Selection bias and dietary supplements

One problem in assessing the merits of taking a specific dietary supplement (ranging from vitamins to  exotic multi-ingredient compounds) is widespread selection bias in the documentation that is supposed to support the use of the supplement in question.  The sheer number of scientific studies combined with variation in research methodologies virtually guarantees that for every supplement a supporting study can be found. For example, the recent issue of Life Extension Magazine (August 2008) has an article on the multiple health benefits of melatonin with 81 references. All these studies discuss either the biochemical properties of melatonin or show beneficial effects. This is what is what is seen. What is not seen are the studies in which melatonin is not effective or has adverse effects.  Or the studies that never got published as a result of “publication bias.” Granted, melatonin seems to be a remarkably effective agent for a diverse number of conditions, including its use as a neuroprotective agent in stroke, but such selective presentation of biomedical research seems to be a mainstay in the marketing of dietary supplements.

Another limitation of such documentation is that the studies that are used to recommend the taking of a supplement often solely address the (short-term) effects of that compound on the medical condition in question. Although it would not be practical to report on all the studies that investigate (chronic)  administration of the compound on other systems in the body, such unrelated adverse effects should not be ruled out when considering prolonged use. It is a major leap from demonstrating beneficial effects of a compound in rodents and preliminarily studies in humans to “recommending” the use of that compound for prolonged use in humans. And it is a giant leap to go from such studies to combining different effective compounds in very high dosages in a single product.

Promoting the use of supplements with a hodgepodge of  encouraging in-vitro studies, small animal studies, and observations in humans is not necessarily wrong, nor constitutes deliberate selection bias. Human biochemistry is extremely complex, and rigorous  research would require enormous resources and longitudinal experiments.  In real life there is a need to make informed decisions based on the evidence at hand. Still, our current state of knowledge and our ignorance about how all that we know adds up for specific individuals should induce modesty and, perhaps, moderation. For those who take supplements as a means to radical life extension, making cryonics arrangements remains the irreplaceable  cornerstone of such a program because it increases the odds to reach a time where truly meaningful (molecular) life extension technologies will be available, aside from the protection cryonics offers against most “lethal” accidents.