It might not be a secret that adding protein to your diet is important for maximizing your strength-training efforts.

As when Ashton Eaton, 2x Decathlon Olympic gold medalist. recently responded to the benign question, “what is your diet?” on Quora concisely highlighting his first principle,

“Protein is king.”

But for those looking to maximizing their results, it pays to go deeper. There is a lot to learn about your protein intake, and diving into the research on protein dosing, source, and timing can lead to fruitful experimentation and better results.

One aspect of protein that might be less appreciated are the roles each individual amino acid play during muscle protein synthesis (MPS). One amino acid worth discussing in detail is leucine.

According to a recent critical review in The Journal of Nutrition about amino acid digestion and muscle protein anabolism, leucine seems to act as a trigger for amplifying protein synthesis.1 If you want to maximize your body’s ability to rebuild muscle after exercise, you’ll need to hit your leucine threshold to trigger mTORC1-activated MPS. Here’s how.

Your Body’s Needs for Building Proteins

To build muscle, your body needs to replace and grow new cells. Growing new cells requires the production (synthesis) of new proteins to do the work of replication and energy production. And taking a relatively simplified view, this process has two essential requirements:

  1. Enough energy to undergo and complete the process of synthesizing new proteins, and
  2. A high enough concentration of the amino acid “building blocks” of the proteins you’re going to synthesize.

The body is constantly experiencing “protein turnover.” This simply means that new proteins are being produced while old proteins are being broken down.

Comparing the rates of these two processes sheds lights on whether muscle tissue is growing (if synthesis is greater than breakdown), or muscle tissue is shrinking (breakdown is greater than synthesis).

The act of exercising, and in particular weight training, stimulates MPS regardless of your protein intake. During high intensity resistance training, your body largely shuts down the process of MPS in order to increase blood flow and supply muscles with the energy they need to perform the task at hand.

During this time, as blood flow increases, amino acid flux increases. This means, as you use your muscles for extended periods of time, your body harvests protein from existing muscle tissue. The concentration of available amino acids in your blood stream increases.

In the period just after resistance training (0 – 1 hour), muscle tissue becomes more sensitive to nutrition and gradually shifts from net negative protein turnover to net positive; i.e., MPS increases.

This happens during exercise even in a fasted state. So why eat protein?

Protein extends MPS post-exercise. Adding protein, especially the essential amino acids not available through protein turnover and required from your diet, increases the duration of MPS and helps provide the body with the ingredients it needs to continue building muscle tissue for longer.

The Leucine Trigger

Leucine, in particular, acts as a strong trigger for protein synthesis.

mTORC1 (mechanistic target of rapamycin complex (1) is a protein complex with a role in controlling protein synthesis. Leucine plays a role in activating mTORC1 signaling. Thus, leucine helps directly trigger a key mechanism in the MPS pathway.

Eating protein that includes leucine can trigger MPS even in the absence of exercise. However, the effects are relatively transient when compared to post-exercise MPS.

Adding leucine to post-exercise protein increases MPS in a dose-dependent manner. That means, increasing leucine content increases mTORC1 signaling and thus increases MPS activation.

This occurs when protein consumption is relatively low. When total protein intake is low, the amount of leucine plays an outsized role in stimulating MPS compared to other amino acids. When protein intake is high, it becomes more like a trigger with a maximum threshold. This means, even if protein intake is high but leucine is low, your body won’t be maximizing MPS. Only when the leucine threshold is reached will your body fully stimulate mTORC1 signaling and maximize MPS.

Meet the Leucine Threshold for Maximum Results

I mentioned earlier that energy was required to build new proteins. This is true up to a point. But once you have enough energy, it was shown that adding additional energy to your diet does not increase post-exercise protein synthesis.(2)

This same concept has similarities to protein synthesis, as well.

Say you set out to build your dream home. You’ve got your blueprints ready. Now you just need some materials to start. Wood. Nails. Screws.

You start building. You will make progress as long as you continue to supply enough materials. The walls will get higher, and the home will take shape.

That is, until you run out of wood or nails.

Similarly, you need to have the basic building blocks in your diet in order to build proteins and grow muscle tissue.

If you have plenty of wood, but your nails run out, you similarly won’t make it to a complete home.

Likewise, if you have plenty of some amino acids, but not enough of some others, you won’t maximize your ability to synthesize new proteins. You need quality protein that has plenty of all required essential amino acids.

So just like in our home-building analogy, your home will get closer to reality as the amount of all of your required building materials increase together.

Until, of course, your home is complete.

When you’re finished, you’re finished. Extra wood and extra nails won’t help anymore.

Here, too, the similarities continue with your protein intake. MPS continues to increase by adding more leucine to your diet as long as it comes along with a balanced, quality protein of all essential amino acids. But once a threshold is reached, it won’t continue to provide additional MPS benefits forever.

This critical review states that the leucine threshold for maximum MPS is roughly 1.8 – 2.0 grams.

MPS continues to increase in a dose-dependent manner upon increasing leucine until this threshold. Consuming more has not shown to provide further gains.(3) In the words of the study’s authors, activation of mTORC1 through leucine appears to act more like an “on/off switch” as opposed to a “dimmer switch.” However, if protein quality is low, leucine supplementation can amplify mTORC1 activation.

Optimization Through Personalization

In addition to other interesting information on protein dosing and timing that is worth reading in this review, one final interesting note pertains to the individualized way in which we respond to protein intake and MPS.

Their review highlights a large variability in individual response to protein and amino acid quality on lean mass growth and fat free mass growth. Some individuals respond extremely well to adding protein to their diet for stimulating MPS, while others appear to be “non-responders.”

Another interesting area of ongoing research is the desensitization of your muscle tissue to amino acid flux over time. For those all-too-familiar with plateaus and stalled progress, understanding your body’s response to repeated exercises may help direct your future workout plans.

For those looking to maximize muscle growth, its important to measure your progress. As the old saying goes, “you can’t improve what you don’t measure.” So track your growth, track your strength, track your protein intake, and analyze your findings to optimize your plan to meet your goals.

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  1. Reidy, P. T., & Rasmussen, B. B. (2016). Role of Ingested Amino Acids and Protein in the Promotion of Resistance Exercise–Induced Muscle Protein Anabolism. The Journal of Nutrition, 146(2), 155–183. http://doi.org/10.3945/jn.114.203208.
  2. Glynn, E. L., Fry, C. S., Timmerman, K. L., Drummond, M. J., Volpi, E., & Rasmussen, B. B. (2013). Addition of Carbohydrate or Alanine to an Essential Amino Acid Mixture Does Not Enhance Human Skeletal Muscle Protein Anabolism. The Journal of Nutrition, 143(3), 307–314. http://doi.org/10.3945/jn.112.168203.
  3. Glynn, E. L., Fry, C. S., Drummond, M. J., Timmerman, K. L., Dhanani, S., Volpi, E., & Rasmussen, B. B. (2010). Excess Leucine Intake Enhances Muscle Anabolic Signaling but Not Net Protein Anabolism in Young Men and Women. The Journal of Nutrition, 140(11), 1970–1976. http://doi.org/10.3945/jn.110.127647.