I doubt you'll gain fat by going on Atkins. You'll likely gain muscle, which you most certainly lost by fasting AND exercising at the same time. Muscle loss isn't good, and it will slow your metabolism. My advice: stop fasting, start Atkins, or LC plan of your choice, and eat when you are hungry, don't starve yourself. You'll probably put on some pounds, probably muscle, so if that bothers you, stay off the scales.