Carbohydrates and Protein-Sparing: How Important Are Carbs for Muscle Growth and Maintenance?

Carbohydrates and Protein-Sparing: How Important Are Carbs for Muscle Growth and Maintenance?

It’s hardly a secret that your muscles need protein for growth. Protein contains the amino acids muscles need to become larger in response to resistance training. Without enough dietary protein, muscle growth stalls. Muscle hypertrophy takes place when myofibrils increase in size in response to a sufficient resistance training stimulus. When myofibrils grow in size so do muscle fibers along with the size of your muscles. This increase in fiber size requires protein synthesis. Therefore, you need enough protein in your diet to supply the building blocks for growth.

Protein isn’t the only dietary component you need for muscle growth – what about carbs? Carbohydrates aren’t directly involved in protein synthesis. Why are they essential? Carbs have a protein-sparing effect. This is most important when you’re restricting calories to lose weight or doing a significant amount of exercise, especially endurance exercise. In a sense, carbohydrates are “anabolic” because they help you avoid muscle protein breakdown during exercise and help you maintain muscle in response to calorie restriction.

 Carbohydrates Protect Against Muscle Breakdown

How do carbohydrates help you hang onto muscle? During long periods of exercise or low-calorie intake, you break down fat. Most cells in your body can use that fat as a fuel source. That’s a good thing if your goal is to lose body fat. Under extreme carbohydrate and calorie restriction, your liver can produce ketone bodies as an alternative fuel source when glucose supplies are low. Ketone bodies are protein sparing too. Of course, it’s not a good idea to drop your calorie or carb intake too low even when you’re trying to lose weight.

Ketone bodies are a fuel source but some tissues in your body still need glucose. Where does that glucose come from? If you’re not supplying your body with enough carbohydrates through diet, muscle protein becomes a source of glucose. Your liver is able to convert protein to glucose through a process called gluconeogenesis. That’s exactly what happens when your energy requirements are high and glucose is in short supply. This even happens to some degree during an overnight fast.

Where does the protein come from? From your muscles, of course. So carbohydrates are protein sparing because they protect your muscle tissue from breakdown. They do this by supplying cells with the glucose they need. Protein sparing refers to any source of fuel that “spares” muscle tissue. This includes carbohydrates but also ketone bodies produced in response to fat breakdown during times where glucose is limited. A high-protein diet is also protein sparing to some degree. When your body has access to lots of dietary protein, your liver can use that protein to make glucose rather than breaking down muscle tissue.

 The Importance of Post-Workout Carbs

Ever notice how so many experts recommend a protein/carbohydrate snack after a workout? Some studies show there’s an “anabolic window period” after a resistance training workout, about an hour in duration, where your muscles need amino acids and carbs for growth and recovery.

Why carbs and not just protein? For one, you’re trying to replace muscle glycogen stores. Secondly, carbs cause a greater insulin spike than protein alone. This helps get amino acids into cells quickly so they can serve as building blocks for growth when your muscles are most receptive to them. This is one time when eating rapidly-absorbed carbs is beneficial because they cause a greater release of insulin. Carbohydrates help with muscle growth here too. One thing to keep in mind – some studies support the importance of nutrient timing and the anabolic window period while others don’t.

Do Very Low Carb Diets Make It Hard to Build Muscle Mass?

You might assume that very low carb diets are a death sentence for muscle growth. Interestingly, an article published in Nutrition and Metabolism suggests otherwise. This article highlighted several small studies showing very low carb diets (daily carb intake as low as 10 grams) resulted in fat loss WITHOUT significant loss of muscle tissue. These studies went so far as to suggest that low carb diets may protect against muscle loss in the presence of adequate amounts of dietary protein. Why might this be?

For one, low carb diets are high in protein. Higher protein means more amino acids are available to be used to make glucose without tapping into the muscle tissue you’ve worked so hard to build. Second, very low carbohydrate diets increase ketone production. Ketones serve as a fuel replacement source for many tissues when there isn’t enough glucose around. Therefore, there’s less need for glucose to be made from muscle protein breakdown through gluconeogenesis when there are ketone bodies around.

The Bottom Line?

Most evidence suggests carbs prevent muscle protein breakdown when you’re dieting or burning lots of calories through exercise. Still, there is some evidence that ketone bodies, produced in response to a low carb diet, reduce breakdown of muscle proteins and may be sufficient to prevent muscle loss. If you’re eating a low carbohydrate diet and also consuming relatively high amounts of protein, the added dietary protein may offer some protection against muscle loss.

What about carbohydrate timing? Do you need to consume simple carbs right after a workout? Whether timing carbohydrate and protein intake have benefits beyond eating a diet that contains enough of these macronutrients is still unproven – but it does make sense from a physiological standpoint. Consuming carbs along with protein helps get amino acids into your muscle cells quickly to help them recover. You’re most “carb tolerant” right after a workout when your insulin sensitivity is high so you can best handle simple carbohydrates without adverse metabolic consequences after a workout session. That’s why a combination of carbohydrates and protein is beneficial, especially after resistance training and after a high-intensity workout. Eat protein but don’t forget that carbs have value too.



Am J Physiol Endocrinol Metab 2001, 280(6):E982-93.

J Appl Physiol 2000, 88(2):386-92.

Nutr. Metab 2006. 3:9


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