Grasping Partial Truth About Carbs, Insulin, and Obesity

Few subjects rouse more passion than the role of carbs in obesity. Legions of tweeters on Twitter will swarm when the subject comes up. They’re eager to prevail when anyone questions the universal effectiveness of low carb diets. A more nuanced but equally passionate (both pro and con) version of this struggle swirls around the carbohydrate-insulin model of obesity. This model suggests that carbohydrates are uniquely fattening because they stimulate insulin secretion. David Ludwig is a passionate advocate for this view.

Others point out that the model “is not always consistent with results from several carefully conducted studies in both animal models and human subjects.”

With a recent commentary in Science, John Speakman and Kevin Hall have stirred the pot once again.

Grappling with Partial Truth

Speakman and Hall start with a discomforting truth:

“The primary cause of common human obesity remains uncertain.”

Certainty sells. Ambiguity does not. Thus, when apostles of the carbohydrate-insulin model offer bold assertions that they can explain three decades of rising obesity, the public likes it. But Speakman and Hall say the model doesn’t hold up to close scrutiny:

“The theoretical basis of the CIM has been refuted by several recent experiments. We suggest that although insulin plays an important role in body fat regulation, the CIM fails because it focuses on the direct action of insulin on adipose tissue after the consumption of a meal containing carbohydrates. Rather, we propose that the role of insulin in obesity may be better understood by considering its pleiotropic action on multiple organs that is driven by factors mostly independent of carbohydrate intake.”

Dismissing Dismissive Views

Ludwig responded quickly on the BetterHumans blog site. He describes the article in Science as another in a series of “dismissive opinions” based on “weak studies” and a “disregard of nearly a century of supportive research.”

Setting aside the passion of his response, Ludwig makes a good point:

“Admittedly, the CIM, like any model of a complex disease, is at best an approximation. As new data accumulate, it will need to be revised. And perhaps new models that better encompass the evidence will arise.”

Complexity Is Frustrating

Obesity is not merely complicated, it is complex. Many factors are interacting in unpredictable ways to yield the rise in obesity we have witnessed over the last three decades. Simple explanations generally turn out to be simply wrong. But exploring (and questioning) these explanations with objective curiosity is not futile. It will lead us to better answers.

In this sense, we see agreement between Ludwig, Speakman, and Hall. We are all struggling with complexity and partial truth about carbs, insulin, and obesity.

Click here for the commentary by Speakman and Hall, and here for the response from Ludwig. For further perspective, this recent review by Ludwig et al and this commentary are excellent.

Struggle, photograph by Robert Demachy / WikiArt

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May 24, 2021