The mechanism that makes older age a genuine protein-intake stratum, not just a demographic label. Anabolic resistance is the age-related blunting of the muscle-protein-synthesis (MPS) response to ingested protein/amino acids — the muscle needs a larger per-meal stimulus to switch on the same building response a younger muscle reaches easily. It is the single cause that ties three otherwise separate findings together, and it is the support factor the older-adult protein target rests on (the effect-modification route on Protein Intake for Older Adults).
(inferred from Bauer et al., 2013; Katsanos et al., 2006; Moore et al., 2014)
The one mechanism, three downstream manifestations
The value here is that a leucine-sensitivity finding, a per-meal dose-response, and a daily-target consensus are the same phenomenon measured at three points on one causal chain — not three separate facts. The chain: aging lowers the sensitivity of MPS to a protein meal -> a higher per-meal leucine share is needed to trigger the response -> a higher per-meal protein dose is needed to saturate it -> a higher daily total follows if every meal must clear the raised threshold.
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Definition + causes (Bauer). «Abnormal muscle protein anabolism may result from inadequate nutritional intake (lower anabolic signal) or from impaired response to nutrients and hormones (lower sensitivity), that is, anabolic resistance» (Bauer et al., 2013). The contributing factors named across the sources: dysregulated intracellular signalling, reduced postprandial nutritive blood flow, subclinical chronic inflammation, greater splanchnic (first-pass) extraction of amino acids, and reduced habitual activity (Moore et al., 2014); the specific initiation machinery (S6K1 / eIF4) is named by Katsanos and Bauer, not Moore (Katsanos et al., 2006).
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It is a FED-state / low-dose defect, not a basal one. Both metabolic sources find no age difference in basal (postabsorptive) MPS: «There was no difference (p = .53) in basal MPS rates between older (0.027 ± 0.04%/h …) and young (0.028 ± 0.03%/h) men» (Moore et al., 2014), and Katsanos found no between-group basal FSR difference. The loss is specifically in the response to feeding, and mostly at low protein doses — old muscle «retains the capacity for enhanced rates of MPS, but only with sufficient nutritional stimulation» (Moore et al., 2014). This is why the fix is dose and distribution, not a lost capacity.
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Manifestation 1 — the leucine threshold (Katsanos). In the elderly, a whey-patterned EAA bolus (6.7 g EAA, 26% leucine = 1.7 g) did not raise MPS, but the same EAA dose enriched to 41% leucine (2.8 g) did: «increasing the proportion of leucine in a mixture of EAA can reverse an attenuated response of muscle protein synthesis in elderly but does not result in further stimulation of muscle protein synthesis in young subjects» (Katsanos et al., 2006). The reversal was insulin-independent (insulin rose equally in both arms) — a direct leucine effect on the synthesis machinery (S6K1 / eIF4 translation-initiation). Leucine is the trigger amino acid; aging raises the trigger dose. (Katsanos was a parallel-group study of both sexes, ~8-10/arm.)
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Manifestation 2 — the per-meal dose breakpoint (Moore). The MPS dose-response saturates at a higher relative per-meal dose in older men — «MPS reached a plateau after ingestion of 0.40 ± 0.19 and 0.24 ± 0.06 g/kg body mass … and 0.60 ± 0.29 and 0.25 ± 0.13 g/kg lean body mass … in older and younger men, respectively» (Moore et al., 2014). The breakpoint sits «~68% and ~140% greater than younger men» relative to body mass and lean mass respectively (Moore et al., 2014); a rightward shift with a lower first-segment slope — the geometric signature of reduced sensitivity to small doses.
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Manifestation 3 — the daily target (Bauer, consensus). Because every meal must clear the raised threshold, the daily recommendation rises above the general-adult RDA. The magnitudes and the renal contraindication live on -> Protein Intake for Older Adults; here the point is only that the daily number is a consequence of the per-meal mechanism, not an independent finding.
These are DIFFERENT quantities — the mechanism unifies the CAUSE, not the magnitudes
The three findings share a cause; they are not commensurable numbers. Leucine g/meal, protein g/kg/meal, and protein g/kg/day are three different denominators, and the synthesis is legitimate only because it claims a shared mechanism, never a shared value. Do not equate 2.8 g leucine/meal with 0.40 g/kg/meal with 1.2 g/kg/day — each answers a different question on a different axis. (The per-meal-vs-per-day denominator table is on Protein Intake for Older Adults.)
Not independent backing — one research programme
(inferred from Bauer et al., 2013; Katsanos et al., 2006; Moore et al., 2014) This is a type-A emergent synthesis, explicitly NOT type-E independent corroboration. The three sources are one research community, so their convergence cannot be counted as independent routes:
- Shared authors. Stuart Phillips is a co-author of both Moore 2014 and the Bauer PROT-AGE consensus; Robert Wolfe’s UTMB group produced Katsanos, and its members recur across the others’ reference lists.
- Shared citation. Bauer’s consensus cites Katsanos; Moore cites Bauer and Katsanos as antecedents. Moore’s own «three times ~0.40 g/kg or ~1.20 g/kg/d» arithmetic landing on Bauer’s 1.0-1.2 g/kg/d target is therefore a within-programme consistency check, not two methods arriving separately.
The convergence raises coherence, not confidence-through-independence. Genuinely independent confirmation (a different school, a hard-outcome trial) is a standing gap — now partly filled by Komar 2015 (below), a different-school chronic-outcome MA; it confirms the DIRECTION on mass but deflates the surrogate optimism on strength.
