Have you ever wondered what happens to the protein you consume after you eat a bite of steak or drink a shake? This post looks at how protein is broken down in your body and the different steps it goes through before ending up as muscle tissue.

Proteins make up more than just our muscles. They form the basis of hair, nails, connective tissue, hormones, enzymes, Ph buffers, fluid balancers and a myriad of other things. These complex molecules are made up of smaller units called amino acids, often referred to as the “building blocks” of protein. Your body uses twenty-one different amino acids to create the proteins it needs. Nine of which are essential and must be obtained through your diet [1] and six of which are conditional (which means your body can synthesize them but not always in the amounts needed).

The Breakdown Begins

The journey kicks off in your mouth where chewing introduces enzymes into your food. Then in your stomach, hydrochloric acid mixes with the enzyme pepsin and starts breaking the proteins down into smaller polypeptides, oligopeptides, and some free amino acids [1].

The Small Intestine: The Hub of Digestion

After leaving your stomach, the partially digested protein enters the small intestine. Various digestive enzymes get to work, further breaking down the protein into even smaller pieces, mainly dipeptides, tripeptides, and free amino acids [1]. The first part of the small intestine, specifically the duodenum and jejunum, absorbs these tiny protein fragments through its walls.

Entering the Bloodstream

Once the small intestine absorbs them, amino acids enter your bloodstream and are transported to the liver and other tissues in your body. The liver, serving as a central processing unit, regulates amino acid levels and adapts their metabolism to meet your body’s demands [1].

The Amino Acid Pool

Before being put to work, amino acids enter what’s called the “amino acid pool.” This isn’t an actual pool but rather a term used to describe the total amount of amino acids available in your bloodstream and cells at any given time. It’s from this pool that your body draws the necessary amino acids for building new proteins [1].

Building Muscle: The Anabolic Process

From this pool, your body draws necessary amino acids for building proteins, particularly during muscle growth (hypertrophy) resulting from resistance training. They are transported to your muscle cells and used to construct new proteins, resulting in muscle growth (hypertrophy) [2]. This process is known as muscle protein synthesis (MPS) and is the driving force behind your gains in the gym.

The Importance of Leucine

Leucine, a key amino acid abounding in meat, dairy, and eggs, actively triggers MPS by signaling the mTOR pathway [2]. To maximally stimulate MPS, you need to consume enough leucine, which is abundant in high-quality protein sources like meat, dairy, and eggs [1].

The Fate of Excess Amino Acids

But what happens if you consume more protein than your body needs? Excess amino acids are typically broken down (catabolized) and used for energy or converted into glucose, fatty acids, or ketones [1]. The nitrogen from these amino acids is removed and converted into urea, which is then excreted through your urine.

Optimizing Protein Intake

To support optimal muscle growth and recovery, current research recommends consuming around 1.6 – 1.8 grams of protein per kilogram of body weight daily [3]. Spreading your protein intake evenly across several meals appears to be the most effective strategy, with a focus on consuming high-quality, leucine-rich protein sources [4].

So, as you enjoy your next protein-packed meal, marvel at the remarkable journey those amino acids will go on.

References:

• 1 Gropper, S. S., Smith, J. L., & Groff, J. L. (2012). Advanced Nutrition and Human Metabolism (6th ed.). Cengage Learning.
• 2 Norton, L. E., & Layman, D. K. (2006). Leucine Regulates Translation Initiation of Protein Synthesis in Skeletal Muscle after Exercise. The Journal of Nutrition, 136(2), 533S-537S.
• 3 Schoenfeld, B. J., & Aragon, A. A. (2018). How much protein can the body use in a single meal for muscle-building? Implications for daily protein distribution. Journal of the International Society of Sports Nutrition, 15(1), 10.
• 4 Mamerow, M. M., Mettler, J. A., English, K. L., Casperson, S. L., Arentson-Lantz, E., Sheffield-Moore, M., Layman, D. K., & Paddon-Jones, D. (2014). Dietary protein distribution positively influences 24-h muscle protein synthesis in healthy adults. The Journal of Nutrition, 144(6), 876–880.