Q. This publication sometimes writes that there is “no clear evidence” about something when other people are publishing articles referencing plenty of supporting studies. How is this possible?

A. Dariush Mozaffarian, MD, DrPH, dean of the Friedman School and editor-in-chief of Tufts Health & Nutrition Letter answers: “While I cannot address your question directly unless I know the specific topics you are referring to, I can offer some general information. Often the situation you describe arises because of the nature of research and a lack of precision in how research is interpreted and reported on.

“There are different kinds of studies. Animal research and in vitro studies (those conducted in test tubes or petri dishes) can help us get some basic mechanistic information or determine physiological plausibility, but they cannot prove that something actually happens in the human body. Observational studies (where groups of people are observed at a particular point in time or over a period of time) help us find associations between behaviors and outcomes, but they cannot prove cause and effect. Meta-analyses and systematic reviews combine data from multiple studies to make a statistically stronger case for a given outcome, but if all of the studies are observational, the results are still not proof of cause and effect. The only way to prove cause and effect is with well-designed randomized controlled trials (RCTs) where (ideally) all variables are held constant except for the one being studied, and the impact of changing that variable is compared to the impact of a placebo. Additionally, confounding factors and the difficulty of controlling all aspects of health and dietary intake mean even good studies can return conflicting results. We may therefore need good data from multiple studies to really be certain.

“While all research adds to our pool of knowledge, reporting on animal studies, lab studies, and observational data (and even single RCTs) as if they provide decisive evidence is irresponsible.

“Here at Tufts Health & Nutrition Letter, we work hard to be clear about what is possible and what is proven. For example, we always use the words ‘is associated with’ when discussing observational studies. You’ll also see a lot of ‘may’ or ‘could’ instead of ‘will’ or ‘can.’ Do keep in mind, however, that saying there is no evidence something works is not the same as saying there is evidence it doesn’t work.”