The classic explanation of obesity is simple: you eat more than you burn. The equation is physically correct, but as a cause it explains nothing. It’s like saying a room is hot because more heat enters than leaves. True, but it doesn’t tell you why.
Dr. Jason Fung proposes flipping the arrow: you don’t get fat because you eat too much. You eat too much because your body is hormonally programmed to store more fat. The key hormone is insulin.
Insulin as thermostat
Your body maintains a fat “set point” regulated by the hypothalamus. It works like a thermostat: it defines how much fat the body should carry and adjusts hunger, satiety, and metabolism to reach that level.
When insulin is chronically high — from frequent meals, refined carbs, or absence of fasting — the thermostat rises. The body responds with more hunger, less satiety, and a slower metabolism. It’s not lack of willpower. It’s physiology.
Why diets fail
Cutting calories without lowering insulin triggers a chain of compensations that make the diet unsustainable: hunger rises, metabolism drops, fatigue and constant cold appear. And when you quit the diet, weight rebounds — because the thermostat never came down.
Fung documents patients whose metabolism dropped to 800 kcal/day after restrictive diets. The caloric equation is still valid, but the body adjusted expenditure to defend the fat level that insulin ordered it to maintain.
What actually works
The goal isn’t eating less — it’s lowering insulin so the thermostat corrects itself. Strategies that work: reduce meal frequency (no grazing all day, even if portions are small), intermittent fasting (18:6), low insulin-load eating (proteins, healthy fats, fibrous vegetables), and minimizing sugars, flours, and ultra-processed foods.
Not all food is equal even with the same calories. Cookies and eggs can have the same energy, but their hormonal impact is completely different. The key isn’t how much you eat, but what that food tells your body to do.
Obesity isn’t a moral failure or a simple calorie surplus. It’s a hormonal alteration corrected with strategy, not willpower.