Why does the same cardio workout burn fewer calories over time?

Nikkiey explains that your metabolism works within a range. It can run more efficiently, conserving calories, or less efficiently, spending more of them. For fat loss you want it less efficient. Lots of repetitive cardio pushes it the other way. Your body recognizes that the activity burns a lot of calories and needs endurance more than strength, so it learns to conserve energy, in part by paring down muscle. She describes long cardio sessions as a point where the body may start breaking down muscle tissue for fuel instead of fat. Less muscle means a slower metabolism and fewer calories burned for the same session.

That is the trap she sees with people who do one or two hours on the treadmill or elliptical every day. They track the number on the machine, for example eating 1,800 calories and burning 600, but Nikkiey says that number has virtually nothing to do with anything. Over months of doing the exact same thing, they have been teaching their body to conserve calories for that activity. The only way to keep getting results is to keep adding duration or intensity, and the look they end up with is often what she calls skinny fat: smaller, but with no muscle tone.

Strength training sends the opposite signal. Nikkiey points out that lifting sessions don't burn many calories on a fitness watch, and that shouldn't matter. Those workouts demand strength and muscle, so your body responds by building muscle and becoming less efficient with calories, which is what she says speeds up your metabolism and helps fat loss.

So if you've been grinding out more and more cardio for less and less change, it isn't a willpower problem. On the show's view, the plan is training your body to burn less. Building muscle is what changes the equation.