When people want to lean up or lose fat the first thing they do is do more cardio. When done properly, cardio can improve your health, help you lose fat faster, and even help you build muscle. But when done improperly, it can do the opposite: impair health, fail to help you lose weight, and negatively impact body composition.
The problem is simple: the more you do a certain type of activity, the more your body adapts to increase efficiency, and the more this occurs, the less energy is needed to continue doing it.
Cardio doesn’t preserve muscle, which is just as important as losing fat.
We instinctively say we want to lose weight, but what we really want to do is lose fat.
This distinction is important because losing weight includes losing muscle, which is to be expected when you combine a calorie deficit with cardio alone, and which is the road to a “skinny fat” physique.
The key here is the inclusion of resistance training in your weight loss regimen, which ensures that you lose maximal amounts of fat and minimal amounts of muscle.
from Muscle for life