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What's the buzz around Leucine and Muscle Protein Synthesis?

by dietitian Clare Wood

Muscle protein synthesis is crucial for most athletes, and is reliant on several nutritional aspects of the post-exercise feeding; the amount of protein, the type protein, the timing of intake to maximise nutrient release into the bloodstream and uptake by muscles, and co-ingestion with carbohydrate to enhance absorption.

The amount of protein required for optimal muscle gains is currently agreed on to be ~20 grams. More is not usually better, as synthesis maxes out, and not much more will actually be used to build muscle. See more about Protein for Athletes.

The type of protein may be what's important. Proteins contain up to 20 different amino acids (AA) of which 11 are essential (EAA), these are the ones that we need to consume because the body cannot make them. After consumption, proteins get digested, broken down into AA's and released into the bloodstream. The liver takes a portion of them for use, except for the branched-chain amino acids (BCAA's), leucine, isoleucine, and valine, which are free to be used directly by the muscle. Research suggests they have individual and collective abilities to stimulate the muscle machinery; however, leucine is understood to be the queen bee of AA's, the most important one for turning on the anabolic signalling pathways to initiate Muscle Protein Synthesis.

Aside from the recent hype over leucine, some think that consumption of all of the EAA's should promote the greatest increases. This has been supported by some research in 2017 from Aguiar et al. which found that supplementing with only leucine at 3.0 g/day post-training did not improve strength gains or muscle mass after 8 weeks of strength training. Which may show that single intake is not better than whole foods that include leucine. So there is still some parts of this story still to be explored.

Research suggests that proteins with a higher level of EAA's, that are rich in leucine are more rapidly digested, resulting in greater rates of muscle hypertrophy. Foods with the full complement of EAA's are also described as having a high biological value (HBV) and are considered to be of a higher quality.

meat will provide lots of amino acidsmeat is naturally high in leucine

Where is leucine found?

The content of leucine in foods varies greatly, but some foods are naturally high in leucine, including meat, poultry, fish, dairy and legumes. Dairy is very effective because the whey portion of milk protein is particularly high in leucine, plus with its faster digestion rate than other proteins, it allows blood leucine levels to peak more quickly after ingestion. A pure form of whey protein, i.e. a supplemental form, in theory is more effective than whole milk protein, casein (which is the more slowly digested milk protein), and soy proteins.

Table of foods and their leucine content

Food type Amount containing ~2g Leucine Protein (g)
Beef and seafood 120 g 25
Chicken 135 g 27
Whole eggs 2 large or 3 small 19
Dairy milk – skim 600 ml 22
Soy milk 900 ml 33
Cheese – reduced fat 2 slices (70g) 22
Cottage cheese 140 g 25
Yoghurt – natural 350 g 20
Whey protein powder 18 g 16
Soy protein powder 25 g 22
Beans, lentils, Chickpeas 350 g or ¾ cup 20-23
Tofu 400 g or 1 ½ cups 48
Nuts (almonds and peanuts) 120 g or 1 cup 26
Bread 9 slices 28
Rice - cooked 6 cups 26

Some practical tips

In summary, for maximal muscle gains along with your training program, in order of importance, athletes must aim for adequate protein intake, optimise the timing of intake post-exercise, aiming for 3-4 g of leucine in that protein, and finally adding some carbohydrate to the snack/meal.


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