Is lower body fat associated with greater reduction in energy expenditure after increased exercise?

What we’ve found so far suggests that when someone with less body fat starts exercising more, their body may adjust by burning fewer calories at rest or during daily activities than expected. This could mean the body becomes more efficient, using less energy to perform the same tasks. It’s not that the person is slowing down—it’s that their metabolism may be adapting in a way that reduces total energy expenditure.

This doesn’t mean exercise is less effective for people with lower body fat. It simply means the body’s response to increased activity might include a compensatory drop in other areas of energy use, like fidgeting, digestion, or even spontaneous movement. The exact reasons for this aren’t clear from the evidence we’ve reviewed, but the pattern appears consistent.

We don’t know if this happens in everyone, or how much it varies between individuals. The evidence doesn’t explain whether this effect is temporary or long-lasting, or how it interacts with diet, age, or muscle mass.

For now, the pattern we’ve seen suggests that if you have lower body fat and increase your exercise, your body might naturally reduce energy use in other ways. This doesn’t mean your efforts are wasted—it just means your body is responding in a way that may make weight loss or maintenance harder than expected. Paying attention to how you feel, how you eat, and how your energy changes over time may help you adjust your approach.