LEUCINE: FUNDAMENTAL FOR MUSCLE MASS

LEUCINE IMPORTANCE FOR MUSCLE PROTEIN SYNTHESIS

Summary in two words
Leucine is the switch that starts muscle protein synthesis, for which all the other essential amino acids are also needed. The importance of leucine increases with age, when the body requires higher doses to achieve the same muscle synthesis.

To optimize muscle protein synthesis, it is necessary:
Choose high quality protein (animal protein is best, then soy)
Reaching the leucine threshold at each meal (2-2.5 g adults)
Consume 1.2 to 1.6 g of protein per kg of body weight (ideal weight) and spread the protein intake over all meals of the day
over 65, the body needs more protein and more leucine: protein 1.6 g/kg body weight, 2.8 -3.5 g leucine per meal

HOW PROTEIN SYNTHESIS OCCURS
Muscle protein synthesis is the process by which muscle builds new protein to grow, repair, and maintain itself functionally. It occurs continuously, but increases significantly after a protein meal and strength exercise

Muscle protein synthesis is controlled by a “switch” inside the cell called mTORC1 (which for convenience will henceforth be called mTOR) that decides whether the muscle should build new protein or remain in maintenance mode. When mTOR is turned off, protein synthesis is low. When mTOR is turned on, the muscle starts making new proteins.

Leucine is the main nutritional stimulator of mTOR. When the concentration of leucine in blood increases after a meal, mTOR is activated and initiates the process of protein synthesis. (1,2)

Leucine is an essential amino acid, meaning it is not produced by our body so it is necessary to introduce it through food. It belongs to the branched amino acids, BCAAs.

IS LEUCINE ENOUGH?
Leucine activates the signal that initiates synthesis, but the presence of the other essential amino acids is also necessary because they are the “building blocks” for building the protein. (3) If leucine is present but one or more essential amino acids are missing, the building signal turns on, but synthesis quickly stops. This explains why leucine alone is not sufficient to build new muscle tissue.

Leucine acts according to a threshold mechanism: below a certain amount per meal, muscle protein synthesis remains low; above the threshold, activation increases markedly.

In young adults, the threshold is about 2-2.5 g of leucine per meal, which is easily achieved with 20-25 g of high-quality protein, such as whey (whey protein), eggs, or meat. (4,5)

Taking more leucine than necessary is not harmful, but it does not produce an additional benefit on protein synthesis.

LEUCINE AND PROTEIN QUALITY
Not all proteins stimulate protein synthesis in the same way. The differences depend on:
content of essential amino acids
leucine content
digestibility and speed of absorption

Whey protein (whey) is particularly effective because it combines a high percentage of leucine with rapid absorption. Eggs, dairy, meat, and fish also provide complete, leucine-rich proteins.

Plant proteins, when taken individually, contain less leucine and may be deficient in one or more essential amino acids. Therefore they must be taken in larger amounts (thus more calories) or by combining different sources (grain + legume). Soy is among the few plant proteins with an amino acid profile close to that of animals.

AGING AND ANABOLIC RESISTANCE
A condition called anabolic resistance appears with advancing age: muscle in the elderly responds less to nutritional stimuli and needs a stronger signal to activate protein synthesis. (6)

For this reason over the age of 65: (7)
Leucine threshold rises to about 2.8-3.5 g per meal
30-40 g of high-quality protein is needed to reach this threshold
the distribution of protein in meals becomes crucial; it is not enough to increase the daily protein intake, it is necessary to make sure that each meal exceeds the leucine threshold.

Over age 65, it is recommended to consume at least 1.2-1.6 g of protein per kg of body weight, considering ideal weight.

