Does Overeating Cause Weight Gain, or the Other Way Around?

A Comment on Opinions from May 2014

pile of spaghetti
What causes people to eat large portions?. Photo: Diane Labombarbe/E+/Getty Images

People have a lot of theories about what is causing the obesity epidemic. (Large portions? People suddenly got greedy?) To tell the truth, it is highly unlikely to be just one thing. But certainly something started us (as a group, at least in the U.S.) eating more around 30 to 40 years ago. I think something made us hungrier. Among the choices that, to me, could have some validity: processed foods tricking our brains into wanting to eat more, exposure to endocrine-disrupting chemicals such as BPA, changes in our gut microbiome, viruses, and one of the leading ones: a dramatic increase in consumption of refined carbohydrates, mainly sugars and processed grains.

One of the many unanswered questions is, "which came first, the overeating or the fat"? The answer may seem obvious, but then again, maybe not. Think about it: a larger person requires more calories to maintain their weight. So if something was causing a person to accumulate more fat, they would have to eat more to maintain that larger body. The calories in/calories out equilibrium that worked to keep that person at a stable lower weight would now be working to keep him/her at a stable higher weight. (This actually happens in growing children, pregnant women, and in certain disorders of fat metabolism.)

This chicken-or-egg proposition is causing a lot of thinking among obesity researchers. A pair of May 2014 articles by Dr. David Ludwig and Dr. Mark I. Friedman, one in the Journal of the American Medical Association (Increasing Adiposity: Consequence or Cause of Overeating?) and one in the New York Times (Always Hungry? Here's Why), brings the question to the fore.

They wonder aloud whether some changes in our eating, notably the increase in refined carbohydrate, may have caused our bodies to accumulate fat, and the overeating followed as a consequence.

Ludwig and Friedman talk about circumstantial evidence that excessive carbohydrate causes weight gain, but admit that there is no definite proof.

They mention some of the research that I've reported on: that low-fat/high-carb diets are the least effective (particularly with insulin resistant folks) at promoting weight loss. That lower-carb diets dampen metabolism less in people who have lost weight. That in people with elevated insulin levels, low-carb diets lower insulin, which accompanies weight loss.

Why is there no real proof that excess carbohydrate promotes problems? Possibly partly because diet studies have many notorious problems, one of which is that people often don't actually follow the diets they were assigned to. So the researchers' conclusions end up being based on the instructions given to the participants, not what they actually ate. As Ludwig and Friedman write in the New York Times article (emphasis mine):

Unfortunately, existing research cannot provide a definitive test of our hypothesis. Several prominent clinical trials reported no difference in weight loss when comparing diets purportedly differing in protein, carbohydrate and fat. However, these trials had major limitations; at the end, subjects reported that they had not met the targets for complying with the prescribed diets. We wouldn’t discard a potentially lifesaving cancer treatment based on negative findings, if the research subjects didn’t take the drug as intended.

They point out that diet studies where at least some of the food is provided have a much higher rate of compliance, and encourage high-quality research to resolve some of the unanswered questions related to diet and obesity.

It's worth noting that Mark Friedman is the vice president of research at the Nutrition Science Initiative (NuSi) the organization founded by Gary Taubes (author of Good Calories, Bad Calories and Why We Get Fat) and Dr. Peter Attila to promote high-quality science on the effects of diet on our bodies. Dr. Ludwig has been studying obesity for many years. He is a Professor of Pediatrics at Harvard Medical School and a Professor of Nutrition at the Harvard School of Public Health.


Ludwig, DS and Friedman, MI. Increasing AdiposityConsequence or Cause of Overeating?. JAMA. 2014;311(21):2167-2168. 2014

Ludwig, DS and Friedman, MI. Always Hungry? Here’s Why. The New York Times. May 16, 2014

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