Protein requirements are elevated in endurance athletes after exercise as determined by the indicator amino acid oxidation method
Participants
n=6 endurance-trained men, 28 ± 4 y, body weight 64.5 ± 10.0 kg, VO2peak 60.3 ± 6.7 ml/kg/min, studied during an acute 3-day controlled training period
Protocol
Days 1-2: controlled diet 1.4 g protein/kg/day + training runs of 10 km and 5 km (day 1 and day 2 respectively)
Day 3: acute endurance exercise (20 km treadmill run) → then test diets providing variable protein (0.2-2.8 g/kg/day) with sufficient energy; protein given as a crystalline amino acid mixture patterned on egg protein with [1-13C]phenylalanine as the indicator
IAAO method: whole-body phenylalanine flux, 13CO2 excretion and phenylalanine oxidation measured at each dose; the estimated average requirement (EAR) = breakpoint of a biphasic linear regression, the recommended intake = upper 95% confidence interval
Key Results
| Parameter | Value | Context |
|---|---|---|
| EAR (estimated average requirement) | 1.65 g/kg/day | biphasic-regression breakpoint of 13CO2 excretion (R² = 0.86) |
| Recommended protein intake | 1.83 g/kg/day | upper 95% CI of the breakpoint |
| Same, from phenylalanine oxidation | 1.53 and 1.70 g/kg/day | similar to the 13CO2-based values |
| Phenylalanine flux | 68.8 ± 8.5 μmol/kg/h | not affected by protein intake |
| Range of tested doses | 0.2-2.8 g/kg/day | full dose-response curve |
| RDA (general population) | 0.8 g/kg/day | the recommended intake here is ~2.3× the RDA (1.83/0.8, arithmetic) |
| Current endurance-athlete recommendation | 1.2-1.4 g/kg/day | based mainly on nitrogen balance; exceeded here |
Breakpoint 1.65 g/kg/day: below it, 13CO2 excretion falls with each increment of intake (amino acids being spared from oxidation); above it, no further change
1.83 g/kg/day is the intake covering the upper 95% CI of that breakpoint in this sample
Authors' conclusion: the metabolic demand for protein in endurance-trained adults on a higher-volume training day is greater than in sedentary peers and than the 1.2-1.4 g/kg/day recommendations derived from nitrogen balance
Mechanisms
Inference, not tested here: endurance exercise raises amino acid oxidation and post-exercise remodelling raises amino acid uptake, so more dietary protein is needed before oxidation of the indicator stops falling
Method point (abstract): IAAO measures the requirement directly from indicator oxidation rather than inferring it from nitrogen balance, which the authors regard as the source of the lower older recommendations
Practical Application
In n=6 endurance-trained men (VO2peak 60.3) on the day of a 20 km run after two controlled training days, the IAAO breakpoint gave an estimated average protein requirement of 1.65 g/kg/day and a recommended intake of 1.83 g/kg/day (13CO2; 1.53 and 1.70 by phenylalanine oxidation) — above the 0.8 g/kg/day RDA and the 1.2-1.4 g/kg/day nitrogen-balance recommendations for endurance athletes. On a higher-volume training day, 1.83 g/kg is the evidence-based target, not a ceiling.
Arithmetic on the abstract's numbers: for an 80 kg athlete 1.65 g/kg is 132 g and 1.83 g/kg is 146 g per day.
Not from this paper; the base's own practice: set the daily target from body mass at ~1.8 g/kg and adjust upward, not downward, in a deficit or on rest days.
Limitations
n = 6, males only, three-day controlled period. The IAAO method is a gold standard for requirement estimation, but this sample is tiny and the window acute.
Requirement was measured after a single 20 km run — one exercise dose, one modality, one day. Whole-training-week requirements are inferred, not measured.
Trained young men; masters athletes, women and athletes in an energy deficit are outside the sample (see REF-725 for the sex question, answered later and separately).
IAAO estimates the intake that maximises whole-body protein synthesis over hours; it does not demonstrate a performance or body-composition benefit at that intake.
Energy intake was sufficient by design — the requirement under a deficit is a different and higher question.
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
"2× lower than endurance athlete needs" -> 1.83/0.8 ≈ 2.3× (arithmetic stated); "safe threshold for 97.5% of the population" -> abstract: upper 95% CI of the breakpoint.
"1.95-2.0 g/kg (Witard 2025) = 156-160 g", "140-160 g/day strategy", "Optimum 146 g baseline recommendation" -> REF-151 says "may exceed 2.0"; house totals removed, the 80 kg arithmetic kept and labelled.
Added the abstract's phenylalanine-oxidation estimates (1.53/1.70), flux (68.8 ± 8.5), participant descriptors (28 ± 4 y, 64.5 kg, VO2peak 60.3) and the trial registration.
Mechanisms labelled inference; "post-exercise requirement higher than rest (confirmed by Moore 2024)" -> REF-152 found the rest day highest at a fixed intake; reworded as a pointer.