The familiar advice for losing weight is simple: eat fewer calories and burn more calories. This idea—often called “Calories In, Calories Out”—is based on a fundamental truth: body weight is affected by the balance between the energy we consume and the energy we expend.
But that does not fully explain what controls the two sides of the equation—or why obesity and long-term weight loss can be so difficult.
Different foods can affect hunger, satiety, insulin, blood sugar, and energy expenditure in different ways. When people deliberately cut calories, the body may respond by increasing hunger and reducing the amount of energy it burns, making long-term weight loss difficult.
The more useful question may therefore be not simply “How can I eat fewer calories?” but “What kinds of foods make it easier to eat less without being hungry?”
This is one reason researchers have become increasingly interested in food quality, insulin resistance, protein, carbohydrate restriction, and ultra-processed foods rather than focusing only on calorie counting.
The principle of energy balance is not controversial. If the body consistently takes in more energy than it uses, the excess energy must be stored. If it consistently uses more energy than it takes in, stored energy must be drawn upon.
But this describes what happens, not necessarily why it happens.
The body actively regulates both hunger and energy expenditure. Eating less can make us hungrier, while weight loss can cause the body to reduce the amount of energy it burns. Conversely, different foods can have different effects on satiety, hormones, blood sugar, and how much we spontaneously want to eat.
This is why telling someone with obesity simply to “eat less and exercise more” may be similar to describing the problem rather than explaining its cause.
Calories matter. But understanding what regulates appetite, metabolism, and fat storage may be more useful than simply counting them.
Insulin is a hormone that helps regulate blood glucose and the storage and use of energy. After we eat—particularly foods containing carbohydrate—insulin levels generally rise. Among its effects, insulin promotes the storage of fat and inhibits the release of stored fat from fat cells.
Researchers including Ben Bikman and Gary Taubes argue that chronically elevated insulin and insulin resistance can make it more difficult for the body to access stored fat for energy. From this perspective, reducing refined carbohydrates and other foods that produce large insulin responses may help lower insulin levels, improve metabolic health, and make weight loss easier.
This does not mean that insulin is the sole cause of obesity or that calories cease to matter. Appetite, genetics, sleep, physical activity, medications, food environment, and many other factors can influence body weight.
The important point is that the hormonal and metabolic effects of food may influence how much we eat and how readily stored energy is used. This is one reason two diets containing the same number of calories may not necessarily feel the same or be equally easy to maintain.
The role of insulin in obesity remains scientifically debated. Researchers such as Gary Taubes, Ben Bikman, and David Ludwig argue that high insulin levels and the hormonal effects of different foods can play an important role in determining hunger, fat storage, and energy expenditure.
Other researchers, including Kevin Hall, have tested some predictions of the carbohydrate-insulin model in tightly controlled feeding studies. Their results have not supported the strongest version of the theory—that reducing carbohydrates produces a large metabolic advantage independent of calorie intake.
But the debate is broader than whether one hormone explains obesity. There is substantial evidence that food composition affects satiety, appetite, blood glucose, insulin, and how much people spontaneously eat.
A reasonable conclusion is that calories matter, but counting calories alone does not explain why people gain weight or why maintaining weight loss can be so difficult.
When people reduce calories and lose weight, the body does not necessarily remain passive. It can respond by increasing hunger and reducing energy expenditure. This biological response is sometimes called metabolic adaptation.
As body weight falls, a smaller body naturally requires fewer calories. But energy expenditure can sometimes decline beyond what would be expected simply from having a smaller body. At the same time, changes in hormones that regulate appetite can make people feel hungrier.
This helps explain a common experience: losing weight may be easier than keeping it off. Maintaining a calorie deficit through willpower alone can become progressively more difficult as the body pushes back against weight loss.
A sustainable approach to weight control should therefore consider not only calories, but also satiety, food quality, metabolic health, physical activity, sleep, and preservation of muscle.
One of the most interesting experiments on this question was conducted by Kevin Hall and colleagues at the National Institutes of Health. Participants lived at the NIH Clinical Center and were randomly assigned to eat either an ultra-processed or an unprocessed diet for two weeks, then switched to the other diet.
Although the diets offered were designed to be matched for calories, sugar, fat, fiber, and several other nutrients, participants were allowed to eat as much or as little as they wanted.
On the ultra-processed diet, people spontaneously ate about 500 more calories per day and gained weight. On the unprocessed diet, they ate fewer calories and lost weight.
The study did not show that calories are unimportant—quite the opposite. The extra calories helped explain the weight gain. But it raised a more useful question: What was it about the ultra-processed food that caused people to eat more calories without deliberately choosing to do so?
This illustrates an important limitation of “Calories In, Calories Out” as practical advice. The foods we eat can influence the number of calories we naturally want to consume.
For many people, a more useful strategy than constantly counting calories may be to choose foods that make it easier to control hunger and maintain good metabolic health.
That generally means emphasizing whole or minimally processed foods, adequate protein, and foods that provide lasting satiety, while reducing ultra-processed foods, added sugars, and refined carbohydrates.
For people with insulin resistance, reducing carbohydrate intake may also help lower blood glucose and insulin levels and improve satiety. Some people find that this allows them to eat less without deliberately restricting calories or feeling persistently hungry.
Exercise remains important, particularly resistance training, and its benefits extend far beyond the calories it burns. Exercise helps preserve muscle, improves insulin sensitivity, and supports cardiovascular and metabolic health.
The goal is not to pretend that calories don't matter. It is to create a metabolic and dietary environment in which maintaining a healthy energy balance becomes easier and more sustainable.
Dr. Jason Fung, a Canadian physician and author of The Obesity Code, argues that obesity is not simply a problem of consuming too many calories, but also involves the hormonal regulation of how the body stores and uses energy.
Fung places particular emphasis on insulin. He argues that chronically elevated insulin can encourage fat storage and make it more difficult for the body to access stored fat. From this perspective, simply telling people to “eat less and move more” may not address the biological forces affecting hunger, metabolism, and body weight.
His approach emphasizes reducing refined carbohydrates and sugars, avoiding frequent eating, and using intermittent fasting in appropriate circumstances to lower insulin levels and allow the body to draw more readily on stored energy.
Fung's broader point is provocative: calories matter, but the hormonal response to food may help determine how hungry we become, how much we eat, and how the body handles the energy we consume.
The Hunger Code examines why obesity cannot be understood simply as eating too many calories and exercising too little. It explores how hunger, hormones, metabolism, the brain’s regulation of body weight, and ultra-processed foods can influence how much we eat and how readily the body stores or loses fat.
Why Do We Get Fat? Part 1 — The Biggest Lie in Obesity Medicine - 13 minutes
Dr. Fung explains why he believes the traditional “Calories In, Calories Out” model does not adequately explain obesity, emphasizing the roles of insulin, hormones, hunger, and metabolism.
Insulin Blocks Fat Burning — The Key to Weight Loss - 13 Minutes
Fung explains how insulin affects fat storage and fat burning, and why he believes lowering insulin through diet and fasting can play an important role in weight loss.
The NIH randomized controlled trial in which participants consumed about 500 additional calories per day on an ultra-processed diet.
Taubes challenges the conventional “eat less, move more” explanation of obesity and argues that hormonal regulation—particularly insulin—plays an important role.
An accessible explanation of insulin resistance and the role insulin plays in metabolism, fat storage, and chronic disease.
Dr. Ludwig examines how food quality, hormones, hunger, and metabolism may affect body weight and challenges the idea that calorie restriction alone is the best approach to weight control.