“Calories In, Calories Out” Doesn't Explain Obesity
The familiar advice for losing weight is simple: eat fewer calories and burn more calories.
Energy balance matters. But it does not fully explain why we overeat, why some foods make us hungrier than others, or why maintaining weight loss can be so difficult.
Different foods affect hunger, satiety, blood glucose, insulin, and metabolism differently. And when people lose weight, the body may respond by increasing hunger and reducing energy expenditure.
The more useful question may be not simply “How can I eat fewer calories?” but “What foods make it easier to eat less without being hungry?”
Calories Matter—but Food Matters Too
If we consistently consume more energy than we use, the excess must be stored. But that describes what happens, not necessarily why.
The body actively regulates hunger and energy expenditure. Protein, carbohydrates, fat, fiber, and food processing can all influence how full we feel and how much we spontaneously eat.
This helps explain why “eat less and exercise more” is often easier advice to give than to follow.
Insulin and the Debate About Obesity
Insulin helps regulate blood glucose and energy storage. It also promotes fat storage and inhibits the release of stored fat.
Researchers including Gary Taubes, Ben Bikman, and David Ludwig argue that insulin resistance and chronically elevated insulin may contribute to hunger, fat storage, and difficulty losing weight. Reducing refined carbohydrates may therefore be particularly helpful for some people with metabolic problems.
Other researchers, including Kevin Hall, have tested the carbohydrate-insulin model in controlled studies and have not found support for its strongest claims of a large metabolic advantage independent of calorie intake.
Insulin is clearly important, but it is not the only factor controlling body weight. Genetics, appetite, sleep, physical activity, medications, food environment, and many other factors also matter.
Why Keeping Weight Off Is Difficult
Weight loss itself can trigger biological responses that make further weight loss harder. Hunger may increase while energy expenditure decreases.
This helps explain why maintaining a calorie deficit through willpower alone can become increasingly difficult.
A sustainable approach therefore needs to consider satiety, food quality, metabolic health, exercise, sleep, and preservation of muscle, not simply calorie targets.
Ultra-Processed Foods and Appetite
A particularly revealing NIH experiment led by Kevin Hall compared ultra-processed and unprocessed diets.
Participants were allowed to eat as much as they wanted. On the ultra-processed diet, they spontaneously consumed about 500 more calories a day and gained weight. On the unprocessed diet, they ate less and lost weight.
The extra calories caused the weight gain—but the interesting question is why the ultra-processed foods caused people to eat more without deliberately trying to do so.
Food affects calorie intake.
A More Practical Approach
Rather than concentrating entirely on counting calories, it may be more useful to choose foods that naturally provide greater satiety.
That generally means emphasizing whole or minimally processed foods, adequate protein, vegetables, and other nutrient-dense foods, while reducing added sugars, refined carbohydrates, and ultra-processed foods.
For people with Insulin Resistance, reducing carbohydrates may also improve blood glucose and insulin levels and make hunger easier to manage.
Exercise—particularly resistance training—helps preserve muscle and improves insulin sensitivity and overall health.
Calories still matter. The goal is to make a healthy energy balance easier to maintain without persistent hunger.
Jason Fung — Insulin, Hunger & Obesity
Dr. Jason Fung, author of The Obesity Code and the newer The Hunger Code (2026), argues that obesity involves more than excessive calorie intake.
His earlier work emphasized insulin and the hormonal regulation of fat storage, along with reducing refined carbohydrates and intermittent fasting.
In The Hunger Code, Fung broadens his focus to why we become hungry and overeat. He examines hormonal signals, the brain's regulation of appetite, eating habits, and the influence of ultra-processed foods.
His central message is that understanding and managing hunger may be more effective than relying on willpower and calorie counting alone.
Although the importance of hunger regulation is well established, some of Fung's explanations of insulin and the body's weight-regulating mechanisms remain debated.
Book
The Hunger Code: Resetting Your Body's Fat Thermostat in the Age of Ultra-Processed Food
Jason Fung (2026). Explores how hunger, hormones, the brain, and ultra-processed foods influence appetite and weight regulation.
Videos
Why Do We Get Fat? Part 1 — The Biggest Lie in Obesity Medicine - 13 Minutes
Dr. Jason Fung explains why he believes calorie counting alone does not adequately explain obesity.
Insulin Blocks Fat Burning — The Key to Weight Loss - 13 Minutes
Fung explains his view of how insulin affects fat storage and why lowering insulin may help with weight loss.
Learn More
Kevin Hall, Ph.D. — Ultra-Processed Diets Cause Excess Calorie Intake and Weight Gain
The NIH randomized controlled trial in which participants ate about 500 additional calories per day on an ultra-processed diet.
Gary Taubes — Why We Get Fat
Makes the case that hormonal regulation, particularly insulin, plays an important role in obesity.
Ben Bikman, Ph.D. — Why We Get Sick
An accessible discussion of insulin resistance, metabolism, and chronic disease.
David Ludwig, M.D., Ph.D. — Always Hungry?
Examines how food quality, hunger, hormones, and metabolism may affect body weight.