Sugar
Sugar is your enemy. Sure it tastes good and you get an instant boost from it, but your body processes it easily and turns it into glucose.
Excess glucose gets stored as fat in your body to fuel it against starvation. Eating too much sugar leads to insulin resistance and that packs on the pounds and destroys your fitness achievements.
Sugar includes the obvious – donuts and candy. But it also includes spaghetti sauce, ketchup, recovery drinks, cereal, canned food, and prepared food.