Leucine Threshold for Building Muscle

Are you taking a whey protein? Are you eating at least 30g protein per meal? Is every protein a complete protein?  Are you even hitting your leucine threshold?!

That's a lot to worry about for the average person looking to improve their physique and most likely information overload for an athlete who just needs you to tell them what to eat.  So why are so many people obsessed with details that don't matter as much as the "big picture" things?  

If you don't understand the physiology and someone throws some great buzz words and maybe some cool marketing at you, it's hard to sift through the haystack for that elusive needle.

The leucine threshold is one topic in particular I want to cover because there has been considerable misunderstanding and misapplication using it as a supplement and it ties some great muscle building and protein-related concepts together.  If you stick with me, you'll come out of this with a LOT more knowledge and some great practical tips.

Protein Synthesis

Protein synthesis is the building up or accrual of new proteins aka building muscle. 

Cool idea bro, just smash some steak and lets get jacked!  However, one major misconception with protein is that what you eat goes right to the muscle tissue.  For the protein you eat, about 50% of those amino acids (the simpler protein form) are absorbed by the gut. Then only about 20% of the remaining circulating amino acids actually make it into the muscle tissue for protein synthesis.

So if you ingested 30g of protein, 15g of that is absorbed into the intestines and then only 20% (or 3g) makes new muscle tissue aka protein synthesis.  We're looking at a whopping 10% of total protein actually going to the muscle building process.  

Protein Turnover

Nope, it's not a new high-protein dessert, it's the process of breaking down and building up new proteins.  Like fat loss, protein synthesis is not a linear process.  In times where protein is low, like between meals and while you are sleeping, protein synthesis is muscle tissue is actually lower.  This is because the available amino acid pool is low (because you haven't eaten) so your organs like the liver and heart get preferential pull off of the available amino acids.  Why?  Because you'd die if you weren't creating new proteins in your internal organs.

Protein synthesis is never simply on or off.  It is highly impacted by food and available amino acids from meals but as noted above, during times where there is not much food in the system, the rates of protein turnover in various tissues changes.  Organs get priority during fasting and the muscles (which account for 50% of protein needs in the body) will pull a lot more from that pool after being fed.  

You can think of this process as similar to fat loss, simply in the reverse way.  For instance, you are never purely storing fat or purely burning it.  After a meal, you are storing some of those calories as fat so between meals or at night (aka fasting) you can free up body fat to use as energy.  Eating a meal stops fat loss, not eating accelerates it.  The TOTAL result comes from how the balance is pulled in one direction.  Even though eating a meal might cause some fat storage, if total calories eaten every day are less than what you burn, you will push fat loss as the primary end result.  

Protein turnover works in a similarly impacted fashion.  Eating a meal causes an uptick in protein synthesis and down regulation in protein breakdown.  Fasting between meals and at night causes an uptick in protein breakdown and decrease in protein synthesis.  However, the TOTAL protein eaten every day is what determines if you push the entire system in muscle gain or muscle loss territory.

The final note here is that protein synthesis in muscle tissue is the most impacted by meals.  Since staying alive is your #1 goal, the protein synthesis rates in organs is not nearly as affected by fasting as muscle tissue is.  While sleeping you might see organ protein synthesis run at 70%  but muscle protein synthesis could be as low as 20-30%.

MTor and the importance of protein in meals

While much of your body tissue is highly sensitive to total calories in terms of repair, muscle tissue is impacted on a greater scale by total protein.

This is controlled in large part by the enzyme call mTor. If you aren't sure why this is important, pay attention: mTor is essentially the master switch of protein synthesis at the initiation phase.  Think if turning the key in your car to start the engine.  You can have a full tank of gas, fresh oil, great tires and a big engine but none of that will run unless you have something to turn it all on.  That is how you can think of mTor: the "on" switch for muscle building.

mTor is sensitive to a few other things, lets see if any of these ring the "muscle building" bell:

-Strength training and exercise


-Total Calories


I'd say that bell would be dinging!  We can initiate protein synthesis through exercise, spiking insulin, eating enough food and of course leucine; the topic of this post.

It should come as no surprise that you won't build much muscle if you don't exercise, eat very little total food and eat very little protein.  Most people can wrap their heads around that. However, what is it specifically about leucine that is so important for building muscle?

Leucine's Role

When you eat protein, leucine is largely free to enter the bloodstream and go straight to the muscle; this is how your muscles sense the ingestion of a meal and whether or not they can begin protein synthesis to create more tissue (build muscle).

According to Dr. Layman, a protein researcher in the  Department of Food Science & Human Nutrition at the University of Illinois, it takes about 2.5g (two and a half grams) leucine to initiate protein synthesis. On the whole, this isn't much at all.

I did some digging around and found some research fromthe  Department oj Biochemietty, University of Chicago conducted in 1951 that determined the amino acid composition of various cuts of beef.  On average, beef was a tad over 8% leucine by total protein.  So if you had a a 5oz steak for example with 40g protein, the leucine content for that meal is about 3.2g.  It meets the leucine threshold but not to the whopping extent most people would assume. 

