Optimizing Dietary Protein

Dietary Strategies for Optimizing Skeletal Muscle Mass

Getting dietary protein into your muscle.

This starts in the mouth with chewing, continues in the stomach with the action of hydrochloric acid and pepsin, and concludes in the small intestine with further enzymatic activity from trypsin and chymotrypsin.  Under these mechanical and chemical influences, protein is reduced to amino acids, dipeptides, and tripeptides which are taken into cells lining the small intestine via active transport and then passed into the bloodstream.  In the bloodstream, these protein substrates are transported in their unbound form.  Upon arriving at a tissue actively synthesizing protein, these compounds are again actively transported from the bloodstream into cells, including muscle cells, where they are assembled into tension-developing structures called sarcomeres.

1. Is there a limit to how much protein can be absorbed from the GI tract in one meal?

The body can absorb nearly all ingested protein, regardless of the amount. However, muscle protein synthesis (MPS) that generates more contractile elements to produce more tension and potentially more muscle volume, appears to hit a maximum of 20–40 grams of uptake per meal, depending on body size and protein quality. Absorption and utilization are distinct: while your gut can absorb more than 30 grams, amino acids that are absorbed but not committed to MPS may be oxidized or used for other metabolic processes rather than sidetracked for muscle building.

The evidence for this plateau of MPS utilization of 20–40 grams per meal comes from studies using tracer methodologies to measure ingested amino acid incorporation into muscle tissue. One such study by Schoenfeld and Aragon (2018) concluded that ~0.4 g/kg/meal of high-quality protein—roughly 20–40 grams depending on body weight—is optimal for MPS in young adults. Beyond this, additional amino acids are more likely to be oxidized or used for other metabolic functions rather than further increasing MPS.  These studies are very challenging and the results are an absolute function of the experimental conditions, including the sampling duration. Hence, the findings about maximal incorporation could have been influenced by the timing of the muscle sampling.

Newer research speaks to just htis point. In a  2023 study published in Cell Reports Medicine, investigators ffound that ingesting 100 grams of protein post-exercise led to a greater and more sustained MPS response than 25 grams, suggesting that the anabolic response may not have a strict upper limit. The key difference between this and the Schoenfeld study lies in the duration of measurement—Schoenfeld tracked MPS for a few hours, potentially missing delayed effects from larger protein doses.

So, while the 20–40 gram guideline is still practical for most people aiming to optimize MPS across multiple meals, it’s not a hard cap, and more research is needed.. 

Schoenfeld 2018: https://rdcu.be/eu05l

Trommelen 2023: https://doi.org/10.1016/j.xcrm.2023.101324

2. How soon after exercise is amino acid muscle uptake detected, and how long does it last?

Amino acid uptake into muscle begins within 30 minutes post-exercise and can remain elevated for up to 24–48 hours, with the most pronounced effect in the first 3–5 hours. This window is often referred to as the “anabolic window,” during which muscle cells are more sensitive to amino acids and insulin, enhancing MPS.

3. What factors determine the rate of amino acid uptake in skeletal muscle?

Several key factors influence this rate:

• Insulin levels: Insulin promotes amino acid transport into muscle cells, especially when paired with carbohydrates.
• Exercise: Resistance training increases blood flow and transporter activity, enhancing uptake.
• Amino acid composition: Leucine, in particular, is a potent stimulator of MPS via the mTOR pathway.
• Muscle fiber type and metabolic state: Fast-twitch fibers and muscles in a catabolic state (e.g., post-exercise or fasting) show higher uptake.
• Age and training status: Younger and well-trained individuals often exhibit more efficient amino acid utilization.

4. What about Creatine?

• Is it degraded (hydrolyzed) by stomach acid? 
• No

• Is it broken down by proteolytic enzymes in the either the stomach or small intestine?

• No

• How and where is it absorbed?

• • It is moved from the lumen of the small intestine (not the stomach) into cells lining the small intestine using a specific active transport system and from there into the bloodstream where it travels unbound.
• What tissues use creatine for their metabolism?
  • All tissues in the human body depend on energy generation in cells that use a combination of ATP and Phosphocreatine to move energy from the mitochondria to organelles that use it for a variety of functions.  In nerves and the brain, the energy is essential to maintain cell membrane electrical gradients that support nerve signal condition.  In muscle, the chemical energy is used to power the contraction of muscle cells, collectively generating tension which is ultimately demonstrated as force production in the whole muscle.
• How does it get from the bloodstream into target tissue cells like muscle?
  • This movement is controlled by and dependent on a creatine-specific active transport system.

Other Questions I’ll Answer Soon!

• What factors control how much creatine is incorporated into the target tissue?