Food studies frustrate me. One week a study says one thing. The next week another study says the opposite. Why do researchers churn out so many contradictory studies – and why does the news media keep covering them?
It's fair to say that many people share your frustration – despite the fact that food studies have a lot of popular appeal. After all, in theory at least, this type of research has the potential to empower people. If you knew what specific foods could prevent certain diseases, you wouldn't need to rely on doctors and prescription drugs.
Unfortunately, the science of nutrition can be complicated and it's prone to spurious findings.
What we eat is only one of many factors that determine our overall health. Genetics, lifestyle and the environment also play a role.
Furthermore, the food we eat is constantly in flux throughout the day, the year and over our lifetimes. So, it's difficult to pinpoint the parts of our diet that are particularly beneficial – or harmful, says Dr. Donald Redelmeier, a staff physician at Sunnybrook Health Sciences Centre and a professor of medicine at the University of Toronto.
But possibly the biggest challenge facing nutritional scientists is that they have to rely on indirect ways of studying the effects of diet on health. And, in particular, it's hard to do food studies as randomized controlled trials – which are considered the most reliable form of medical research.
In these trials, patients are randomly divided into two groups, with one getting a particular treatment – such as a new drug – and the other receiving a placebo or dummy pill. The researchers can often draw fairly reasonable conclusions about the treatment based on what happens to the two different groups.
However, there's no way researchers could convince large groups of volunteers to stick to a particular diet for many years. (Of course, they can study the effects of different foods in animals – but the results don't translate well to humans.)
As a result, nutrition experts must resort to a less reliable form of research known as an observational study. Under this approach, researchers study people as they go about their normal lives. By getting volunteers to answer questions about their diet and lifestyle, the researchers try to determine if those who eat different foods are more or less likely to get certain diseases.
The biggest weakness with observational studies is that they find only "associations" – they can't prove cause and effect. Any connection between a particular food and a certain disease is speculative at best.
Redelmeier says the findings can be misinterpreted for a lot of reasons, including:
An association between a food and a health outcome might happen through random chance. Look hard enough and you will spot imaginary links that don't really exist. As Redelmeier explains: "People in Switzerland are 50 per cent more likely to die of gun injuries than people in Canada, but it has nothing to do with eating cheese fondue."
People may not accurately remember what they ate. Some studies try to correct for faulty memories by asking volunteers to keep food diaries. But participants may improve their regular diets when they know they are being closely monitored. So, inaccurate dietary information can still be a problem even in well-designed studies.
To find out if a food helps prevent long-term chronic diseases, thousands of people must be followed over many years. The longer a trial lasts, the greater the likelihood that some of those volunteers will drop out or change their dietary habits before the study is completed.
If some people have a strong preference for eating one type of food (such as veggies), then the same people may also avoid eating other things (such as fast food), Redelmeier says. This means it's difficult for researchers to conclude that certain foods help guard against particular diseases. Instead, the real "protective" benefit may come from the fact that they didn't eat something else that had the potential to cause harm.
Many food companies stand to profit from positive findings. Even a hint of a health benefit will likely get hyped – placing an unjustifiable emphasis on a single type of food.
Despite these challenges, it's still worthwhile trying to study the effects of food on health, according to Dr. Walter Willett, chairman of the nutrition department at the Harvard School of Public Health.
When done carefully – and confirmed with follow-up research – dietary studies can potentially have a huge impact on public health.
Willett points to all the years of work that led nutritional experts to conclude that trans fats can contribute to the development of heart disease.
Even so, one meal laden with trans fats isn't going to be solely responsible for triggering a heart attack. In the world of nutrition, the poison is in the dose.
For that reason, Redelmeier says the best thing to do is focus on having a lot of variety in your diet – "everything in moderation" – and don't get distracted by contradictory studies and the latest food fads.
Paul Taylor is a Patient Navigation Advisor at Sunnybrook Health Sciences Centre. He is a former health editor of The Globe and Mail. You can find him on Twitter @epaultaylor and online at Sunnybrook's Your Health Matters.