When you first get into Ketosis you will lose some significant weight really quickly. That's one of the best benefits and one of the main Signs of Ketosis. The bad news is that the beginning weight loss is mostly water. The good news is that means you are on your way to becoming a full fledged fat burner. At this point the extra calories you burn by exercising and working out will be from fat. If you are not in Ketosis yet you'll burn sugar while you exercise. This is a good thing too. While You're Exercising: Obviously you burn more energy while you are working out, but not as much as you would think. After you Exercise: Your body will need energy to recover, maintain and build muscles.( You might be hungry. Plan to eat some healthy fat.) You'll also Increase Your BMR: Exercise combined with a good Keto diet will increase your muscle mass and improve the blood circulation and the function of your vital organs. This will increase the amount of calories you burn even while you are resting.