What is Leucine: Benefits, Sources, and Risks

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What is Leucine: Benefits, Sources, and Risks

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What Is Leucine?

Leucine, also known as L-leucine, is one of the most important amino acids for muscle protein synthesis. 

It is one of the 20 amino acids that make up the proteins in living organisms. It is considered an essential amino acid because our bodies cannot make this amino acid. It can only be acquired from dietary sources. 

Leucine is one of “the trio” of branched-chain amino acids (BCAA). The other two are valine and isoleucine (1).

Animal studies show that leucine deficiency is associated with unhealthy weight loss and eventual death. Thus, leucine is important for a healthy life (2).

Adequate amounts of leucine should be consumed every day to maintain good health. 

Leucine benefits

Leucine contributes to muscle mass

Among BCAA’s, leucine is the principal amino acid that promotes proteins synthesis.

In animals, the use of leucine supplements alone improved muscle protein synthesis (3).

Similarly in humans, its use alone has been shown to promote muscle protein synthesis in healthy individuals (4). 

Leucine activates protein synthesis by stimulating the formation and activation of protein complexes needed for protein synthesis (5).

The mammalian target of rapamycin (mTOR) is the most studied signaling pathway for leucine-driven protein muscle synthesis (6).

Leucine-rich proteins such as whey protein are shown to significantly increase muscle protein synthesis in animals (7).

An increase of leucine by isolated protein e.g. whey protein, ingestion after physical exercise is considered more anabolic for muscle protein synthesis than whole food protein sources. This phenomenon is termed the “leucine trigger hypothesis” (8).

Thus, isolated protein supplements such as whey, casein, and soy protein after strength training can be useful in anabolic protein synthesis. 

In a randomized control study in humans, leucine-containing whey protein beverages increased muscle protein synthesis after resistance training. 

The use of high leucine even with low protein intake was able to induce protein synthesis. 3-5 g leucine-containing whey protein supplement after resistance training is ideal to promote muscle protein synthesis (9).

An active leucine metabolite called beta-hydroxy beta-methyl butyrate (beta-HMB) has been used in several studies to understand the role of leucine in protein synthesis. 

beta-HMB use increased physical performance in endurance-trained cyclists (10).

The use of beta-HMB in older individuals can increase muscle strength and mass but some studies show no improvement with beta-HMB in older individuals (11).

Among essential amino acids, leucine is the most important promoter of muscle protein synthesis. 

Can leucine promote weight loss?

Leucine can promote weight loss. Several animal studies found that leucine intake resulted in reduced food intake which could be a possible way of weight loss (12,13).

Also, leucine intake led to increased energy expenditure thereby, lowering fat and body weight (14).

Some studies proposed that leucine may influence the body’s hunger mechanisms. For example, leptin is a protein known to suppress hunger. An increase in leptin will lower the appetite. Plasma leptin can increase after leucine intake, which can reduce hunger (15). But not all studies show this leucine intake effect on leptin (16).

In humans, high protein intake results in greater weight loss compared to a low protein diet (17).

Intake of a high protein diet containing a minimum of 10 g of leucine per day resulted in significant weight loss compared to consumption of a low protein diet (18,19).

Considering that a normal human diet contains leucine with other amino acids, the use of a protein diet with high leucine may provide weight loss benefits. 

Current studies show that the use of a protein diet with leucine can be beneficial in weight loss.  

Leucine can control glucose metabolism

Leucine not only promotes protein synthesis but also mediates glucose metabolism. It has shown to be a potent insulin secretagogue i.e. causes the secretion of insulin.

Several animals studies show an increase in insulin after leucine use with or without exercise (20).

In mouse models of diabetes, leucine was effective in inducing glycemic control (21).

Long-term leucine intake with high-fat diet intake in animals increased insulin sensitivity by improving the insulin signaling pathways and reducing oxidative stress (22). However, this also led to body weight increase in animals on a high-fat diet. 

But another similar study found that the use of leucine supplements reduced high fat diet-induced obesity as well as hyperglycemia (high blood glucose). Leucine ingestion also reduced total cholesterol levels (23).

The use of leucine reduces insulin resistance, improves insulin sensitivity, and may also influence insulin secretion (24,25).

A combination of leucine, zinc, and chromium supplements significantly increased oxidative status, reduced inflammation, and oxidative stress in animals of diabetes disease models. These effects were most probably due to reduced blood glucose levels (26).

