A Giant Leap of Faith About Diet and Exercise in PNAS

“This study confirms what I’ve been saying, which is that diet is the key culprit in our current epidemic,” says Barry Popkin. He’s talking about a new study in PNAS. With a giant leap of faith, one can use this study for a perfect expression of confirmation bias about the role of diet and exercise in the rise of obesity. This is quite useful for folks with passionate convictions about bad food as the primary cause of for high rates of obesity.

But Andrew Brown of the Arkansas Children’s Research Institute offers a note of caution. Commenting in Science about the interpretation of this study, he points out that it relies on “assumptions stacked upon assumptions.” Then he goes on to say:

“It very well could be that small changes in physical activity have outsized effects on energy intake itself.”

Likewise, biostatistician Jeff Goldsmith of Columbia University echoes these concerns, noting that the sample skews toward highly developed nations. “I do worry about trying to make broad claims when the sample is skewed,” he says.

The Irresistible Lure of a Simple Answer

A simple answer to a complex problem is like catnip. People cannot resist it. This is especially true when the subject is the rise of obesity. Like Popkin, many people search to confirm their staunch convictions about the culprits in this “epidemic.” The food is medicine man, Dariush Mozaffarian, was happy to latch onto this study and tell the Washington Post:

“It’s clear from this important new research and other studies that changes to our food, not our activity, are the dominant drivers of obesity.”

A Pesky Limitation

In the midst of this headlong rush to confirm their biases about obesity’s primary cause, these advocates overlook a pesky limitation. This study is simply not about causality. The authors of the study know this and state it explicitly:

“The data in this study are cross-sectional and we lack detailed dietary data for most of the populations in this dataset. We therefore cannot establish causality in the relationships between economic development, body fat percentage, and dietary intake.”

So yes, this is a good and interesting study about energy expenditure and obesity. But it tells us very little about the primary cause or causes for the rise in obesity we have witnessed over the last four decades.

The folks with an easy answer to that question have a proposition to sell you. Buyer beware.

Click here for the new study in PNAS, here and here for further reporting on it. For perspective on the complexity of obesity’s causes, click here.

Leap, painting by Norman Rockwell / WikiArt

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July 16, 2025