Energy Pathways 101

“As a follow up question from yesterday’s post, which breaks down first – muscle or fat.  If you have good muscle mass and some excess fat, if you begin to do more cardio – will the fat breakdown first or the muscle.  Or do they both happen at the same time?”

The body has several energy pathways it uses during exercise, which burn through all three stores of fat, muscle and glycogen (carbohydrate storage).  We sometimes do aerobic activity that requires oxygen (think: long runs), and sometimes anaerobic that doesn’t require oxygen (think: sprints).  Most times we use both energy pathways within the same session and all three macronutrients.  Therefore, these individual energy systems are never really turned on or off, but become used more or less in order to balance energy use during a workout.

At rest, fat metabolism is the source of energy.  When we exercise, carbohydrates are the main source of energy.  Protein metabolism accounts for a small proportion of energy expenditure–except if in a starvation mode.

So then, one reason never to get too low in consumption of carbs is because the muscle glycogen will be used to provide energy for the workout versus the muscle itself.  Once glycogen stores run out, you don’t ever use only muscle or only fat, but instead, both energy sources are required to power up the workout.

On the nutrition front, to preserve lean body mass and muscle, take in sufficient protein (.8–2grams per kg of body weight) with sufficient carb (moderate exercise: 5-7g per kg/body weight).

On the exercise front, both resistance and cardio training can help preserve lean muscle mass while toning up.


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