Caloric balance is the primary determinant of weight change, and macronutrient composition directly influences changes in body composition.

Study: Reduced adipose tissue with limited loss of lean mass after weight loss: results from the Prostate Active Lifestyle Study.

When these men lost weight by eating less and exercising more, most of what they lost was fat, not muscle — which means calories control weight loss, but what you eat and how you move affects whether you lose fat or muscle.

Study: A small switch in perspective: Comparing weight loss by nutrient balance versus caloric balance

Both diets helped people lose weight, but eating more protein and healthy fats made people lose more fat and gain more muscle than just counting calories alone.

Study: The Effects of Continuous vs. Intermittent Caloric Restriction on Fat Loss: A Randomized Controlled Trial

Both groups lost the same amount of fat, even though one ate fewer calories overall—proving total calories matter most for weight loss. The group that took diet breaks kept more muscle, suggesting what you eat and when you eat it can affect muscle vs. fat changes.

Study: Very low-calorie diet in candidates for bariatric surgery: change in body composition during rapid weight loss

When people eat way fewer calories, they lose weight mostly from fat, but some muscle too — which means total calories control weight loss, while what you eat affects whether you lose more fat or muscle.