Can too much cardio make you lose muscle?

Nikkiey frames it as adaptation. Your body gets good at whatever you ask of it. With long cardio sessions, it reads the signal as "we need endurance, not muscle and strength," so it learns to conserve calories by paring down muscle. Because muscle is directly tied to your metabolism, that efficiency slows it down. Strength work sends the opposite message: build muscle, which is what speeds your metabolism up. She also warns that excessive running can tip into overtraining, where the body is constantly fatigued, inflamed and unable to recover, which she says makes building muscle almost impossible.

Fuel makes it worse. In her catabolic vs anabolic episode, Nikkiey says that if your body is adapted to endurance-style workouts like cardio and boot camps and you aren't eating enough to support them, you live in a catabolic state, breaking down muscle and storing body fat, especially over weeks and months. Fasting or skipping food before lifting does the same. The scale goes down, but so does your muscle, and that's the skinny fat look so many women are trying to escape. She notes that underlying medical conditions affecting hormones may change how this applies to you, so that's worth raising with your doctor.

If you've been doing more cardio and getting softer, that's not a willpower problem. It's the plan.