When people increase their physical activity, their bodies reduce other forms of energy expenditure, such as resting metabolism and spontaneous movement, which limits the overall increase in daily calorie burn.

We analyzed the available evidence and found that when people increase their physical activity, their bodies may reduce other forms of energy expenditure, like resting metabolism and spontaneous movement, which can limit how much total daily calorie burn actually increases [1]. This pattern was supported in 49 studies or assertions, with only one assertion offering a contrary view. What we’ve found so far suggests that the body might adapt to higher activity levels by becoming more energy-efficient in other areas — for example, by moving less unconsciously throughout the day or lowering the energy used at rest. This doesn’t mean the extra exercise has no effect, but rather that the total energy burned over the course of a day may not rise as much as expected. The body appears to compensate in subtle ways, which could help explain why some people don’t lose as much weight as predicted when they start exercising more. This doesn’t happen in everyone, and the degree of compensation may vary by individual, age, fitness level, or how the activity is increased. But the pattern we’ve seen across many observations points to a tendency for the body to balance energy use across different systems. The evidence we’ve reviewed leans toward the idea that increasing physical activity doesn’t always lead to a proportional rise in total daily calorie burn, because other parts of energy expenditure may decrease to offset it. In everyday terms: if you start working out more, your body might respond by making you feel a little less fidgety or slightly more tired during the day — which could mean you burn fewer extra calories than you think.