L-leucine is one of the most popular bodybuilding supplements on the market. L-leucine is an essential amino acid that needs to be obtained through our diet. As a building block for protein, it is crucial for protein synthesis and other metabolic functions.
The studies that have been done on L-leucine’s impact on the body have shown that it’s involved in protein synthesis as well as other metabolic functions. It may also help our body regulate blood sugar levels, muscle growth and repair, bone tissue growth and repair, growth hormone production, and even wound healing .
While there are plenty of claims, including improving athletic performance, building muscle mass and aiding recovery after exercise, the studies out there are still relatively limited in scale and scope. More studies into the effects of L-leucine are needed.
Reported Effects of L-Leucine
1. Aid Muscle Recovery
A recent systematic review and meta-analysis of studies into supplementing L-leucine found that it can be effective on outcomes of exercise-induced muscle damage, as long as the extent of muscle damage was low-to-moderate. It goes on to state that the supplementation strategy was most effective when a high daily intake was taken for an extended period of time, and it was particularly effective if the supplement was taken before the damaging exercise. 
Another study into L-leucine supplementation and muscle damage induced by exercise showed that supplementing before exercising did, in fact, decrease muscle soreness and muscle fatigue occurring for a few days after the exercise. 
In this case, test subjects were asked to do squats. The test only had 30 participants, and wider studies are needed before drawing any hard conclusions. However, the findings do suggest that L-leucine may be useful for muscle recovery following exercise.
2. Promote Weight Loss
You’ve probably seen claims that L-leucine can help you lose weight, shed excess body fat, and even treat obesity. There have been numerous studies in this area, and the findings aren’t as straightforward as some people claim.
One study, published in The Journal of Nutrition, found that leucine supplementation may help reduce the loss of lean tissue and increase the loss of body fat.  The way that leucine appears to modulate insulin signaling and glucose use by the muscles helps maintain a stable environment even during energy-restricted periods, thus helping your body burn fat during exercise.
However, a couple of other studies on the effects of leucine and weight gain in animals shows us a different story. In two studies on rats, leucine supplementation was shown to have a negative effect on weight loss. In the first, leucine supplementation was found to not prevent weight gain in male rats,  while another showed that L-leucine supplementation actually promoted obesity in rats by promoting a pattern of gene expression that favors fat accumulation. 
3. Boost Immunity
One of the areas that’s shown some promise over the last few years is the effects of L-leucine supplementation on the immune system.
As an essential amino acid, L-leucine plays an important role in the development of an immune response in the body. To ensure that the T cells (immunity cells) have enough nutrients and energy, L-leucine activates something called the mTORC1, a critical regulator of T cell proliferation, differentiation, and function, all of which lead to an efficient and effective immune response. 
Further studies on the effects of L-leucine on pig’s immune systems have been done, and scientists are hopeful about finding even more benefits to the immune system from taking L-leucine supplements. 
Common Supplements that Contain L-Leucine
Complete proteins naturally contain L-leucine and other amino acids. Foods that are naturally high include chicken, fish, eggs, and milk, as well as yogurt and soybeans.
L-leucine is also readily available for supplementation.
You can find pure L-leucine in tablet, fluid, and powder form. Many BCAAs include L-leucine, as do many multivitamins, and proteins.
Side Effects of L-Leucine Supplementation
Taking high doses of single amino acids for long periods of time can cause an imbalance in your body’s nitrogen levels. This can negatively affect your metabolism and kidney function. Very high doses can sometimes cause hypoglycemia, as well as pellagra, the symptoms of which can include hair loss and skin lesions.
We still don’t know much about the effects of women taking L-leucine supplements while pregnant or breastfeeding. Until scientists collect more data, it’s best to stay on the side of caution and not take any amino acid supplements while pregnant or breastfeeding.
Most people take in a few grams of L-leucine every day from the food they eat, particularly if their diet includes animal-based protein like poultry, meat, or fish.
When it comes to supplementing L-leucine, start with a low dose and increase slowly, somewhere between 2.5 and 5 grams per day.
One study published in The Journal of Nutrition, while not providing concrete upper limits, cautiously estimated that the L-leucine upper limit is around 35g per day for an individual male weighing 70 kg. 
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- Zampieri TT, Torres-Leal FL, Campaña AB, Lima FB, Donato J Jr. L-leucine supplementation worsens the adiposity of already obese rats by promoting a hypothalamic pattern of gene expression that favors fat accumulation. Nutrients. 2014;6(4):1364‐1373. Published 2014 Apr 2.
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