In humans, few studies have investigated the direct role of leucine in glucose regulation. High protein intake is associated with better glucose control in type 2 diabetes patients (27,28).

Oral intake of leucine in healthy individuals is associated with increase insulin secretion after resistance training (29,30).

In women with diabetes type 2 high levels of leucine are associated with a lower risk of diabetic nephropathy (31).

Leucine causes increased insulin secretion and improves insulin sensitivity. Therefore, leucine may provide glucose control 

How much leucine per day?

The amount of leucine for muscle growth has been a matter of debate. 

For example, earlier studies suggested recommended dietary intake (RDI) of leucine for the general population to be 30 mg per kg of body weight (~15 mg per lbs of body weight) (32). Others recommended even more leucine at 45 mg per kg (~22 mg per lbs of body weight) for the general population, regardless of the lifestyle (33).

For healthy individuals, WHO recommends daily leucine intake of 39 mg/kg of body weight (~20mg per lbs of body weight) (34).

Leucine constitutes about 5-10% of the total proteins. For individuals doing physical or strength training, protein intake of 1.5-2.0g per kg (~ 0.8 -1.0g per lbs) of body weight per day will provide the necessary leucine (35).

For the elderly, 3g leucine in 25-30g of protein per meal, 3 times daily is recommended to preserve muscle mass loss (36).

Leucine risks

Leucine is generally a safe amino acid since it is one of the building blocks of proteins found in the body. Leucine supplements are routinely consumed by athletes and bodybuilders. 

Leucine intake is well-tolerated in both animals and humans with no serious adverse effects (37).

Some people may experience pellagra symptoms (diarrhea, inflamed skin) with high leucine intake. Thus, high consumption can be harmful. 

Currently, for healthy adult young and elderly men, the upper limit is 500mg per kg of body weight per day (38).

If you have any health condition, you should consult your doctor before taking any dietary supplement including leucine. 

Food sources of Leucine

Most of the research on leucine was done using leucine supplements however, leucine is naturally available through food sources. 

The majority of the leucine sources are of animal origin, including meat, poultry, eggs, and dairy, and some vegetables (39).

Common sources of leucine are given below (40).

Leucine-rich foods

Animal productsLeucine g per 100 g of food
Adult bovine’s rump1.9
Baked ham1.7
Chicken’s breast, without skin1.96
Bresaola2.7
Lamb1.5
Pork low-fat steak1.7
Pork’s sausage1.2
Rabbit2.0
Raw Ham2.2
Turkey’s breast2.0
Chicken egg1.0
SeafoodLeucine g per 100 g of food
Salmon1.5
Tuna 1.9
Drained tuna in oil2.0
Trout1.0
Cod1.5
Farmed sea bream, filets1.6
Herring1.3
Mackerel1.6
Mullet roe2.8
Mussel0.8
Clam0.7
Anchovy1.3
Sardines1.6
Octopus0.7
Cheese, Milk, and YogurtLeucine g per 100 g of food
Cow’s milk partially skimmed0.4
Asiago2.0
Cow ricotta1.0
Emmenthal2.7
Feta1.5
Grana cheese2.8
Greek yogurt, low-fat0.7
Mozzarella cheese1.4
Parmesan2.9
Dried Fruits and NutsLeucine g per 100 g of food
Pine nuts2.0
Sweet almonds, dried1.5
Pistachios1.4
Cashew 1.3
Hazelnuts, dried0.9

Low leucine diet

The low leucine diet is mostly plant-based low protein foods. 

VegetablesLeucine g per 100 g of food
Chard0.09
Eggplant0.07
Fresh lettuce0.11
Forest asparagus0.21
Fresh ripe tomatoes0.03
Cabbage0.11
Bell pepper0.03
Spinach0.32
Zucchini/courgettes0.13
FruitsLeucine g per 100 g of food
Pineapple 0.02
Apple0.01
Apricot0.02
Banana0.05
Avocado0.31
Blueberry0.05
Grapes0.01
Melon0.02
Orange0.02
Peach0.02
Raspberry0.05
Strawberry0.04

The Gist 

Several animal and human studies have shown that leucine is one of the prime stimulators of muscle protein synthesis. It can also promote weight loss, increase insulin sensitivity, and help with glycemic control. 

The main sources of leucine are animals products. 

High doses of leucine can be harmful. The upper limit for young and elderly men is set at 500 mg per kg body weight per day.