Protein requirements may be lower on a training compared to rest day but are not influenced by moderate training volumes in endurance trained males
Participants
Endurance-trained men (sample size not given in the abstract); IAAO protocol under 3 conditions: recovery day (REST), after a 10 km run, after a 20 km run.
Protocol
Single suboptimal protein intake: 0.93 g/kg/day, the same on every study day
IAAO: Indicator Amino Acid Oxidation method — phenylalanine excretion (F13CO2) as an inverse proxy for whole-body protein synthesis; phenylalanine net balance
3 conditions: rest day vs recovery after 10 km vs recovery after 20 km
Key Results
F13CO2 (inverse proxy of protein synthesis): greatest on REST, lower after exercise → whole-body protein synthesis higher after training at the same intake
Phenylalanine net balance: lowest on REST → at a fixed suboptimal intake, the rest day is the day the body retains least
10 km vs 20 km: no difference → moderate variations in training volume did not change the response
Single-point F13CO2 was indistinguishable from past IAAO studies that used multiple protein intakes (validates the single-intake design)
Authors' conclusion: protein requirements may be greatest on recovery days and are not influenced by moderate training volumes in endurance athletes
Mechanisms
Inference, not tested here: after exercise, muscle takes up more amino acids for repair and remodelling, so less phenylalanine is oxidised at the same intake; on a rest day that pull is absent and more is oxidised. Volume within the moderate range (10 vs 20 km) did not change the oxidative loss enough to shift the requirement.
Practical Application
In endurance-trained men eating a fixed 0.93 g/kg/day, whole-body protein synthesis (inverse F13CO2) was lowest and phenylalanine net balance was lowest on the rest day compared with recovery after a 10 km or 20 km run, with no difference between the two run distances — so protein requirement may be highest on recovery days and does not rise with moderate volume. Do not cut protein on rest days; if anything, the rest day is the day to hold it up.
Consistent with the review in REF-151: ~1.8 g/kg/day as the advocated intake, possibly exceeding 2.0 g/kg/day on rest days and during carbohydrate-restricted training (those numbers are REF-151's, not this paper's).
Not from this paper; the base's own practice: an athlete tracking protein should keep the rest-day total at or above the training-day total rather than scaling it down with the training load.
Within moderate volumes a longer session did not raise the requirement — Inference, not tested: a 90-min and a 3-h endurance session likely carry the same daily protein target; very high volumes and depleted states were not studied.
Limitations
IAAO in endurance-trained males only, acute measurement over single days — not a training-block or body-composition outcome.
The training-day-versus-rest-day comparison used moderate volumes; the finding explicitly does not extend to high-volume or depleted states.
This entry sits in apparent tension with [REF-156] and [REF-725], which report elevated post-exercise requirements. The likeliest reconciliation is that they answer different questions — acute post-exercise requirement versus whole-day requirement — but that reconciliation is inference, and SYNTH-22 is the procedure for handling it.
Small sample (n not in the abstract), males, one laboratory; women were not studied and are addressed separately in REF-725.
Carbohydrate availability was not the manipulated variable here; the interaction is covered in REF-726.
Single suboptimal intake design: the study infers requirement direction, it does not estimate a breakpoint.
How this entry was checked and changed
Why this level
regraded 2026-09-27 from A: small single study (IAAO)
Content correction 2026-09-27
"Confirms Witard 2025: 2.0 g/kg on recovery day vs 1.8 g/kg on training day" -> REF-151 says ~1.8 g/kg/day advocated and "may exceed 2.0" on rest days; reworded and attributed to REF-151.
"160 g / 144 g" example and "90 min Z2 and 3 h long ride require equal protein" -> house arithmetic/inference; relabelled (base practice / inference), numbers removed.
Mechanisms labelled inference; the abstract's validation point (single-point F13CO2 matches multi-intake IAAO studies) added.
Sample size: not in the abstract; stated as such instead of implied.