Most protein arguments online get muscle-building and healthspan confused. They are different questions, with different answers, and conflating them has produced both bad bro-science ("only animal protein works") and bad pop-nutritionism ("animal protein gives you cancer"). The trial evidence is clean enough to separate the two.
What the trials actually show
The first thing to know is that for muscle protein synthesis (MPS), gram-for-gram, animal protein wins. Van Vliet, Burd, and van Loon (2015) reviewed the direct head-to-head MPS trials and concluded that, at matched protein doses, soy and pea protein produce a smaller acute MPS response than whey or beef ( van Vliet, Burd, & van Loon 2015 ). The two mechanistic reasons are:
- Leucine content. Whey is roughly 12% leucine by weight; soy is about 8%; pea is about 7%. Leucine is the primary trigger for the mTOR-driven MPS response. Hitting the per-meal "leucine threshold" of about 2.5-3 g requires more grams of plant protein.
- Protein quality (DIAAS). Whey scores DIAAS 1.09; pea protein 0.82; soy 0.91; rice 0.59. DIAAS captures both the amino-acid profile and the digestibility of each amino acid in adult humans. Lower DIAAS means a higher proportion of the protein you eat doesn't make it into the systemic free amino acid pool.
The Morton 2017 meta-analysis (49 trials, n=1863) is the cleanest summary of total daily protein intake versus hypertrophy in resistance-trained adults ( Morton et al. 2017, n=1863 ). Above 1.6 g/kg/day total protein, additional protein produced no further hypertrophy benefit. Source quality mattered more at lower per-meal doses, less as total daily intake climbed past the saturating threshold.
Phillips 2016 ("Protein requirements beyond the RDA") synthesizes the practical implication: total daily protein matters most up to about 1.6 g/kg/day; per-meal distribution matters in the 20-40 g range; protein quality matters most at the low end of per-meal dose, less at the high end ( Phillips et al. 2016 ).
How plant protein closes the gap
Three documented strategies bring plant-protein MPS up to whey-equivalent territory:
Higher per-meal dose. The simplest fix. At 40 g per meal of pea or soy isolate, MPS responses converge with 25-30 g of whey. The math is straightforward: you need more grams to hit the same leucine quantity.
Leucine or EAA fortification. A 25 g dose of pea protein with 2 g added leucine produces an MPS response indistinguishable from 25 g of whey in mechanistic studies. Some commercial plant blends now add free-form leucine specifically for this reason.
Blending. Pea + rice combines pea's lysine with rice's methionine, producing a complete amino-acid profile that approximates whey. Soy on its own is already complete, which is why soy isolate is the most-studied plant protein for MPS endpoints.
For practical purposes: if you're already eating 1.6 g/kg/day total and distributing it across 4 meals at 30-40 g each, the source choice matters less than people think. If you're at 1.0 g/kg/day with two meals of 50 g, the source choice matters a lot.
The healthspan twist
This is where it gets interesting, and where most arguments break down.
Levine et al. 2014 (n=6381 NHANES adults) found a striking age-dependent inversion in the protein-mortality relationship ( Levine et al. 2014, n=6381 ). In adults aged 50-65, high animal-protein intake (defined as ≥20% of calories) was associated with a 4x increase in cancer mortality and a 75% increase in all-cause mortality versus low intake. Above age 65, the same high animal-protein intake was associated with a 60% reduction in cancer mortality and a 28% reduction in all-cause mortality.
The proposed mechanism: IGF-1. Animal protein elevates IGF-1 more than plant protein. In middle age, elevated IGF-1 is associated with cancer-promoting cell proliferation. In older age, elevated IGF-1 protects against the muscle wasting and frailty that drive most age-related mortality.
This is observational data, not causal. The signal is consistent with rapamycin and caloric-restriction work suggesting that growth and longevity are partial trade-offs, and the trade-off shifts with age and frailty risk. The clean implication is that the optimal protein-source mix is not constant across the lifespan.
For lifters under 50 chasing hypertrophy: animal protein remains the most efficient lever, and the IGF-1 signal is probably worth the trade-off given the muscle-mass-equals-mortality-protection signal in older age. For adults 50-65 not actively training hard, a higher proportion of plant protein looks more defensible. Above 65, the calculus reverts: higher animal protein, hit the leucine threshold, fight anabolic resistance.
Anabolic resistance and per-meal leucine
The reason the over-65 protein recommendation looks different is anabolic resistance: the same dose of leucine produces a smaller MPS response in older muscle than in younger muscle. The work-around is dose-up. Where 25 g of whey reliably triggers MPS in a 30-year-old, the equivalent stimulus in a 70-year-old is closer to 40 g.
This puts the older population in a tough spot for plant protein. They need a higher leucine quantity per meal already, and plant protein's lower leucine fraction makes hitting that threshold via plant alone (without fortification) impractical for most appetite levels.
The practical translation: a 70-year-old hitting 1.2 g/kg/day from mostly plant protein would need to eat far more total grams of food than a 30-year-old at 1.6 g/kg/day from mostly whey. Whey or animal protein, or fortified plant protein with added leucine, is the realistic compliance path.
What this means for protein-source choice
The decision tree:
- Lifter, under 50, hypertrophy goal: animal protein primary. Whey for convenience, beef/eggs/fish for the rest. Hit 1.6 g/kg/day across 4 meals at 30-40 g each.
- Adult, 50-65, general health: mixed sources, lean toward plant. Soy isolate or pea+rice blends are the workhorses. Total target 1.0-1.4 g/kg/day. Source matters more than gram count once you're past 1.0 g/kg/day.
- Adult, over 65, frailty risk: animal protein primary again. The anabolic-resistance and IGF-1 calculus both favor it. Push to 1.2-1.6 g/kg/day, with 30-40 g per meal across 3-4 meals.
- Vegan, any age: 1.6 g/kg/day total, 40 g per meal of pea or soy isolate, or 25 g plant protein plus 2 g added leucine. The evidence supports this approach, though it requires more grams and more attention to per-meal threshold.
The TMAO conversation deserves its own treatment. Animal protein produces more TMAO than plant protein. The cardiovascular signal from elevated TMAO is real but observational, and the supplement-dose contribution is small relative to dietary baseline. It is not a reason to avoid whey at 30 g per meal. It is one reason to think twice about red meat as the primary protein source seven days a week.
