The amino acids responsible for triggering muscle growth are the branched-chain amino acids (BCAAs): leucine, isoleucine and valine. Nonetheless, one of these is the main driver of muscle protein synthesis: leucine. Leucine is the golden key of MPS and should be a main focus in your nutrition.
Research conducted by Dr. Layne Norton has shown that in order to stimulate muscle protein synthesis, you need to consume between 2-3g of leucine per meal.  The interesting part is that muscle protein synthesis does not have a linear relationship with leucine intake. In other words, greater leucine consumption does not directly correlate with increased muscle protein synthesis.
There is a threshold point; a minimum amount of leucine that you need to consume. If you do not hit it, you will not stimulate muscle protein synthesis, but once you hit it, the effects will be maximized very quickly. For the majority of people, MPS is stimulated with 2-3g of leucine.
The next question on your mind might be, "How much protein do I need to get these 2-3g of leucine?" Once again, the answer is a little more complicated than that. Not all proteins are created with equal concentrations of amino acids. Some contain more leucine than others.
Whey protein, with a 12% leucine content, has the highest leucine concentration. Below, you will find a table that compares several protein sources based on leucine content, the amount of protein from each source you need to consume to obtain 3 grams of leucine and the calories in that serving. It is important to note that 3 grams is the upper limit. Most people will stimulate muscle protein synthesis with 2.5 grams of leucine.
|Protein Source||Leucine % (w/w)||Protein needed to obtain 3g Leucine (g)||Serving to obtain 3g Leucine (g)||Calories per 3g Leucine (g)|
|Whey||12||23||30 (Depending on your whey protein supplement)||110|
As noted in the table above, the protein source can be just as important as the amount of protein consumed. For example, if you need 3 grams of leucine to stimulate muscle protein synthesis this can achieved this by consuming 25g of protein from whey protein. However, if you consume 280g of cooked lentils, you will obtain the same 25g of protein but only obtain 1.8g of leucine and will likely not stimulate muscle protein synthesis.
The source of protein is essential to consider if you are on a diet, and want to maximize the anabolic response to each meal while keeping calories low. Tofu protein has a higher leucine content than chicken; you need 34grams of tofu protein to obtain 3g leucine and stimulate MPS.
However, in order to obtain 34g of protein from tofu you will consume 340 calories, 20g of fat and 10g of carbs. You are far better off consuming 185 calories from a 135g serving of chicken (which will also give you 3 grams of leucine).
In summary, you should plan to obtain 2-3 grams of leucine per meal so that you can hit the leucine threshold and stimulate muscle protein synthesis. In order to obtain this amount of leucine, you will need between 25-40 grams of protein, but this is highly dependent on the protein source.