Leucine is an essential amino acid and further classified as a branched-chain amino acid (BCAA).
While all amino acids serve many important functions in the body, leucine uniquely contributes to muscle growth.
Before you hit up Amazon for a 5-star rated leucine supplement, let’s review how leucine fits into the amino acid family, how much you need, where to get it and what it can do for you.
Amino Acids 101
Amino acids are the building blocks of protein. There are 20 amino acids in total and they are further classified into subcategories based on whether or not they need to be obtained through food and their function (*).
Non-essential: can be synthesized by the body under normal circumstances.
Conditionally Essential: subclass of non-essential amino acids, normally non-essential but during times of illness or stress, become essential.
Essential: cannot be synthesized in the body, must be obtained through food.
Branch-chained: subclass of essential amino acids used preferentially by muscles for fuel. They are named for their chemical structure.
Amino Acids by Subtype
Leucine Requirements and Food Sources
The Recommended Dietary Allowance (RDA) for leucine is 42 mg/kg of body weight per day for adults 19 years of age and older (*).
According to the USDA nutrient database (*), the top food sources of leucine are all animal proteins and include:
#1 Chicken and Turkey: 6 oz chicken leg has 5160 mg
#2 Beef, bison and lamb: 6 oz skirt steak has 5007 mg
#3 Pork: 6 oz pork chop has 5150 mg
#4 Beef Liver: 6 oz beef liver has 4500 mg
#5 Tuna, salmon and other fish: 6 oz tuna filet has 4133 mg
5 Amazing Leucine Benefits
#1 May Help to Maximize Muscle Gain
Building muscles involves exposing muscles to stress like heavy lifting or running long distances. The muscles break down as a result of the stress and the body builds them back up to be stronger than they were before the exercise. This is training the muscle to be able to withstand the stress during the next workout – it is essentially a survival mechanism so your body doesn’t fall apart with repeated activities.
A complete amino acid profile is beneficial for muscle growth. However, studies show that leucine is the most potent anabolic signal [*]. Leucine is the strongest activator of mTOR which is your body’s switch for muscle growth [*].
In older people, studies have shown that 4+ grams of leucine is necessary for maximal muscle growth [*].
Leucine provides both the substrate for muscle protein synthesis and the key anabolic signal. The leucine threshold hypothesis is the idea that small quantities of leucine on their own can signal muscle protein synthesis.
Studies have shown that adding just a few grams of leucine to casein protein — even when ingesting 20+ grams of total protein — increase muscle growth [*]. This study even showed that 10g of total protein was equivalent to 25g if leucine content was matched [*].
In totality, these studies lay waste to the notion that you can maximize muscle protein synthesis on a standard vegetarian diet. Some plants do contain substantial protein. But they are woefully deficient in leucine.
Whereas just a few oz of beef liver can get you all the leucine that you need.
#2 May Improve Blood Sugar Control
Controlling blood sugar is essential to weight loss and overall good health. Chronically elevated blood sugar contributes to inflammation as well as several chronic conditions including diabetes, insulin resistance, metabolic syndrome, obesity, and heart disease. When blood sugar is high, the body will slow down or stop burning fat entirely.
High protein, low carbohydrate diets enhance satiety (feelings of fullness) in individuals, increase muscle protein synthesis and enhance glycemic control. Researchers conducting a review of high protein diets concluded that higher amounts of BCAA’s in general and leucine specifically in high protein diets are a key element in these positive changes (*).
#3 May Help to Preserve Muscle Mass
Muscle loss, also known as muscle wasting, occurs for a variety of reasons including as a part of overall weight loss, illness, aging, and reduced activity. Muscle is metabolically active tissue that supports movement and activity and also plays a role in immune function. Loss of muscle mass is associated with poor health outcomes.
BCAA’s, including leucine, account for approximately 35% of muscle tissue. Supplementation of combined BCAA’s and leucine individually has shown to prevent muscle loss in a variety of situations including caloric deficit and illness (*,*,*).
#4 Might Reduce Muscle Soreness
After exercise, it is not uncommon to experience delayed onset muscle soreness (DOMS) as a result of the breakdown of muscle tissue previously described that occurs during exercise.
We’ve already established that leucine and the other branched-chain amino acids reduce damage during exercise by increasing protein synthesis and reducing protein degradation.
Some studies have found that supplementation with BCAA’s prior to exercise both reduces muscle damage and DOMS (*,*).
#5 May Improve Psychomotor Functioning and Reduce Fatigue During Exercise
Psychomotor functioning is just a fancy way of saying “thinking on your feet”. Supplementation with BCAA’s may help reduce perceived fatigue and improve psychomotor functioning during exercise.
A 2011 study showed that supplementation with BCAA’s improved mental reaction time among soccer players and may be useful as a performance aid in soccer and other sports that require quick reaction time (*).
Another small study on cyclists showed that perceived fatigue was lower among those supplementing with BCAA’s during a 60-minute ride as compared to results with a flavored placebo.
Get your BCAA’s
The branch chained amino acids are important for building muscle and preventing muscle loss. Leucine appears to be a key element in this equation.
Animal protein supplies ample BCAA’s and therefore should provide all the benefits of leucine as listed here. Just one more reason to get on the carnivore diet train…the diet highest in the world in leucine.
For instance, just a few oz of beef liver will have all the leucine you need.
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