Today I would like to look at what happens when we start running low on carbohydrate as you would do in longer duration cardio sessions.
Your body needs carbohydrate to survive. The brain cannot function without it so if your carbohydrate stores are running low your body will preserve it and look for another fuel. It is impossible to convert fat into carbs, so the only other source is to use protein.
Protein is not stored in the same way as fat but is used as building blocks for all tissue in the body, with a high proportion being muscle tissue.
So, in order for you to keep exercising at a high rate, you stand a chance of losing precious muscle tissue. I call it precious because in terms of your metabolism and fat loss, maintaining or increasing muscle may possibly be one of the most important things.