The Ultimate Guide To Leucine Supplementation

Leucine Molecule

When it comes to building muscle, no amino acid is more important than Leucine.

It’s one of the three Branched Chain Amino Acids (BCAAs)–the other two being Isoleucine and Valine–which all together comprise just over 30% of total muscle protein.

Unlike the other two BCAAs, however, Leucine directly stimulates muscle protein synthesis.

Naturally, this has made it the central focus of most of the research on BCAAs over the years.  As it turns out, Leucine is actaully the only BCAA that the average person is likely to derive much benefit from.  The other two are kind of lackluster and don’t really seem to do much of anything.

Leucine, on the other hand, can be extremely useful given the right set of circumstances.

Isoleucine and Valine tend to get included by default but pretty much all the benefit of BCAA supplements is attributed to Leucine alone.

There’s very little scientific evidence (actually, there’s none) that indicates that the other BCAAs do much of anything.

This lack of scientific evidence, despite the widespread popularity of BCAAs, has lead to quite a bit of controversy.

You see, over the years, BCAA supplements have become mainstream and, like most mainstream supplements, this has brought out the critics.

The argument against BCAAs is typically premised on one or more of the following:

  • The benefits of BCAAs are over-blown
  • Some of the studies are industry-influenced (not very valid)
  • You don’t need BCAAs if you eat enough protein

To be clear, these are all true statements.  The benefits of BCAAs have been exaggerated and over-blown by the mainstream supplement industry.

Some of the studies are most likely industry influenced because the benefits are too extreme in those studies.  And, of course, you definitely don’t NEED to take BCAA supplements if you eat enough protein.

Go ahead and throw that BCAA supplement out the window…But pickup some Leucine because you could most likely benefit from it…

Why then, has it been unfairly grouped in with Isoleucine and Valine and deemed “ineffective” by so many?  Who knows!

I completely agree that BCAA supplements are overrated and don’t deserve anywhere near the amount of attention and dollars that people give them…But Leucine is awesome, and I’ll explain why a little later.

Let’s just take it from the top…

What Is Leucine?

Leucine is a one of nine essential amino acids–meaning the body can’t synthesize it and it must therefore come from diet–and one of the three Branched Chain Amino Acids (BCAAs).

It’s chemical structure looks like this:

leucine chemical structure

It is widely considered to be the most important BCAA because it’s the only one that has any sort of significant impact on muscle protein synthesis which is, after all, the reason most people use BCAA supplements.

In addition to being used as a building block of protein, Leucine directly stimulates muscle protein synthesis through activation of Mechanistic Target Of Rapamycin, otherwise known as mTOR.

mTOR is a type of signaling molecule called a “protein kinase” which basically just means it attaches to other proteins and changes their function by adding a phosphate group to their chemical structure.  This process is called Phosphorylation, and it’s required for a ton of different cellular processes.

Stimulating muscle protein synthesis is just one of those processes.

mTOR serves as the key component in two protein complexes:

  1. mTOR complex 1 (mTORc1)
  2. mTOR complex 2 (mTORc2)

Each of these complexes is an important regulator of various important processes, but mTORc1 is the one that controls cell growth and proliferation.

the effects of mTOR

So, mTORc1 is responsible for all things anabolic.  It directly controls processes such as:

  • Muscle Protein Sythesis
  • Fat Storage
  • Mitochondrial Biogenesis

mTORc2, on the other hand, deals mostly with matters of cell survival, and not so much muscle growth.

So, for all intents and purposes, mTORc1 is what we’re referring to when we use the term “mTOR” in the context of building muscle.

Okay, now that we have a solid understanding of what mTOR is, back to Leucine…

how leucine triggers muscle growth

The diagram above shows how Leucine stimulates mTOR which in turn stimulates muscle protein synthesis via a few different enzymes.  You don’t need to sit there and memorize the names of the various signaling molecules and enzymes involved in this process.  You just need to know one thing.

Leucine stimulates mTOR which tells the body to build muscle.

Now, one important piece that’s often left out when supplement companies try to sell you on Leucine is that mTOR also functions as a nutrient sensor and won’t trigger muscle growth unless the correct nutrients (i.e. amino acids) are present in the cell.

In other words, Leucine may signal the body to build muscle, but it can’t be done without the necessary building blocks of protein, amino acids.  This is why Leucine alone is not a substitute for a poor diet.

It can help preserve muscle mass in instances of low protein intake (more on that later), but Leucine alone will not help you build muscle if you’re not getting enough protein through your diet.

Getting Leucine Through Your Diet

Leucine is found in a variety of foods.  Some are considered complete sources of protein, providing all nine essential amino acids.  Others are considered incomplete sources of protein, providing less than all nine essential amino acids.

