Beyond Protein: the Anabolic Effects of Carbohydrates

When you think of anabolic macronutrients, the first thing that probably comes to mind is protein. What does it mean when a nutrient is anabolic? Anabolic refers to metabolic pathways that make larger compounds from smaller ones. In the weight-lifting community, anabolic means building new muscle protein to increase muscle size. Isn’t that the ultimate goal of bodybuilders and fitness gurus alike?

No doubt about it, protein is an anabolic macronutrient. When you consume dietary sources of protein, they’re broken down in your digestive tract to amino acids. The newly-released amino acids are absorbed by your small intestines and enter your bloodstream where they’re used for, among other things, to build muscle. Muscle is constantly undergoing turnover as muscle tissue is broken down and rebuilt. To do this, your muscles need amino acid building blocks.

To build muscle and avoid losing muscle tissue, you have to supply your muscles with enough amino acids so that muscle synthesis exceeds muscle breakdown. If you don’t, your muscles enter a catabolic state and begin to break down lean body mass. So, getting adequate dietary protein not only stimulates muscle protein synthesis but suppresses muscle protein breakdown.

Protein turnover and protein balance (also referred to as nitrogen balance since protein contains nitrogen) are influenced by the composition of your diet (amount of total energy you’re consuming and grams of protein) as well as how much your muscles are being stimulated by exercise. Whether you do resistance or endurance exercise, or both, you need more protein than a sedentary person to maintain protein or nitrogen balance. No surprises here, right?

Protein Isn’t the Only “Anabolic” Nutrient

Although protein is the ultimate anabolic macronutrient in the minds of most people, it’s not the only one. Carbohydrates are the primary source of energy used to fuel exercise, especially high-intensity exercise, but carbohydrates also play a role in maintaining the anabolic environment your muscles need to grow and become stronger. Whether you’re doing high-intensity interval training or high-intensity resistance training, muscles need sufficient glycogen stores to supply your muscles with energy and maximize how hard you can work. Glycogen is built from dietary carbohydrates. It’s a storage-form of carbs your muscles can tap into to fuel muscle contraction. Having sufficient muscle glycogen stores is important when you’re trying to lift heavy weights. The last thing you need is for your muscles to “poop out” because they’re glycogen depleted. When you can’t lift as hard due to a fuel shortage, you limit muscle growth.

Another reason carbohydrates are anabolic has to do with their protein-sparing effect. If you’re eating a reduced-calorie diet to lose weight, getting enough protein and healthy carbohydrates is important for preserving lean body mass. Though most cells in your body can use fat for fuel, it likes to have a source of glucose around. Plus, red blood cells can only use glucose and your brain and the retina of your eye strongly prefer glucose, although they can use ketone bodies during periods when glucose is in short supply. Where does this source of glucose come from? From protein.

During periods when you’re not consuming enough calories or during long endurance workouts, if you don’t have available glucose, your body, more specifically your liver, makes it from amino acids. The process the liver uses to make glucose from amino acids is called gluconeogenesis. The source of the amino acids, in some cases, is muscle tissue. So, if your body needs glucose and you’re depriving it of carbohydrates, it will tap into your hard-earned muscle tissue. Carbohydrates are protein sparing because when your body has a glucose source available (from carbohydrates) it won’t break down muscle. Dietary protein, too, is muscle sparing because if you have dietary amino acids available, your liver can use these amino acids to make glucose rather than cannibalizing muscle tissue.

Finally, carbohydrates create an anabolic environment by stimulating the release of insulin. The one time you want insulin hanging around in your system is after a resistance-training workout. Insulin is the major hormone that drives glucose and amino acids into cells, including muscle cells that need them to grow. According to some studies, there’s a window period after a workout when your muscles are “primed” to grow as long as they have proper nutritional support. That’s why fitness gurus tell clients to eat a protein/carb snack within an hour after a workout.  Post-workout carbohydrates stimulate insulin release, even more so than a straight protein meal. This sends much-needed amino acids into muscle cells. The best time to consume high-glycemic carbohydrates is right after an exercise session when your muscles can maximally benefit from insulin release.

What about fats? Are they protein sparing too? Research shows fatty acids from dietary fat do have a protein sparing effect but less than carbohydrates.

The Bottom Line?

You may not think of carbohydrates as being anabolic like protein, but they are. Carbs work “behind the scene” to help you maintain lean body mass and maximize the intensity of your workouts. This doesn’t mean you should add sugar and processed carbs to your diet. Save the high-glycemic carbs for after a workout, and choose whole food sources of carbs like fruits, vegetables and moderate amounts of whole grains the rest of the time. Aim for a balanced diet that doesn’t overly restrict any one macronutrient. If you’re trying to build lean body mass, make sure you’re consuming enough calories. You can’t usually build muscle when you’re in a calorie deficit.

References:

J Strength Cond Res. 2004 Feb;18(1):174-9.

Curr Opin Clin Nutr Metab Care. 2009 Jan;12(1):66-71. doi: 10.1097/MCO.0b013e32831cef75.

Metabolism Clinical and Experimental 55 (2006) 501 – 507.

JAMA. 1916; LXVII(17):1233. doi:10.1001/jama.1916.02590170041017.

Journal of the International Society of Sports Nutrition 2013, 10:5  doi:10.1186/1550-2783-10-5.

“Effect of different levels of carbohydrate, fat and protein intake on protein metabolism and thermogenesis.” E. Jequier. Institute of Physiology, University of Lausanne, 7, rue du Bugnon, 1005 Lausanne, Switzerland.

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