The Leucine Threshold: Why Not All Protein Portions Are Created Equal

By Eugene Capitano, DC MSc (Neuroscience & Psychology of Mental Health)
ACSM-Certified Exercise is Medicine® Practitioner and Personal Trainer
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Most people think building muscle is all about eating more protein. But the science tells a more precise story, how much protein you eat per meal matters as much as how much you eat in a day. The key player is a single amino acid called leucine, the switch that turns on your body’s muscle-building machinery.

When you eat protein, your digestive system breaks it into amino acids. Leucine signals a molecular pathway known as mTORC1, which tells your muscles, “time to grow.” Once enough leucine enters your bloodstream, it crosses a threshold, the point at which muscle-protein synthesis (MPS) is fully activated.

Below this threshold, the signal is too weak to activate muscle protein synthesis (MPS). Once the threshold is met, MPS is maximally stimulated — a concept often called the “muscle-full effect.” However, the process isn’t as simple as an on/off switch.

Think of it more like a dimmer: when you’re young and metabolically responsive, a small amount of leucine can turn the light fully on. But with aging or anabolic resistance, the dimmer becomes less sensitive, you need a stronger signal (more leucine or higher-quality protein) to achieve the same brightness.

The Science — The Leucine Threshold & the Muscle-Full Effect
Research shows that muscle growth depends on hitting a precise per-meal protein target that changes with age.

• Under 39: Around 20–25 grams of high-quality protein (≈ 2 g leucine) is enough to fully activate muscle-protein synthesis (MPS).
   
• Ages 40–59: You need about 25–30 grams (≈ 2.2–2.5 g leucine) to offset the gradual decline in anabolic sensitivity that begins in midlife.
   
• 60 and older non active: The target rises to 30–40 grams (≈ 2.8–3.0 g leucine) per meal to overcome age-related anabolic resistance. If active the thresholds would be similar to ages 40-59.
   

Once that threshold is reached, your muscles enter the “muscle-full” state. Extra protein won’t increase growth. The key is not eating more at once, but hitting this threshold repeatedly at separate meals throughout the day.

Research shows that muscle growth depends on hitting a precise per-meal protein target that changes with age. Once that threshold is reached, your muscles enter what scientists call the “muscle-full” state—the point at which muscle-protein synthesis (MPS) has been maximally stimulated and  protein no longer adds to growth.

• Under 40: Around 20–25 grams of high-quality protein (≈ 2 g leucine) is enough to fully activate MPS. This amount easily comes from a serving of whey protein, eggs, chicken, or fish.
   
• Ages 40–59: You need about 25–30 grams (≈ 2.2–2.5 g leucine) to offset the gradual decline in anabolic sensitivity that begins in midlife. Evenly distributing this amount across meals produces stronger muscle-building responses than saving most protein for dinner.
   
• 60 and older: The target rises to 30–40 grams (≈ 2.8–3.0 g leucine) per meal to overcome age-related anabolic resistance. Pairing this with resistance training two to three times a week keeps muscles responsive and maintains strength and metabolic health.
   
• 75 and frail: At this stage, appetite and chewing ability often decline, so 30–40 grams of high-quality or leucine-enriched protein—often delivered as a whey-based shake—helps preserve mobility and independence. Protein intake should be guided by a healthcare provider if there are kidney or metabolic concerns.
   

The takeaway: It’s not about eating more protein at once. It's about hitting your threshold at each meal, several times a day. Each time you reach this threshold-hitting meal is a new opportunity to turn the muscle-building “switch” back on.

Studies using gold-standard tracer methods show that once the leucine threshold is met, MPS plateaus. For example, 40 g of whey protein doesn’t build more muscle than 20 g in young adults at rest. Beyond the threshold, the extra amino acids are diverted to non-muscle uses or oxidized for energy.

The real advantage comes from hitting the threshold at several meals per day, not from overloading one. This rhythm of repeated MPS stimulation drives muscle maintenance, metabolic balance, and healthy aging.

Resistance training dramatically extends your anabolic window — up to 24 hours of enhanced sensitivity. Eating a threshold-hitting protein meal after training amplifies the muscle-building response. For older adults especially, combining strength work with proper protein distribution is the single most powerful strategy to counter sarcopenia.

• Under 39: 20–25 g protein per meal; total ≈ 1.0-1.2 g protein / kg body weight / day.
   
• 40–59: 25–30 g per meal; total ≈ 1.2–1.4 g /kg.
   
• 60 + active: 25–30 g per meal; total ≈ 1.2–1.6 g /kg.
   
• 60 + sedentary: 30–40 g per meal; total ≈ 1.4–2.0 g /kg.
   
• 75 + frail: 30–40 g per meal; total ≈ 1.4–1.8 g /kg. (consider shakes); monitor kidney function.
   

Each meal is an independent opportunity to trigger MPS — three strong meals beat one massive one every time.

Leucine is the spark that ignites muscle growth, but you need the right dose at the right time. Don’t spread protein thinly. Don’t rely on one big dinner. Rather, eat a balanced, leucine-rich meals throughout the day and pair them with regular resistance exercise.

That’s how you preserve muscle, strength, and metabolic vitality — at any age.