Welcome to Fueling Field Notes, where I’m documenting my journey to preparing for the CSSD (Certified Specialist in Sports Dietetics) exam. These posts are equal parts study notes, science breakdowns, and practical takeaways from the field.
If you have been following along in this study journey, you know that protein holds many roles in the body. It can serve as an energy source, but it’s not efficient. Carbohydrates and fat hold awards for supplying energy.
Protein’s real value lies in everything else it does, from building muscle and enzymes to regulating hormones and supporting immune function.
We have covered carbohydrate and fat metabolism. Now let’s look at how the body handles protein, what happens to amino acids during exercise, and why protecting protein from being used as fuel is one of the most important goals of smart endurance nutrition.
When we consume protein, it’s broken down into amino acids in the GI tract and absorbed into the bloodstream. From there, the liver takes the lead. It’s the primary site of amino acid metabolism and serves as a monitor and regulator of the body’s amino acid supply, directing amino acids where they are needed most.
About 20% of absorbed amino acids are processed directly by the liver. The liver uses them for two main purposes: anabolism and catabolism.
Anabolism refers to building processes. The liver takes simple molecules like amino acids and constructs complex molecules such as liver enzymes, plasma proteins like albumin, and other essential compounds. This is the constructive side of metabolism.
Catabolism refers to breakdown processes. When amino acids are in excess or when the body needs energy, the liver breaks them down into usable components. The nitrogen-containing amino group is removed and eventually converted to urea for excretion in urine. The remaining carbon skeleton can be used to produce ATP through the TCA cycle, converted to glucose through gluconeogenesis, or in some cases converted to fat and stored.
Unlike carbohydrates, which are stored as glycogen in the liver and muscle, and fat, which is stored in adipose tissue, protein has no dedicated storage site. There is no reserve pool of amino acids sitting in the body waiting to be used.
The closest thing to a storage site is skeletal muscle. The body can break down muscle protein to release amino acids when needed. But this comes at a high cost.
Muscle protein breakdown impairs the muscle’s ability to function, generate force, and recover. The body goes to considerable lengths to protect skeletal muscle from being used as fuel, and this protection depends heavily on the availability of carbs and fat.
When adequate carbs and fat are consumed, they provide a protein-sparing effect.
In simple terms, when the body has enough energy from other preferred sources, it does not need to break down muscle tissue for fuel. Protein is free to do the very important things only protein can do.
When an athlete is underfueled and not meeting their caloric needs, this protection breaks down. Protein must step in to meet energy demands, and all of its other essential functions begin to suffer.
The body uses protein for energy during exercise through two main pathways: direct oxidation of amino acids for ATP production, and gluconeogenesis.
When muscle cells are broken down for energy during exercise, the amino acids most commonly used are the branched-chain amino acids (leucine, isoleucine, and valine), along with aspartate, asparagine, and glutamate.
Fun fact: The stress hormone cortisol stimulates muscle protein breakdown, which is why periods of high physical or psychological stress increase amino acid oxidation.
Endurance exercise is a form of short-term physiological stress, and the result is an increased oxidation of BCAA leucine in particular. The degree to which this happens depends heavily on fueling status.
When an endurance athlete exercises with low glycogen stores, such as during low-carb diets or poorly-fueled training sessions, leucine oxidation increases significantly. A higher-carb diet with near-maximal glycogen stores has the opposite effect, reducing the need to break down muscle for fuel.
Dietitian note: This is exactly why well-fueled workouts and recovery nutrition are such important priorities for endurance athletes. Every time you train underfueled, you increase the physiological stress on muscle tissue and raise the likelihood that protein is being used for energy rather than repair and adaptation. Minimizing that stress is not just about performance in the moment. It is about protecting the long-term investment you are making in your training.
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