The Harvard School of Public Health, Boston, USA
“Scientific research is very complex. There are many different types of research studies, and each has its distinct strengths and weaknesses. Studies can provide established relationships (evidence) as well as probable and possible links, for example, between nutrient intake and disease risk reduction. Nutritional science has adopted the rules of evidence-based medicine, considering randomized, placebo-controlled trials (RCTs) as the best providers of scientific evidence as they infer strong causal relationships. However, experts criticize that as the criteria of evidence-based medicine do not fit into a nutritional context, the effects of nutrients and dietary supplements cannot be measured like those of the drugs in evidence-based medicine (see also Looking for evidence in nutritional science and What insights can be expected from micronutrient supplementation studies?). Consequently, meta-analyses using RCTs can be of limited validity, particularly when they have questionable inclusion criteria for studies, disregard certain trials, and compare studies that are not really comparable (see also Do antioxidant supplements increase mortality? and Analyzing the Benefit/Risk Ratio of Antioxidant Supplements).
One of the most crucial things to keep in mind is how a given study referred to in the mass media fits into the entire body of evidence on a topic.
- Was the study done on animals or humans?
Mice, rats, and monkeys are not people. To best understand how nutrients affect human health, studies must almost always be carried out on humans.
- Are they simply reporting the results of a single study? If so, where does it fit in with other studies on the topic?
In nutrition science, only very rarely would a single study be influential enough for people to change their behaviors based on the results.
- How large is the study?
Large studies often provide more reliable results than small studies.
- Did the study look at real disease endpoints?
Chronic diseases, like heart disease and osteoporosis, often take many decades to develop. To get around waiting that long, researchers will sometimes look at markers for these diseases, like narrowing of the arteries or bone density. These markers, though, don't always develop into the disease. In addition, it is important that the discussed disease endpoint was the primary objective of the investigation and not a side product that the study was not designed to investigate.
- How was diet assessed?
Some methods of dietary assessment are better than others. Blood concentrations of nutrients, for exam-ple, are more valid than nutrient intake amounts. Good studies will have evidence that the methods have validity.
The starting point for many (if not all) mass media reports on supplements is that the major vitamin trials have, for the most part, produced disappointing results, and even evidence of harmful effects in some cases. Scientific research is, however, much too complex to be properly discussed in a balanced way in simple newspaper articles. The majority of the most popular studies used by the media to produce stories about harmful effects of micronutrient supplements do not provide conclusive, generalizable information:
- They were carried out on people who were ill or exposed to a high health risk (e.g. smokers and asbestos workers), so they cannot be regarded as trials of preventive measures;
- The micronutrients were often given in very high doses in these studies (many times the amounts that are recommended by health authorities), which does not reflect typical consumer behavior;
- They mainly used single micronutrients, although it is known that the interaction of nutrients in combina-tions (e.g., multivitamins) can significantly contribute to their health benefits.
Some famous examples of such trials are two studies from the 1990s which suggested that high doses of beta-carotene (10 times the recommended dose) taken for years by heavy smokers and asbestos workers increased their risk of lung cancer (1,2). Not all trials of beta-carotene have showed this harmful effect. However, many newspapers turned this into a general warning against supplements containing beta-caro-tene, disregarding the fact that potential harmful effects were shown in groups at a high-risk of lung cancer taking excessive doses. Sometimes, the media even present positive results in a uniformly negative way. The SU.VI.MAX trial (3), for example, showed a reduction in cancer risk by 31% and overall mortality by 37% among men taking a combination of antioxidant vitamins and minerals. These effects were not apparent in women, because they (unlike with the men) tended to already have sufficient blood concentrations of the nutrients at the beginning of the study. Typically, media reports only mention that there was no effect in women – and so the study is mostly represented in a negative fashion.
Although many mass media reports are written in a one-sided and simplistic way, some of the points in the negative statements about dietary supplements are valid: ideally, a balanced and varied diet should cover micronutrient requirements. Dietary supplements can close gaps in nutrient intake but they cannot compen-sate for a generally unhealthy lifestyle. Regarding supplement dosage, consumers should follow the nutrient intake recommendations of public health authorities.”
Based on: Harvard School of Public Health. The Nutrition Source – Nutrition Research and Mass Media: An Introduction. Published online 2012.