Foods that rich in Leucine, whether complete or incomplete sources of protein, include:

  • Eggs
  • Soy Beans
  • Beef
  • Pork
  • Chicken
  • Tuna
  • Cheese (Parmasean)
  • Pumpkin Seeds
  • Shrimp
  • White Beans

You may notice that most of these foods are meats.  If you’re a vegetarian or vegan, you need to pay special attention to your Leucine intake.

Unless you’ve specifically designed you diet to provide a lot of Leucine from the few non-meat foods that contain a lot of Leucine, you’re probably not getting enough to support your muscle-building goals.

The Recommended Daily Intake (RDI) for Leucine is 14mg/kg/day which roughly translates to a 175 lb person (79kg) consuming just under 1200mg of Leucine per day.

Recently, this recommendation has come under question, as some experts feel the RDI for Leucine should be closer to 50mg/kg/day.  That would translate to a 175 lb person consuming roughly 4,000mg of Leucine per day.  And that’s just for sedentary individuals.

Those who exercise regularly should be getting a lot more Leucine than that!  Of course, getting all that Leucine from your diet could be extremely challenging.  Unless you eat a lot of meat, you could probably benefit from some extra Leucine.

“But wait, I already get enough protein…”

As we’ve established, one of the main criticisms of Leucine supplementation it that you don’t need it if you’re getting enough protein already through your diet.

Although we’ve already established that many people probably aren’t getting enough Leucine to support their fitness goals, there is some evidence which indicates adding some Leucine on top of your protein (which already contains some Leucine) can actually enhance the muscle-building effects.

In 2005, an interesting study out of the Department of Human Biology at Maastricht University (Nethlerlands) was conducted to determine the muscle-building impact of Whey Protein and Leucine.

Subjects were given either:

  1. Carbohydrates
  2. Carbohydrates + Protein
  3. Carbohydrates + Protein + Leucine

The graph below shows the calculated Fractional Synthetic Rate (FSR) for each group.  Put simply, FSR is a measure of protein synthesis.

leucine muscle protein fractional synthesis rate

Carbs and Protein resulted in more muscle protein synthesis than Carbs alone, but Carbs, Protein, and Leucine resulted in the most.

The main takeaway from this study is that Leucine can enhance muscle protein synthesis beyond protein alone, even if that protein already contains some Leucine.

This particular study kind kind of pokes a gaping hole in the whole “I don’t need Leucine, I get it from my protein”.  Apparently, a little more can actually help!

The Case For Leucine Supplementation

Like I said earlier, the ever increasing popularity of BCAA and Leucine supplements has brought with it a fair amount of skepticism, some based in fact and some that just seems like a general attack on the supplement industry.

A few of examples of the claims you generally see attached to Leucine are:

  • Muscle-Building
  • Physical Performance Enhancement
  • Mental Fatigue Reduction
  • Muscle Preservation

Out of all of these, only the last one–Leucine Preserves Muscle Mass–is really accurate.

Although usually labeled as a “muscle-builder”, Leucine actually doesn’t just build muscle.

If that were the case, you could just sit there eating Leucine by the spoonful all day and you’d get more and more muscular.  Sadly, it doesn’t work like that.

Leucine will only stimulate muscle protein synthesis up to a point.  That point depends on a lot of different factors, but the most important ones are:

  1. How much dietary Leucine (in the form of protein) you’re taking in
  2. How physically active you are

If you’re hitting the gym on a consistent basis (and pushing yourself when you’re in there), you should aim for 100mg/kg/day.  That translates to a 175lb person taking in around 8g of Leucine each day.

Given that the Leucine content of dietary protein is tyipcally around 5% or so, that same 175 lb person would need to consume 160g of protein daily.

For some people, this is entirely realistic.  For others, it’s pretty tough.  It probably has more to do with your schedule and how much you like to eat in one sitting than anything else.

If you have time to sit around and eat all day, then you can easily get all the Leucine you need from your diet.

If you’re like most people, however, and sometimes one thing leads to another and all the sudden it’s night time and you realize you hardly ate…then you might want to consider supplementing with Leucine.

What Are The Benefits Of Leucine

The benefits that Leucine supplements has to offer mostly have to do with supporting the growth of muscle and preserving muscle mass during times where protein intake may be compromised.

But let’s just take it from the top…


Leucine And Muscle Building

It’s well established that Leucine induces muscle protein synthesis via activation of mTOR (a signaling molecule), but it’s not the ‘muscle-builder’ it’s often made out to be.

Leucine supplementation has been shown to improve muscle protein synthesis in older adults who weren’t consuming enough dietary protein as well as older adults who consume the RDA.  This makes it useful for combating age-related declines in muscle mass.

