypothesis that these are rich in healthy, desirable elements, including antioxidants and fibre. However research on the controlled ingestion of individual “desirable” dietary supplements has so far seen discouraging results. Research in the 1980s to determine if dietary supplementation with the antioxidants beta-carotene, vitamin A and vitamin E protect smokers against contracting lung cancer was called off when it was noticed that smokers taking beta-carotene showed an increased risk of lung cancer. Also, recent research to test if vitamin E and selenium supplements prevent prostate cancer was ended when it was noticed that the people on vitamin E showed increased risk of prostate cancer.

The recent Swedish study (VI Sayin and others in Science Translational Medicine , 2014) found two antioxidants – vitamin E and a drug called N-acetylcysteine – at doses found in multi-vitamin and dietary supplements speed up the progression of lung cancer in mice and in human cell lines. When mice with early lung cancers were fed the antioxidants, tumour growth killed the mice twice as fast. The antioxidants knocked out a tumour growth suppressor normally present in the cancer cells. These findings suggest that people with small undiagnosed lung tumours – which are more likely in smokers but may occur in anyone – should avoid taking supplementary antioxidants because they may accelerate tumour growth.

However, we must be cautious in interpreting this Swedish work. The strains of mice used in the research are genetically predisposed to easily developing lung cancer, and consequently the results may not translate readily to humans. Secondly, these results do not cast suspicion on foods that are naturally rich in antioxidants, for example tomatoes and berries. You would have to eat very large amounts of such foods to reach the levels of supplement used in the Swedish study. Also, foods are complex, containing many other substances that may influence how antioxidants affect the body.

A correlation between omega-3 fatty acids and increased risk of prostate cancer was published in the Journal of the National Cancer Institute (2013). The study involved 35,000 men over the age of 50. Omega-3 fatty acid blood levels in 834 of these men diagnosed with prostate cancer were compared to levels in 1,393 men randomly chosen from the 35,000 participants. Men with the highest levels of omega-3 fatty acids had a 43 per cent higher risk of developing prostate cancer and a 71 per cent higher risk of developing high-grade prostate cancer, which is more likely to be fatal.

Oily fish is rich in omega-3 fatty acids, and many people eat fish to enhance their fatty acid levels. However, diet was not studied in this research, and it is not known whether the omega-3 fatty acids came from food or from health supplements. Commenting on these results, Marji McCullough, epidemiologist with the American Cancer Society, said: "In general it's better to try to get nutrients from food . . . People don't need to eliminate fish from their diet."

What do we make of all this? I am no fan of dietary health supplements. I believe that our long evolutionary history has adapted humans to absorb all necessary nutrients from an adequate diet, and so supplementing intake levels beyond this is unwise. My advice to the average healthy person free of medical conditions is simple: eat a wide variety of fresh, whole foods in moderation, mostly plants, and take plenty of aerobic exercise.

William Reville is an emeritus professor of biochemistry at UCC. understandingscience.ucc.ie