We often hear from different camps in the "diet wars" claiming that their style of eating is superior to all the others. Usually, it’s based on a single health measure: short-term weight loss. Now obesity doctor Yoni Freedhoff and National Institutes of Health researcher Kevin Hall have arrived to dispel the squabbling.
Parsing the best available research on diet and weight loss, they point out that diets like low-fat, Paleo, low-carb, and gluten-free return about the same weak results. At best, this usually amounts to a few kilos of weight loss. And we really have no idea about the long-term sustainability of these diets, and what they do to the body over many years (beyond just moving the pounds on the scale).
This graphic from their paper — which draws on data from a large trial comparing the low-fat, low-carb, and Mediterranean diets — says it all:
What you see in the first chart is a steep drop in weight within the first six months on the low-fat and low-carb diets. Then the weight slowly edges up and plateaus. At the two-year mark, the low-carb dieters look almost undistinguishable from the low-fat dieters: there’s only about a 4-pound difference in weight loss between the groups — a difference that's too small to have an impact on health.
The large curve in the second chart is a mathematical model of how energy intake changed over the course of the two-year trial. Put simply, you’ll note that dieters can’t sustain a steep drop in calories, and slowly eat more until their energy intake plateaus along with their weight. (The other two slopes in the graphic show self-reported data about what people were eating, which the authors point out are clearly flawed since they don’t align with the observed weight regain in the trial.)
This study doesn’t come in isolation. Just about every study comparing prescribed diets over a year shows the same thing: marginal differences among diets and weight losses that are significant at the beginning of the experiment, followed by pounds slowly creeping up and weight plateauing.
"Crowning a diet king because it delivers a clinically meaningless difference in bodyweight fuels diet hype, not diet help," the authors write. "It’s high time we started helping."
According to Hall, helping would mean moving away from the focus on bodyweight in the diet wars toward a better understanding of the other physiological effects of diet interventions. For example, researchers should look at why some people respond better to certain diets than others, and what can be done to help people stick to a healthier pattern of eating over the long run. This could include studying the biological differences among people, "as well as the environmental and social factors that influence a person’s ability to sustain a permanent behavior change," Hall said in an email.
We also need studies that look at the sustainability of diets and lifestyle changes over many years (most of the long-term studies stop at two years). Getting this data would be hugely helpful when it comes to treating chronic diseases like obesity and its co-morbidities, Hall said. And right now that’s data we just don’t have.