Unfortunately, there’s not much evidence that it can increase the rate at which you build muscle if you’re already getting enough protein.

One study found no significant difference in muscle mass between subjects supplementing with Leucine (10g/day for 12 weeks) and subjects who didn’t supplement with Leucine, but did the same workouts.

A similar failure was found in another study as well.

If we take a step back and look at the research as a whole, it’s pretty clear that Leucine can only increase muscle mass in people who eat a low-ish protein diet.

Leucine And Exercise Performance

Supplement companies have touted BCAA supplements as performance enhancers basically since the dawn of BCAA supplements.  Because it’s the most important of the BCAAs, Leucine has also been alleged to improve exercise performance.

The story goes something like this:

Leucine (and the other BCAAs) block Tryptophan uptake in the brain because they compete for the same transporter.  This reduction in Tryptophan results in a subsequent reduction of Serotonin, because Tryptophan is a precursor to Serotonin.

It’s an interesting hypothesis, but the research is less than encouraging.

On the one hand, Leucine supplementation has outright failed to improve exercise performance in at least one study.  Another study, however, found that 4g/day increased strength over 12 weeks (measured by 5-rep max).

Based on these two studies, it would appear that Leucine doesn’t reliably improve performance, but may encourage better strength gains over longer periods of time.

This is pretty consistent with what we already know to be true about Leucine.  It supports optimal muscle growth.

Leucine For Fasted Cardio

If you do fasted cardio or any type of fasted training, you should be aware that muscle breakdown occurs at a much higher rate when you train fasted than when you train fed.

Of course, you’re probably doing it for the fat-loss benefits, so you may not even be thinking about muscle preservation.  But let me just be very clear…

Muscle preservation should ALWAYS be the goal.

If you want to lose fat, then you want to build muscle.  Why allow your hard earned gains to become collateral damage in the fight against fat?  Especially when all you really need is Leucine.

Leucine can help preserve muscle without massively spiking Insulin levels and negating the fat-loss benefits of a fasted state.  So basically, Leucine supplementation makes Fasted Cardio actually worth it!

If that’s not a super legitimate use for Leucine, I don’t know what is.

Leucine While Cutting

The same reason why Leucine is particularly useful when training fasted is also the reason why it’s super useful when cutting.  Leucine preserves muscle in instances when muscle would normally be broken down at an accelerated rate.  Calorie restriction is definitely a muscle destroyer.

Leucine supplementation has been shown to preserve muscle in instances of protein restriction, when muscle loss would normally occur.

Obviously, the severity of the muscle loss depends on how much you’re cutting your calories by, but if you place yourself in a deficit for long enough, you’re sure to lose SOME muscle.

Leucine can help minimize the muscle breakdown.  Considering having some with your lowest protein meals or before and after training throughout your cut, and see if you don’t hold onto a little more muscle than you thought you would.

Leucine Dosage

Given that we all take in Leucine in varying amounts through our diet, finding your ideal dose isn’t quite as simple as copying the dose used in studies.  With most supplements, there’s what we call a “clinical dose”.

The clinical dose of any supplement is simply the dose at which it has been proven effective in scientific studies.

In the case of Leucine, this dose is much more of a range.  Depending on your dietary needs, that range is probably somewhere between 3 and 5 grams, once or twice a day.

For example, if you’re dieting and your concerned about losing muscle, consider taking 5g of Leucine per day.  If you’re supplementing with Leucine specifically for the purposes of Fasted Training, take 5g prior to your workout and throw another 5g or so in with your protein after your workout.

Of course, you don’t have to take Leucine in 5g increments.  It just seems convenient.

Does Leucine Have Any Side Effects?

No one has ever reported any sort of adverse effects from ingesting a Leucine supplement, even at high doses.

Of course, this makes perfect sense because Leucine is found in tons of the foods we eat on a regular basis.  Supplementation is simply a way to get a little more in there.

Assuming the Leucine you’re using is from a reliable, trustworthy company, it’s highly unlikely that you would experience any sort of side effects.

The Bottom Line On Leucine

It’s absolutely true that the benefits of BCAA supplements have been over-hyped by mainstream supplement companies, but Leucine can actually be quite useful given the right set of circumstances.

  • Fasted Training
  • Preserving Muscle Mass While Cutting
  • Not Getting Enough Dietary Protein
  • Amplifying The Muscle-Building Effect Of Protein

If any of those describe your situation, Leucine is worth having in your arsenal.

I’m Matt Theis, founder of SuppWithThat, Momentum Nutrition, and Singular Sport. I created SWT to separate the science from the hype and publish accurate, research-based information on supplements. If you like what I have to say here, feel free to check out my supplements at and

Click to comment
To Top