Calories In, Calories Out Is Still True, But More Complicated
Why movement, muscle, and meal quality all change how the calorie equation works
I was in a debate recently about why people often say they can eat bread, cheese, and desserts in Europe without gaining weight, but not in the United States.
Their take was that “there’s something in the food here making us fat.”
My response was simple. The food is not that different. What changes is how much we move. In many European cities, it is normal to walk 8,000 to 10,000 steps a day just getting to work, running errands, and using public transit. In the U.S., the same day might involve fewer than 4,000 steps and a lot of sitting.
Walking an extra 5,000 steps burns roughly 200 to 250 calories for most adults. Over a week, that is about 1,400 to 1,750 calories, close to half a pound of body fat. Over a year, the difference can be more than 20 pounds, even if the food is exactly the same.
That balance of calories in versus calories out is the core of weight change. But the way our bodies burn calories is more complicated than a simple math equation.
What’s Driving the Debate
“CICO is not a fad, it’s physics,” said Kevin Hall, a senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases, in a January 2025 NIH interview. “But the body is not a simple math equation.”
The principle is clear. Burn more calories than you consume, and you lose weight. Consume more than you burn, and you gain.
The reality is that metabolism is always adjusting to what you eat, how you move, and how your body changes.
Metabolism in Motion
Basal metabolic rate is the energy your body uses at rest. It accounts for most daily calorie burn. That number can change.
Dieting can slow it down, a process called adaptive thermogenesis. Illness or poor sleep can do the same.
Gaining muscle can have the opposite effect. Muscle burns more calories at rest than fat tissue. The more muscle you have, the more energy your body uses even when you are not moving. Strength training can raise daily calorie needs and help maintain weight loss.
A 2024 review in Obesity found that energy expenditure can drop by 10 to 15 percent during prolonged calorie restriction, making weight loss harder over time.
A more recent study found that after very low‑calorie diets or bariatric surgery, resting energy expenditure dropped by about 322 kcal per day beyond what would be expected
The Quality Question
Not all calories are equal in how they affect hunger and energy.
A 2023 American Journal of Clinical Nutrition study found that people eating higher protein and higher fiber meals consumed fewer calories later in the day than those eating ultra-processed meals with the same calorie count.
Practical Takeaways
Experts recommend using CICO as a framework, not a strict rulebook. Tracking can be helpful, but food quality, muscle mass, sleep, and stress all affect the outcome.
“Calories matter, but they are not the whole story,” said Hall. “The foods you choose and the lifestyle you lead will determine how well the math works for you.”
Bottom Line
CICO still applies. It is also incomplete. Energy balance drives weight change, but metabolism, behavior, and body composition make the equation a moving target.
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