Cardio exerts two benefits: It burns calories and affects hormone levels in the body. Specifically, cardio helps raise levels of norepinephrine. Yet when you do cardio it makes a big difference in how your body handles the hormonal changes. Cardio on an empty stomach allows norepinephrine to readily target fat cells, which triggers maximal fat-burning. On the flip side, if you eat before doing cardio, and particularly if you eat carbs, the fat-blocking hormone insulin rises, making your body less effective at burning fat.
The prevailing formula for a long time on how much fat you’re going to burn was calories in minus calories out, based on your basal metabolic rate (BMR) and exercise efforts, explains strength and performance specialist Joel Seedman, Ph.D., owner of Advanced Human Performance in Atlanta. But with all the different biochemical reactions in the body, hormonal response, and endocrine function, there are an infinite number of factors that can affect how your body is storing and breaking down calories.