Leucine: Uses, Benefits, Side Effects, And Dosage

As educators in the sports nutrition space, we talk about protein A LOT.

Those of you who have been reading along and taking a personal interest in your recovery know that protein is a very important dietary component to keep the legs churning strong day after day.

We cannot, and often do not, talk about protein without also talking about the amino acid, leucine. Protein and leucine are very intimately connected. So, what’s the big deal? Why is leucine so important? Well, we’re glad you asked!

Leucine And Muscle Protein Balance

The first thing we must understand is how muscle responds to exercise. All muscle tissues are composed of proteins. Most famous among them are the contractile proteins, actin and myosin.

These two proteins work together to shorten muscles – what you observe as a muscle contraction.

Muscle contractions are not just for the weight lifters, as they occur during any and all self-engaged movements of the human body, including running, cycling, swimming, hurling yourself over a wall, or typing on a keyboard.

As athletes, we’re sure you’re all well aware of post exercise muscle soreness (also known as delayed onset muscle soreness, or DOMS), which is due, in part, to exercise-induced muscle damage (or EIMD).

“Exercise-induced” simply means that exercise caused the muscle damage.

In other words, muscle tissue proteins, like actin and myosin, have been broken down – a process appropriately named muscle protein breakdown.”

Muscle protein breakdown (MPB) can be a cause of both a physical trauma, such as exercise, or the body’s self-regulatory processes, such as during fasting or starvation.

Muscle Protein Breakdown During Fasting

Let’s talk about fasting and starvation a little bit. To your conscious mind, fasting just means you want to eat all the food, but to your body, it is simply a period of low energy availability.

To make up for the lack of energy being consumed as food, the body begins to breakdown bodily tissues, mostly fat tissue, but muscle proteins as well. Unlike when you are at rest and you only feel like you have been food deprived after several hours, the body senses a lack of energy shortly after exercise begins and increases muscle protein breakdown.

Shortly after initiating exercise and for a brief period after a brief duration of exercise, muscle protein synthesis (MPS) is elevated as well (particularly with resistance training), in a balancing act with muscle protein breakdown.

Muscle protein synthesis is pretty much what it sounds like, the opposite of breakdown or the repair of muscle proteins. It is the process by which we restore muscle tissue back to its former glory.

The reason you often feel sore after training is because processes like muscle protein breakdown are more active and often outweigh the total muscle protein synthetic response during exercise and the hours thereafter.

During the rest and recover phases of training (e.g., the 20 something hours or more between training sessions), rates of muscle protein breakdown are greatly reduced, and muscle protein synthesis has a chance to catch up.

After light sessions, this may be just a few hours or a day (e.g., “my legs feel a little tender or tight”), and after tough sessions this may be upwards of 3 days (e.g., “I literally cannot get myself out of this chair!”).

For bodybuilders or powerlifters, they can spare a few days to recover while they just train a different body part or a different lift.

Endurance athletes, however, need their legs day in and day out, so in many ways it is more important for endurance athletes to focus on enhancing muscle protein synthesis following exercise and tip the scales in favor of their recovery.

Leucine for Exercise Recovery

One of the signals for increasing muscle protein synthesis and decreasing muscle protein breakdown is to increase the amount of available protein in the body via the diet.

All proteins are made from little nitrogen-carriers called amino acids. When the amino acids are not a part of body proteins, they form the free amino acid pool.

While the amino acid pool is rich with amino acids, the body senses that it does not need to break down muscle proteins, because enough amino acids are already free and ready to go.

Conversely, when the free amino acid pool is low, the body thinks it needs to add amino acids by breaking down existing proteins, such as muscle proteins.

By consuming protein in the diet, we increase the free amino acid pool, and stave off muscle protein breakdown. Protein also provides the amino acid building blocks necessary for muscle protein repair.

One of these amino acids is not just a structural component of protein, but it is a signal to initiate muscle protein synthesis.

What Does L-Leucine Do?

The rate limiting step to muscle protein synthesis is called “translation initiation.” If you remember learning about transcription and translation of DNA in your latest biology class, great! You may be able to imagine what translation initiation is. If not, no worries.

We’re not going to go into the nitty gritty details of it. Just know that leucine is able to increase translation initiation, the most important step of muscle protein synthesis.

Well what’s the difference between increasing leucine and increasing the free amino acid pool? The research is pretty convincing that leucine represents the majority of the rebuilding capacity of dietary protein.

In a series of experiments by Dr. Garlick and colleagues at the University of Illinois, a number of conclusions about protein and the role of amino acids in muscle protein synthesis were established and confirmed (1).

1. First, protein increases muscle protein synthesis.
2. Second, the essential amino acids (of which there are 9, including leucine) increase muscle protein synthesis.
3. Third, the non-essential amino acids do NOT increase muscle protein synthesis.
4. Fourth, 3 of the essential amino acids known as the branched chain amino acids which also includes leucine increase muscle protein synthesis.
5. Fifth, 2 of the branched chain amino acids (isoleucine and valine) do NOT increase muscle protein synthesis.
6. And sixth, leucine increases muscle protein synthesis.

This paints the picture that 19 of the amino acids in protein are used as building blocks, while just 1, leucine, signals the increase in muscle protein synthesis.

