The Carbohydrate-Insulin Model of Obesity: Beyond ‘Calories In-Calories Out’

A spate of recent reviews claim to refute the CIM,^1,32,33,48 or dismiss any special metabolic effects of macronutrients,⁴⁹ but these attacks are premised on a misunderstanding of physiological mechanisms, misinterpretation of feeding studies and disregard for much supportive data. In animals, dietary composition has been shown to affect metabolism and body composition, controlling for calorie intake, in a manner consistent with the CIM predictions. Admittedly, the evidence for these effects in humans remains inconclusive.

Limited evidence notwithstanding, the conventional model has an implicit conflict with modern research on the biological control of body weight. The rising mean BMI among genetically stable populations suggests that changing environmental factors have altered the physiological systems defending body weight. After all, inexorable weight gain is not the inevitable consequence of calorie abundance, as demonstrated by many historical examples (eg, the United States, Western Europe, and Japan from the end of World War II until at least the 1970s).

Diets of varying composition, apart from calorie content, have varying effects on hormones, metabolic pathways, gene expression, and the gut microbiome in ways that could potentially influence fat storage. By asserting that all calories are alike to the body, the conventional model rules out the environmental exposure with the most plausible link to body weight control. What other factors could be responsible for such massive changes in obesity prevalence? The conventional model offers no compelling alternatives.

High-quality research will be needed to resolve the debate, which has been ongoing for at least a century.⁵ In 1941, the renowned obesity expert Julius Bauer described a key component of the CIM (the reverse direction of causality depicted in Figure B), writing in this journal: “The current energy theory of obesity, which considers only an imbalance between intake of food and expenditure of energy, is unsatisfactory…. An increased appetite with a subsequent imbalance between intake and output of energy is the consequence of the abnormal anlage [fat tissue] rather than the cause of obesity.”⁵⁰ In view of the massive and rising toll of obesity-related disease, this research should be given priority.

https://bit.ly/2KIjZCG