The chronic-outcome test — does the acute MPS surrogate translate? (Komar)
(Komar et al., 2015) Everything above is acute MPS — a surrogate. Komar 2015 (gold SR+MA, 16 RCTs, 999 subjects aged >=65, leucine 2-7.8 g/d for >=10 days) is the chronic clinical-outcome test the metabolic sources’ own open question demanded. It is a type-F surrogate->outcome upgrade, not independent (type-E) corroboration: the Vienna group is a different school, but it cites Katsanos as its rationale, so agreement on the leucine premise is downstream, not a separate route. The verdict is split, and the split is the value:
- Mass — the surrogate translates, but only in the deficit stratum. Leucine-rich protein raised lean body mass +0.99 kg overall [95% CI 0.43, 1.55; p=0.0005], but subgroup analysis puts the whole effect in the sarcopenic stratum (LBM +1.14 kg [0.55, 1.74] in sarcopenia vs -0.05 kg [-1.55, 1.46], null, in healthy elderly). Body weight follows the same pattern (+0.75 kg in sarcopenia, null in healthy). So the anabolic-resistance fix buys measurable mass where there is a deficit to repair, and ~nothing in already-replete healthy elderly -> a repletion-not-enhancement reading (Deficiency Repletion vs Enhancement).
- Strength — the surrogate does NOT translate (the decision-relevant discordance). Neither «hand grip strength nor knee extension strength were affected by leucine supplementation in a fashion significantly different from control interventions» — grip WMD +0.23 (pounds/inch2) [-0.26, 0.73; p=0.36; I2=65%], knee extension +0.07 Nm/kg [-0.26, 0.40; p=0.68; I2=48%]. A leucine-driven MPS/mass gain that does not show up as measured strength is a surrogate-outcome discordance -> Surrogate Outcomes: the chain MPS -> LBM moved while the more patient-important endpoint (strength/function) did not.
- But the strength null is under-powered, not a clean refutation. Komar attributes it to design — «most outcome parameters could be extracted from only a small number of trials potentially insufficient to yield significant results» — and other SRs with broader inclusion (younger, exercise co-intervention) did find grip / leg-press gains. Grip I2=65% is a disperse null. So strength sits at insufficient evidence, not confident no effect (the expectancy test fails) — hold the discordance as a live warning, not a settled null.
- The chronic data cannot isolate leucine, and locates no dose knee. Komar’s own limitation: the effect is «not due to this distinct amino acid but at least in part to other ingredients» and «an increase in total energy consumption might be a prerequisite … with respect to parameters such as LBM». So at the chronic-outcome level the exposure is leucine-rich protein + energy, not isolated leucine — a caveat on the leucine-specificity that Manifestation 1’s acute design could hold constant but a whole-food chronic trial cannot. And with doses spanning 2-7.8 g/d at high heterogeneity, no chronic dose-response knee is locatable; the >=2 g/d inclusion floor is a design choice, not a confirmed threshold -> The Underivable Optimum. Glucose markers (FG, FI, HOMA, albumin) were all null in the pooled analysis; that pool spans the two DM2 trials (Leenders, Scognamiglio), though Komar reports no diabetes-specific subgroup (inferred from Komar et al., 2015) — so the insulin-independent MPS pathway did not read out as glycaemic benefit.
Decision relevance
- The fix follows the mechanism: raise the per-meal stimulus, not just the daily total. Three levers, all mechanism-grounded: (1) enough protein per meal to clear the breakpoint (~25-30 g), (2) enough leucine per meal (a high-quality/animal or leucine-enriched source), and (3) resistance exercise, which improves muscle sensitivity to nutrients and partly reverses anabolic resistance -> Protein and Resistance Training for Muscle and Strength. Spreading protein across meals matters because of the threshold — a day’s protein loaded into one meal wastes the doses that fell short.
- It reframes the elderly protein question as effect-modification, not preference. An older adult is not the same curve, shifted for taste — the MPS dose-response curve is measurably different (route (b) on the transportability menu), which is what licenses a stratum-specific target rather than the population RDA.
- It is the mechanism behind two other pages. The lean-mass loss when protein falls at meals in a short eating window (Time-Restricted Eating) and the inadequate protein intake secondary cause of sarcopenia (Sarcopenia Definition and Diagnosis) are both anabolic resistance meeting a low or poorly-distributed intake.
Limits — surrogate, small, single-school
(Katsanos et al., 2006; inferred from Moore et al., 2014)
- Surrogate endpoint — now partly cashed one step down the chain. The mechanism (Manifestations 1-3) is measured as acute MPS / fractional synthetic rate — a surrogate for mass, itself a surrogate for function and mortality -> Surrogate Outcomes. Komar (above) advances the surrogate ONE step: chronic leucine-rich protein does raise lean mass (in sarcopenic elderly), but the further step to strength was null, and no fracture / disability / mortality trial exists — so the loop is now open on function and hard outcomes, closed only on the mass surrogate in the deficit stratum, and even that cannot separate leucine from protein/energy.
- Small, acute. Moore is a retrospective pool of small tracer studies (men only); Katsanos is a tiny acute parallel-group study (n~8-10/arm, both sexes); single boluses over 3-4 h. Whether the acute breakpoint predicts chronic muscle maintenance is the authors’ own stated open question.
- Sponsor exposure. The programme’s funding sits with companies selling the implied products (Katsanos: Ajinomoto, an amino-acid maker; Moore: US Dairy / GlaxoSmithKline) — a directional-bias flag under symmetric standards, noted not to dismiss the tracer data but to hold the supplement/leucine emphasis loosely.