Does doing a lot of cardio slow down your metabolism?

Nikkiey explains it as adaptation. When you do a lot of cardio day after day, your body learns that it needs endurance, not muscle or strength. Muscle is part of your metabolism, so your body saves energy by turning muscle down, and your metabolism slows with it. That is why she says the calorie number on the treadmill doesn't matter much in the moment. Over weeks of repeated cardio, your body is adapting to burn less.

Strength training sends the opposite signal. Nikkiey points out that those workouts may not burn many calories while you do them, but they demand muscle. Your body responds by building it, and muscle is metabolically active tissue that keeps burning calories even at rest. She also says muscle is what changes your body composition and gives you a toned look. She is clear that women will not get bulky from lifting, and says the only way that happens is with steroids.

She also adds a caveat. She is not asking you to give up activities you love, such as mountain biking, and she understands the cardio high. The point is that hours of cardio aren't the path to a leaner, stronger body. If you've been grinding on the treadmill and getting nowhere, the plan was working against you, not your effort.