REF-725 · Williamson et al., 2023 · Medicine & Science in Sports & Exercise, 2023 Oct 1;55 · Paper · PubMed
Endurance athletes need ~1.85 g/kg/day after training
And the requirement is the SAME for women and men (1.61 vs 1.60 g/kg/day breakpoint, p = 0.94)
Participants
n=15 endurance-trained athletes: 8 males (30 ± 3 y, 78.6 ± 10.5 kg, VO2peak 75.6 ± 7.5 ml/kgFFM/min) and 7 females (30 ± 4 y, 57.7 ± 5.0 kg, 77.5 ± 7.1 ml/kgFFM/min), the females studied in the midluteal phase.
Protocol
2 days of controlled diet (1.4 g protein/kg/day) and training (10 km/day run for males, 5 km/day for females), then a 20-km run before an at-home indicator amino acid oxidation trial
Three protein intakes — 0.2 (suboptimal), 1.2 (moderate) and 2.0 (excess) g·kg⁻¹·d⁻¹ — given as a crystalline amino acid mixture containing [1-13C]phenylalanine; whole-body phenylalanine flux and oxidation (PheOx, the reciprocal of whole-body protein synthesis) measured from breath and urine
A modified biphasic linear regression found each participant's PheOx breakpoint to generate an estimated average intake maximising whole-body protein synthesis, per sex
Notably run in a home-based setting rather than a metabolic ward
Key Findings
| Breakpoint (EAR) | Recommended intake (upper 95% CI) | |
|---|---|---|
| Males | 1.60 g·kg⁻¹·d⁻¹ | 1.81 g·kg⁻¹·d⁻¹ |
| Females | 1.61 g·kg⁻¹·d⁻¹ | 1.89 g·kg⁻¹·d⁻¹ |
PheOx differed between all three protein intakes (P < 0.01) with no effect of sex (P = 0.63)
The breakpoint did not differ by sex (p = 0.94). The two breakpoints are one hundredth of a gram apart.
Authors' conclusion: endurance athletes taking protein toward the upper end of current consensus (~1.85 g·kg⁻¹·d⁻¹) will maximise whole-body protein synthesis in post-exercise recovery regardless of sex.
This sits above the older endurance guidance of 1.2-1.4 g·kg⁻¹·d⁻¹ and lines up with REF-156, which reached 1.83 g·kg⁻¹·d⁻¹ by the same method in men.
Practical Application
In 8 male and 7 female endurance-trained runners (females midluteal) studied after a 20-km run by indicator amino acid oxidation, the protein intake that maximised whole-body protein synthesis was 1.60 g/kg/day in men and 1.61 in women (p = 0.94), with recommended intakes of 1.81 and 1.89 g/kg/day — so give female and male endurance athletes the same per-kilogram target, ~1.85 g/kg/day on training days. There is no evidence here for a separate female number, and inventing one is the more common error.
The working target is ~1.8-1.9 g·kg·d on training days, which is meaningfully higher than the 1.2-1.4 g·kg·d many endurance athletes still carry from older guidance.
Per kilogram, not per meal-plan. A 58 kg runner needs ~105 g/day and a 79 kg runner ~145 g/day — the same rule producing very different plates.
Limitations
n = 15 (7 women), single study, acute. IAAO estimates a requirement over hours, not a training-season outcome; no performance or body-composition endpoint was measured.
Three intake levels only (0.2, 1.2, 2.0 g·kg·d) — the breakpoint is interpolated from a coarse grid.
Home-based administration improves ecological validity but loosens control over diet and activity compared with a metabolic ward.
Females were tested only in the midluteal phase; a null sex difference in this design does not establish that the requirement is stable across the cycle.
Running only (20-km run after run training); trained endurance athletes around 30 — not masters, not novices, not athletes in an energy deficit.
How this entry was checked and changed
Content correction 2026-09-27
Byline "Williamson, McKay, Ross, Peeling, Burke, Devlin, Bandegan, Moore & Trommelen, 2023" -> Williamson E, Fung HJW, Adams CP, West DWD, Moore DR (Crossref).
Sport "running, cycling, endurance" -> running (20-km run after 2 days of run training).
Added: females studied in the midluteal phase only; controlled diet 1.4 g/kg and VO2peak per group; PheOx no sex effect (P = 0.63); distribution rule marked as not from this paper.