Doctor Jason Fung Still Doesn't Understand Obesity | What the Fitness | Biolayne

Norton references tightly controlled human randomized controlled trials that equate calorie and protein intake while varying carbohydrate and fat ratios. These studies consistently show no meaningful difference in fat loss, with a trivial advantage sometimes seen for low-fat diets. This evidence supports the principle that energy balance, not insulin-mediated fat trapping, is the primary determinant of fat loss. Therefore, he implies that one should focus on controlling overall calorie intake and ensuring adequate protein, rather than fixating on carbohydrate restriction.

Norton points out that GLP-1 receptor agonists like semaglutide and tirzepatide are now the gold standard for obesity treatment. These drugs not only suppress appetite but also enhance glucose-dependent insulin secretion, meaning they increase insulin levels after eating. According to the carbohydrate-insulin model, this should worsen obesity by locking away fat and increasing hunger. Yet clinical reality shows the opposite: patients lose significant weight and experience reduced hunger. Proponents of the model sometimes argue that long-term basal insulin decreases, but Norton explains that this is a secondary effect of weight loss improving insulin sensitivity, not a contradiction of the acute insulin-raising effect. The fact that a drug class that raises postprandial insulin is the most effective weight loss intervention directly falsifies the model's central prediction.

Norton emphasizes that when you look at the gold standard—human randomized controlled trials where food intake is strictly controlled, calories and protein are matched, and only the ratio of carbs to fats varies—the results are clear: there is no meaningful difference in fat loss. In fact, a slight, trivial benefit is often seen for low-fat, higher-carb diets, which is the opposite of what the carbohydrate-insulin model would predict. This directly refutes the notion that insulin's effect on fat storage overrides energy balance. The model's proponents often dismiss such studies by claiming they don't reflect real-world ad libitum eating, but Norton's point is that if the mechanism were correct, the effect should be observable under controlled conditions. The absence of a metabolic advantage in these tightly controlled experiments indicates that the primary driver of fat loss is calorie balance, not insulin levels per se.

Norton acknowledges that people with obesity have altered hunger and reward responses, making them more prone to overeat. However, he argues that this does not negate the fact that overeating itself leads to weight gain. He frames it as a chicken-and-egg issue, but ultimately, the energy surplus is what drives fat accumulation. The carbohydrate-insulin model's attempt to reverse causality is, in his view, categorically false and is used to promote a specific dietary ideology.