This 2017 study challenges the assumption underlying the Calories In, Calories Out (CICO) model that increasing physical activity leads to a proportional rise in calories burned. Using doubly labeled water to measure real-world energy expenditure across 332 adults, the researchers found that total daily energy expenditure increases only modestly with activity before reaching a plateau. Beyond a certain point, exercising more does not result in burning more total calories.

The findings suggest that the body actively compensates for sustained increases in physical activity by reducing energy spent on other physiological processes, keeping total energy output within a narrow range. This helps explain why exercise alone often produces far less weight loss than predicted by simple CICO calculations and why “eat less, move more” models routinely fail in practice. Human metabolism is not a passive calorie-counting system, but a regulated one—requiring more than a simple reduction in calories to produce weight loss.