What the Research Says
What the Protein Research Actually Shows
Protein has the strongest evidence of any macronutrient for body composition, satiety, and healthy ageing. The optimal amount is higher than most people consume — and higher than official guidelines.
Why Protein Gets Special Treatment
Of the three macronutrients, protein is the only one with a clearly defined functional role in tissue maintenance and repair. It's also the most satiating and has the highest thermic effect. The research literature on protein is unusually consistent — the debates that remain are about optimal amounts and timing, not whether it matters.
What Replicates Strongly
Muscle protein synthesis requires approximately 1.6–2.2g of protein per kg of bodyweight per day for active individuals. A landmark 2017 meta-analysis by Morton et al. (49 RCTs, 1,800 participants) found muscle gain from resistance training plateaued above ~1.62g/kg/day, with no benefit from higher intakes. Sedentary older adults show similar thresholds. Official guidelines (0.8g/kg RDA) are set to prevent deficiency, not optimise muscle maintenance or anabolism.
Protein is the most satiating macronutrient per calorie. Multiple RCTs show high-protein conditions produce greater reductions in ad libitum food intake, hunger ratings, and next-meal consumption relative to isocaloric high-carb or high-fat conditions. The mechanism involves GLP-1, PYY, and CCK — hormones that signal satiety to the hypothalamus. Effects are dose-dependent up to approximately 30% of total calories.
Per-meal protein threshold for muscle protein synthesis is approximately 20–40g. Leucine is the key anabolic trigger; a meal needs to clear ~2–3g leucine to maximally stimulate MPS. Amounts above ~40g don't add further acute MPS but contribute to total daily intake. Spreading protein across 3–4 meals outperforms two large meals for MPS over 24 hours.
Protein preserves muscle mass during caloric restriction. When calories are reduced for fat loss, high protein (≥1.8g/kg) consistently preserves lean mass relative to lower-protein diets at equal caloric deficit. This effect is important for ageing: the combination of sarcopenia (muscle loss with age) and insufficient protein intake is a primary driver of functional decline.
Plant proteins are effective when combined to provide complete amino acid profiles. Complete essential amino acid availability is the key variable — not source per se. Leucine content of plant proteins is often lower, so total protein needs may be ~10–20% higher for plant-based diets to achieve equivalent MPS. Soy protein shows the strongest evidence among plant sources.
What the Research Can't Tell You
Protein requirements vary based on age, training status, total caloric intake, and health conditions. Older adults (65+) may need the upper end of the range (2.2g/kg) to overcome anabolic resistance. The most useful personal experiment is tracking protein intake alongside strength, body composition, and satiety for 4–6 weeks to find your effective range.