It's no mystery that a diet full of fried foods, giant portions, decadent desserts, alcohol, and sugary soft drinks will lead to weight gain. And there's little question why the pounds pile up when you take in more calories than you burn in physical activity. But how do you explain weight gain when your lifestyle includes regular exercise and a healthy diet that is controlled in calories? Gaining weight is absolutely maddening, especially when you really don't understand why the needle on the scale keeps going up.