Leucine
The muscle builder king. Leucine is the most anabolic amino acid. It directly activates mTOR, the master switch for muscle protein synthesis. Of the three branched-chain amino acids (BCAAs), leucine is the most important for building and maintaining muscle. It's essential for athletes, elderly preventing sarcopenia, and anyone wanting to preserve lean mass.
Key Functions
mTOR Activation
Directly activates mTOR pathway. This is THE switch for muscle protein synthesis. No other amino does this as strongly.
Muscle Protein Synthesis
Triggers building of new muscle proteins. Need ~2.5g leucine per meal to maximize this effect.
Prevents Muscle Breakdown
Signals to preserve muscle during fasting or calorie restriction. Anti-catabolic.
Insulin Secretion
Stimulates insulin release even without carbs. Helps shuttle amino acids into muscle.
Energy Source
Can be used directly by muscles for fuel during exercise. Metabolized in muscle, not liver.
Wound Healing
Supports tissue repair post-surgery or injury. Often given in clinical settings.
The Leucine Threshold
You need to hit a minimum leucine threshold per meal to maximally stimulate muscle protein synthesis:
Leucine-Rich Foods
Whey Protein
Highest leucine content. Fast absorbing. ~2.5g leucine per 25g whey.
Chicken Breast
~2.5g leucine per 100g. Lean protein. Very high quality.
Beef
~2g leucine per 100g. Complete amino profile. Also provides iron, zinc, B12.
Eggs
~0.5g per egg. Need several eggs to hit threshold. Perfect amino profile.
Fish
Tuna, salmon ~2g per 100g. Also provides omega-3s.
Greek Yogurt
~1g per 100g. Convenient, also contains casein for slow release.