This also sheds some light on why eating enough protein in a meal is important. even you cut that portion down to 3oz, you'd be getting about 1.92g leucine which does not meet the threshold for protein synthesis.  In the role of body composition, muscle gain and strength accrual, it becomes exceedingly clear why adequate protein, including leucine, is so important.

Leucine as you age

When you are younger and developing, we are not nearly as sensitive to leucine as we are as adults.  That is because we rely much more on insulin to stimulate growth of tissue.  Physiologically speaking this is a much smarter and fool-proof method of achieving growth because a developing child doesn't need to rely on protein as the sole stimulator of growing new tissue and putting on muscle, they simply need enough total calories.  Since insulin is spiked by protein and carbohydrates alike, a growing child can develop and create new tissue from eating a wider variety of food sources. Ever wonder why a teenage boy can eat a seemingly endless supply of carbs and still be lean and grow muscle?  He can thank his increased mTor insulin sensitivity for turning on the muscle building process, even when protein is low.

When you are an adult, mTor is primarily sensitive to leucine.  We circle back around now to the average person needing to hit adequate protein and the leucine threshold.  You simply can't build muscle off of eating enough food and spiking insulin, that 2.5g leucine threshold needs to be met.

As you age even further, we are not as sensitive to insulin and rely even more on adequate protein and leucine to get these effects.  So for middle aged and older folks, you HAVE to get your protein in.

Additional Notes on Leucine and Protein

-Body size does not impact the leucine threshold in adults very much.  Leucine threshold is not as related to lean body mass as it is to blood volume.  Since blood volume doesn't change very much even with big weight disparities, the leucine threshold doesn't change much either.  Blood leucine needs to about triple to switch mTor - so for large and small adults alike, the 2.5g to 3g leucine threshold is perfectly fine.

-Simply spiking leucine and flipping the mTor switch alone does NOT mean you are going to build muscle.  There are plenty of products out there like BCAAs that contain a few amino acids with leucine as a primary ingredient.  If all essential amino acids are not available, flipping the switch doesn't do anything. You need enough total protein to actually now give the muscles something to turn into new tissue.  Remember that mTor flips the switch, but you still need gas in the tank to get somewhere.

-Increasing leucine in a meal doesn't turn muscle building on more.  Supplementing leucine in a meal that already contains an adequate amount is simple wasted money and leucine. Increasing leucine artificially above the other essential amino acids will even DROP the blood levels of other amino acids, limiting your ultimate muscle building potential.   For all those people adding leucine to a meal or spiking their whey with it is both wastful and counter-productive.

-Protein quality does count for hitting that leucine threshold.  Consider above that I noted beef was about 8% leucine.  In contrast, a whey protein is around 11% leucine, which means you can meet that threshold with less total protein. A food like peanut butter is not only is low in protein for it's total calories but to meet the leucine threshold you'd need about 10 tablespoons which also happens to be about 1,000 calories.  So choose wisely!

The above is why I so often recommend pea protein to vegetarians and vegans to help meet protein needs.  A quality pea protein powder is about 9% leucine so it is quite effective at not only meeting the threshold but helping boost the overall protein intake.

-Once the leucine threshold is met, you want to make sure your total protein needs are met.  Don't just eat the minimum to break the threshold.  Protein needs are based off of lean tissue, exercise and activity status and goals.  For strength training individuals, eating between 1.8 to 2.2 grams per kilogram (or 0.8 to 1 gram per pound) is ideal.  Eating high quality and complete proteins that meet these windows will also meet the leucine threshold and cover all bases!

-While total daily protein is the most important in terms of losing or gaining muscle, the meals that book-end your day are also quite important.  If you have slept 8 hours you've been fasting for at least that long.  When you wake up, protein synthesis is at an extreme low and a high-protein meal rich in leucine helps stimulate mTor and reverse the catabolic process of fasting.  Likewise, before bed, a complete and slow-digesting protein can provide amino acids to help reduce catabolism (or muscle breakdown) while you sleep.  Optimizing both of those meals can help push muscle growth even further and ensure you are recovering optimally.

Leucine wrap up

Congrats, you now know more about leucine than most people!

It's important to understand how meeting that meucine threshold impacts your protein synthesis and ability to grow new muscle.  Falling for the traps of ingesting extra isolated amino acids and spiking meals and shakes with leucine are a waste of time.  Eating inadequate or poor quality proteins is also hurting process.  I love peanut butter but a smear on some toast isn't going to stimulate muscle growth.  You need somwhere around 30g on average to meet the leucine threshold and provide enough total amino acids for your muscle to have something to work with.

Breakfast and dinner are both important, they book-end your day and can help maximize your results.

As you age, the leucine threshold and total protein are vitally important.  I know wine and chips sound SO GOOD at the end of a long day but if you have body composition and strength goals, your protein intake must be addressed.

Just to be clear (since someone always asks!), eating more than 30g protein won't make you fat, it won't hurt your kidneys and you won't simply pee it out.  More protein can help increase recovery and protein synthesis and a larger meal will take longer to digest, which means the amino acids will trickly into your bloodstream over a longer period of time.  Don't feel bad about having an adult-sized protein portion, 2oz of chicken is for weenies!

All this simply strengthens the argument for eating optimal protein, and of course, the argument for eating steak!