Leucine Dosage

The next questions that need an answer are how much leucine is optimal for stimulating muscle protein synthesis in humans, and how much of that leucine are we obtaining in different protein sources that we might consume after exercise already.

Optimal amounts of leucine have been suggested to be 0.05 grams of leucine per kilogram of bodyweight per meal (2).

“Per meal” is important to note here because the increase in muscle protein synthesis after consuming a source of leucine lasts for only about 3 hours. This is one of the reasons why you may hear suggestions to do something like eat 6 meals a day to promote recovery.

By stimulating muscle protein synthesis with leucine every 3 hours, you are maximizing the time spent repairing muscle proteins. However, doing so just 3-4 times per day is sufficient for most athletes.

Athletes with a high training load may be interested in increasing their meal frequency both to keep muscles repaired and because it gets hard to meet the energy needs of the training while eating only 3 meals per day. But that’s another topic for another day!

Again, leucine optimally increases muscle protein synthesis at a dose of 0.05 grams per kilogram body weight per meal, and that translates to about 2.5 grams for a 50 kg athlete and increases by 0.5 grams for every 10 kg increase in body weight, so a well-trained female marathoner may need 2.5 grams of leucine while a trained male cyclist may need 3.75 grams of leucine at 75 kg body weight.

What Foods Contain High Amounts of Leucine?

Okay, so now that we know how much leucine we need. How much can we find in some common foods or sources of protein? Check out this table!

Food/Protein Source	Percent Protein as Leucine	Grams Leucine per Serving*	Number of Servings* to Meet Needs of a 50 kg Athlete
Beef (85% ground)	7.8%	1.2	2.1 (6.3 oz)
Casein Supplement	9.2%	2.3	1.1 (27.2 g)
Chicken (breast)	7.5%	2	1.25 (3.75 oz)
Egg (Whole - large)	8.6%	0.54	4.6 (4.6 eggs)
Fish (Salmon - wild)	8.1%	1.8	1.4 (4.2 oz)
Milk (2%)	9.5%	0.77	3.2 (3.2 cups)
Pork (loin)	8.5%	1.9	1.3 (3.9 oz)
Soy Supplement	8.4%	2.1	1.2 (29.8 g)
Whey Supplement	10.5%	2.6	1.0 (24.1 g)

*For meats, one serving = 3 oz. For supplements, one serving = 25g (~1 scoop). For egg, one serving = 1 whole egg. For milk, one serving = 1 cup.

When To Take L-Leucine

Now that you know all about leucine’s role in recovery and how much of some common protein sources to eat in order to get necessary amounts of leucine, let’s talk about the importance of the post exercise period.

After exercise, the muscles are primed to absorb nutrients like carbohydrates and leucine.

By exploiting the muscles willingness to suck up nutrients, we can potentiate recovery. Leucine also causes a release of insulin, which speeds carbohydrate absorption even further.

Adding leucine-rice protein, like whey to carbohydrate in a post workout shake increases rates of muscle glycogen resynthesis more than an isocaloric amount of carbohydrates alone, confirming the practicality of consuming protein and carbohydrate together after exercise.

The optimal ratio of post exercise carbohydrate and protein is 4:1. However, even when using a high-leucine protein like whey, this ratio either suggests too much carbohydrate or not enough leucine.

We have 2 options:

1. We can deviate from what has been found to be the “golden ratio” of 4:1 or
2. We can find a way to get more leucine, while maintaining the 4:1 ratio.

What happens if we just say, “Screw it. I’m getting some protein, I don’t need that extra leucine nonsense!” I’ll tell you what happens, a whole lotta nothin’!

Comparing doses of 0, 10, 20, and 40 grams of whey protein (approximately 0, 1, 2, and 4 grams of leucine) consumed post exercise, researchers found that 20 and 40 grams were good enough to increase muscle protein synthesis, but 0 and 10 grams were not different from one another.

Conversely, adding leucine to just 6 grams of whey protein increases muscle protein synthesis to the same degree as a full 25 gram serving of whey (4)!

Don’t say screw it! Have your cake, and eat it too with comprehensive recovery nutrition featuring all the carbs, necessary protein, and importantly, enough leucine!

References:

1. Garlick, P. J. (2005). The role of leucine in the regulation of protein metabolism. The Journal of nutrition, 135(6), 1553S-1556S.
2. Norton, L., & Wilson, G. J. (2009). Optimal protein intake to maximize muscle protein synthesis. AgroFood industry hi-tech, 20, 54-57.
3. Witard, O. C., Jackman, S. R., Breen, L., Smith, K., Selby, A., & Tipton, K. D. (2013). Myofibrillar muscle protein synthesis rates subsequent to a meal in response to increasing doses of whey protein at rest and after resistance exercise–. The American journal of clinical nutrition, 99(1), 86-95.
4. Churchward‐Venne, T. A., Burd, N. A., Mitchell, C. J., West, D. W., Philp, A., Marcotte, G. R., ... & Phillips, S. M. (2012). Supplementation of a suboptimal protein dose with leucine or essential amino acids: effects on myofibrillar protein synthesis at rest and following resistance exercise in men. The Journal of physiology, 590(11), 2751-2765.