What it is:
Leucine is an essential amino acid found in various protein-rich foods (e.g. meat, chicken, fish, eggs, and dairy). It is involved in many crucial roles in the body, such as muscle building, growth, repair, and as an energy source. Not only is it classified as essential due to our body's inability to produce it, but it is also a specialized branch-chain amino acid (BCAA) which means its unique structure allows us to utilize it for energy and protects our muscles from breaking down.
Purported claims:
Increased Muscle Protein Synthesis (MPS), recovery, and exercise performance
Increased muscle mass preservation
Increased satiety, weight management, and glucose control
What the science says:
A high level of interest in leucine intake has occurred in the wellness industry due to its ability to signal our bodies to build and grow muscles by activating critical cellular pathways. New research points out that of all the amino acids our body needs, leucine is especially essential for building and repairing muscles because it activates a critical protein complex called mTORC1. This activation flips the switch to initiate the building of new proteins in our muscles. But it does not occur within a vacuum, as the other essential amino acids are also required to support optimal Muscle Protein Synthesis (MPS).
Claims surrounding leucine’s ability to help manage weight and glucose control are due to its complex ability to stimulate insulin release and glucose uptake into muscles. However, studies are mixed, and lone leucine supplementation has not been shown to increase insulin sensitivity or glucose uptake beyond whole protein consumption benefits.
Although there is no official RDA for leucine, the International Society of Sports Nutrition estimated a daily intake of 700–3000 mg of leucine per kg of body weight is needed to maximize muscle synthesis. However, specific populations may need higher recommendations, such as older adults with higher protein requirements.
Research has shown leucine supplementation may benefit specific populations, such as older adults, and those on a calorie-restricted diet—currently, most research centers around those not consuming enough protein overall. If protein intake is optimal, the evidence does not support additional leucine supplementation as advantageous in healthy adults.
A couple of small studies (Churchward-Venne, 2013) have shown that a low-protein supplement combined with high leucine levels has comparable MPS stimulation as a high-protein supplement with equivalent leucine. However, the evidence is lacking that supplementing leucine translates to long-term lean muscle mass. This may be a case of a theory having research evidence that is not significant when translated over to real-life situations. Just because the hypothesis of a ‘Leucine trigger’ might be true, it doesn’t mean it’s significant enough to warrant recommending isolated supplementation. However, resistance training and high levels of protein intake do have abundant evidence supporting muscle growth and maintenance.
To stimulate MPS, you can focus on whole-item animal foods highest in leucine, such as chicken, beef, fish (tuna), dairy, and eggs. As leucine is most abundant in animal food sources, vegetarian and vegan diets may benefit from supplementation, as ~2.5g of leucine is needed per meal to gain the benefits of muscle synthesis.
Plant foods that contain leucine include:
| Amount | Protein (g) | Leucine (g) |
Spirulina | 14g | 8g | 0.69g |
Edamame | 100g | 11g | 0.68g |
Chickpeas | 100g | 7g | 0.5g |
Lentils | 100g | 5g | 0.4g |
Quinoa | 100g | 8.1g | 0.5g |
Tofu | 100g | 7g | 0.6g |
Cannellini Beans | 100g | 8g | 0.68g |
Although leucine is found in plants like legumes, nuts, and seeds, it can be challenging to source. Whey protein is a popular dietary supplement that is also a rich source of leucine but is only suitable for those who consume dairy. It is a convenient way to obtain optimal protein and leucine levels at ~2.7g of leucine per 25g of whey protein. However, for those who follow a plant-only diet, supplementing with a vegan protein powder is likely essential to meet leucine requirements.
Our take:
The current evidence suggests meals comprising 30 g of high-quality protein with ~2.5 g of leucine will optimally stimulate protein synthesis. Research also suggests muscle synthesis occurs just as well with whole food proteins as with nutritional supplements.
To stimulate MPS, begin by optimizing your whole-food protein intake and resistance training program. But, if you struggle to meet your daily requirements through whole foods, let your nutrition advisor know, and they can build a plan to meet your needs that may include supplemental protein powders such as whey or vegan options which contain leucine.
Will this benefit you?
Likely not. If you are protein deficient and struggle to consume the correct amounts from your meals, supplementing with a protein powder may assist you in muscle protein synthesis. But if you already consume enough protein to stimulate muscle protein synthesis, adding leucine supplements has not been shown conclusively to provide additional benefits.
Still curious to try it? If you do, here’s what to keep an eye on:
Supplementing isolated leucine can lead to an imbalance that the body is unable to regulate, leading to clinical consequences such as pellagra. Working with a health professional is valuable when beginning a supplement program.
Extreme levels of leucine may cause a significant increase in blood ammonia concentration.
References and additional reading:
Krause's Food & the Nutrition Care Process. Elsevier Inc, 2017. Print.
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