If you eat a carbohydrate-rich meal (lots of pasta, rice, bread, or French fries, for example), your body releases insulin to help with the influx of all this glucose into your blood. As well as regulating blood sugar levels, insulin does two things: It prevents your fat cells from releasing fat for the body to burn as fuel (because its priority is to burn off the glucose) and it creates more fat cells for storing everything that your body can’t burn off. The result is that you gain weight and your body now requires more fuel to burn, so you eat more. Since insulin only burns carbohydrates, you crave carbs and so begins a vicious cycle of consuming carbs and gaining weight. To lose weight, the reasoning goes, you need to break this cycle by reducing carbs.
In a way, moderate-intensity physical activity is that "magic pill" a lot of people are looking for, because the health benefits go beyond keeping your waistline trim: Not only can it reduce your risk of cancer, stroke, diabetes and heart attacks, but studies have shown that physical activity can significantly improve the moods of patients with major depressive disorders.