This is especially true if you're following one of the ultra-low-carb diets that are so popular now. Carbohydrates are the primary energy source for physical activity, and decades of research has shown that low-carb diets don't adequately support strenuous physical activity or athletic performance for extended periods of time. In contrast, a diet moderate in carbs will supply enough energy for the average Joe to stay active and still burn fat. Endurance exercise requires more carbohydrates than strength training does, but in neither case will training be optimized without sufficient carbs.
Problem 4: Diets cannibalize your muscle.
Diets also tend to pay too little attention to supporting muscle mass during periods of caloric restriction. This is important for more than just aesthetic reasons. And when you lose muscle, your basal metabolic rate drops, and you don't burn as many calories.
Most people try to burn off the fat first and then build the muscle. To do that, you have to lower your calories so far that you don't have the energy to train hard in the gym. You burn more muscle than fat, lowering your metabolic rate and setting the stage for weight gain.
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