MUSCLE MASS AND LONGEVITY
Today we know that muscle mass and strength, are among the strongest predictors of longevity and health. Let's try to understand why muscle is so important:
- muscle is a metabolic organ: the liver and muscles are the only stores of glycogen (glucose) in the human body. Therefore, the more muscle mass we have, the better we can manage blood glucose (excess blood glucose). This results in better insulin sensitivity and thus reduction of metabolic diseases such as diabetes
- muscle is now considered an endocrine organ, secreting myokines that decrease inflammation and improve immune function and brain health
- muscle contraction increases bone density, meaning fewer fractures, a major issue in old age
- Having good muscles allows you to maintain your autonomy
- strength training needed to maintain good muscle mass also works on balance, decreasing the risk of falls
- muscle is a reservoir of protein, during an illness people with greater muscle mass are more likely to survive (8)

SUPPLEMENTATION WITH ESSENTIAL AMINO ACIDS
If you are taking essential amino acids in a protein-free/poor meal, you need to pay attention that to the amount of leucine in your intake. The threshold for activating muscle protein synthesis is:
2.5 to 2.8 g up to 65 years of age
2.8 - 3.5 g over 65 years old

If essential amino acids make up the entire protein intake, the total amount of essential amino acids will have to be:
8-10 grams up to age 65, including at least 2-2.5 g leucine (6)
10-12 grams over 65 years, including at least 2.8 -3.5 g leucine (7)

FOODS WITH LEUCINE
The absolute richest food in leucine is whey, whey protein. To get 2.5 grams of leucine, 20 grams of whey, which contain only 78 calories, is sufficient. In this table are the twenty foods most effective in providing 2.5 g of leucine in the lowest caloric intake. In order of increasing caloric intake. In the third column are the grams of food needed to provide 2.5 g of leucine

The complete list can be viewed at this link. For the printable version, click here.

Fact: Whey is prescribed along with slimming drugs as an optimal protein source for maintaining muscle mass during weight loss. Because of this, demand has increased so much, which has led to an increase in price. Just to give you an idea: the purchase cost of whey isolate in the last year has increased by 50%. For this reason I have currently decided to sell only whey concentrates on Live Better (available from February 2026).

KAISERSCHMARREN LIVE BETTER
The kaiserschmarren with 28 g collagen, 3 eggs and 40 g Greek yogurt 0% contains 2.78 g leucine. The savory version with 100 g cottage cheese, 2 eggs, 28 g collagen and 15 g Parmesan contains 3.36 g leucine. In the recipes we will publish in the future, the leucine content will always be indicated.

For the kaiserschmarren recipe, click here.

HOW MUCH PROTEIN TO CONSUME PER DAY
The new U.S. guidelines, the official document based on the latest scientific evidence, recommends that adults consume 1.2-1.6 g of protein per kg of body weight (considering ideal weight). (9)

Over age 65, anabolic endurance requires more protein, thus 1.6 g/kg body weight.

Protein content per 100 grams of food
100 g eggs = 12.5 g protein = 143 kcal
100 g beef sirloin = 23 g protein = 132 kcal
100 g chicken = 21.23 g protein = 114 kcal
100 g collagen powder = 93 g protein = 372 kcal
100 g sea bream = 18.9 g protein = 105 kcal
100 g sea bass = 18.4 g protein = 97 kcal
100 g shrimp = 13.6 g protein = 72 kcal
100 g squid = 16 g protein = 92 kcal
100 g salmon = 20 g protein = 142 kcal
100 g anchovies = 20.35 g protein = 131 kcal
100 g mackerel = 18.6 g protein = 205 kcal
100 g amberjack = 23 g protein = 146 kcal
100 g parmesan = 32.4 g protein = 397 kcal
100 g emmenthal = 28.5 g protein = 403 kcal
100 g pecorino =26 g protein = 409 kcal
100 g cow/buffalo mozzarella = 18.7/16.7 = kcal 253/288
100 g cow/sheep ricotta = 8.8/9.5 g protein = 146/157 kcal
100 g cow/wheat whole yogurt = 3.8/6.4 g protein = 66/115 kcal
100 g whole milk = 3.3 g protein = 64 kcal
100 g soybean flour = 36.8 g protein = 469 kcal
100 g fresh borlotti beans = 10.2 g protein = 143 kcal
100 g cooked chickpeas (dry) = 7 g protein = 132 kcal
100 g cooked lentils (dried) = 6.9 g protein = 109 kcal
100 g fresh/frozen peas = 5.5 g protein = 64 kcal
100 g canned cannellini beans = 6 g protein = 86 kcal
100 g tofu = 8 g protein = 76 kcal