What Happens To Protein Metabolism During Exercise?
Let’s take a few minutes to talk about how exercise modifies protein metabolism and metabolic activity in your body. In order to get the basics of protein metabolism down, I encourage you to check that out first
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So you have the basics down. Now you’re wondering what happens when you work out. I’ll start by saying the answer is not simple. The processes that regulate protein synthesis and breakdown are still being researched and investigated by science. What we do know tells us that this process is complex.
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Before I get started, I want to briefly mention how you should think of protein metabolism. There are processes that increase protein synthesis (anabolism) and processes that increase protein breakdown (catabolism). This building and breakdown balance, if you will, is in a constant state of flux based on many factors.
First, your metabolic state is dictated and partially determined by many things that occur prior to your arrival to the gym. Your dietary intake, including both net calories and macro breakdown (fat, carbs, proteins) in the last 12-24 hours significantly affects this. When your last meal was will partially determine the balance of sympathetic and parasympathetic activity before you get started. The ratio of circulating insulin and glucagon and another half dozen hormones I will mention shortly play a major role. How much you’ve exercised in the last few days also matters and whether it was resistance training or endurance training or some combination. Any supplements you're on are also a factor. Finally, your body composition including lean muscle mass, body fat percentage and basal metabolic rate are also helping determine your protein metabolism. This is all happening all the time every day, even before you get to the gym.
So now you get to the gym and you’re getting started, so what changes?
Well the first thing is as soon as you start moving, your body releases epinephrine (aka adrenaline) and it’s very close cousin, norepinephrine. This is called the sympathetic state. When these hormones are released, your body is entering the so-called “fight or flight” state. Your heart rate goes up, your heart pumps more blood every time it contracts, your lungs open up and you breathe faster, and, your body begins shunting blood away from your gastrointestinal tract and to your skeletal muscle. This causes your to begin mobilizing glycogen stores to increase circulating blood sugar and fat break down also begins to occur. Finally, protein synthesis is suppressed and protein degradation is increased, increasing available resources for glucose production. This change in protein metabolism depends, in part, on your available glycogen stores.
What Hormones are released?
In addition to epinephrine, a variety of other exercise-induced hormones are released that help modulate total body metabolic activity. This includes cortisol, human growth hormone, insulin like growth factor and thyroid hormone. I’m not going to discuss these hormones specifically other than that they play a significant role in protein metabolism during exercise.
What about intensity?
The next step is the type and intensity of exercise. Resistance exercise is more metabolically demanding on skeletal muscle than endurance or cardiovascular exercise. More importantly, the intensity and duration of exercise is a major determinant of the state of protein metabolism. When you begin exercising, you mobilize your bodies glycogen. Glycogen is your body’s store of glucose but it is a short term solution only and is quickly depleted. If you exercise long enough and/or intensely enough, you will deplete your body's glycogen stores. Alternatively, if you just walk a few blocks, you may not mobilize all your glycogen. Once depleted, your body will mobilize other resources to maintain adequate blood sugar and prevent hypoglycemia. It does this by breaking down body fat and skeletal muscle protein. So if you exercise intensely and for long enough, you will have a net loss of muscle protein. This is caused by a combination of decreased protein synthesis and increased protein breakdown.This catabolic activity is maintained as long as glucagon, epinephrine and cortisol are circulating and/or having low blood sugar.
What about when you're done?
After exercise ceases and the catabolic hormones slow down, net protein breakdown will begin to slow and protein synthesis will increase. This is when hypertrophy begins to occur and is most prominent in the muscles that endured the most resistance while exercising. This new protein synthesis is best augmented by having a readily available supply of dietary protein and amino acids. Increasing dietary protein intake will also help minimize any protein breakdown that is still occurring.
In general, protein synthesis is suppressed during exercise. Concurrently, protein degradation is increased to make amino acids available for new glucose synthesis. This effect is modified by the increase in epinephrine and other hormones released during exercise.