A snapshot of nitrogen balance in endurance-trained women
Abstract
Indirect estimates of the mean daily protein requirement for female endurance athletes are 1.2–1.4 g·kg−1·day−1; however, an empirical estimate using nitrogen balance is absent. A 72-h nitrogen balance was determined during the mid-follicular phase of 10 female cyclists and triathletes training for 10.8 h·week−1 (SD 2.8) following 2 habituated protein intakes: (i) normal habitual (NH) (protein 85 g·day−1), and (ii) isocaloric high-protein (HP) (∼2-fold increase in protein). Total 72-h nitrogen intake was determined from Leco total combustion of ingested food samples. Nitrogen loss was determined from micro-Kjeldahl analysis of 72-h total urinary nitrogen and representative resting and exercise sweat output, plus estimates for fecal and miscellaneous losses. Habituated (steady state) protein requirement was estimated from the mean regression of adapted nitrogen balance vs nitrogen intake. Mean (SD) 24-h dietary protein and energy intake was NH: 1.4 g·kg−1·day−1 (0.2), energy: 9078 kJ·day−1 (1492), HP: 2.7 g·kg−1·day−1 (0.3) 8909 kJ·day−1 (1411). Average 24-h urinary nitrogen and sweat urea nitrogen outputs were 13.2 g·day−1 (2.4) and 0.33 g·day−1 (0.08) in NH; 21.5 g·day−1 (3.9) and 0.54 g·day−1 (0.12) in HP, respectively. Nitrogen balance was negative in NH (−0.59 gN·day−1 SD 1.64) but positive in HP (2.69 gN·day−1 SD 3.09). Estimated mean protein requirement was 1.63 g·kg−1·day−1 (95% confidence interval: 1.1–3.8). In conclusion the snapshot of follicular phase dietary protein requirement conformed with previous estimates for men, but was higher than previous nonempirical estimates for endurance-training women; low self-selected energy and carbohydrate intakes may explain the higher than expected nitrogen turnover, and consequently protein requirement.
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These protein requirements are within the mid-range of current sports nutrition guidelines (1.4-2.0 g·kg−1·day−1).
“noting these requirements are within the mid-range of current sports nutrition guidelines (1.4–2.0 g·kg−1·day−1) [229,230]”
From International society of sports nutrition position stand: nutritional concerns of the female athlete (2023) · cites this paper as [230]
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- Authors
- Houltham, S. D., & Rowlands, D. S.
- Affiliations
- Massey University (NZ)
- ORCID
- Rowlands, David S.
- Venue
- Applied Physiology, Nutrition, and Metabolism
- Publisher
- Canadian Science Publishing
- Published
- 2014-02
- Volume
- 39
- Issue
- 2
- Pages
- 219-225
- ISSN
- 1715-5312 (ISSN-L)
- 1715-5320
- OpenAlex
- W1855806657
- Cited by
- 28
- References
- 29
- Citations by year
- 2025: 2 · 2024: 2 · 2023: 4 · 2022: 3 · 2021: 6 · 2020: 2 · 2019: 3 · 2017: 2 · 2016: 2 · 2015: 2
- FWCI
- 0.77
- Type
- journal article
- Language
- en
- Open access
- closed
- Locations
- Applied Physiology, Nutrition, and Metabolism (publishedVersion)
- PubMed (publishedVersion)
- Funders
- Massey University
- Topics
- Life Sciences › Biochemistry, Genetics and Molecular Biology › Cell Biology › Muscle metabolism and nutrition
- Health Sciences › Medicine › Orthopedics and Sports Medicine › Sports Performance and Training
- Health Sciences › Medicine › Complementary and alternative medicine › Cardiovascular and exercise physiology
- Keywords
- nitrogen balance, protein requirements, female endurance athletes, high-protein diet, urinary nitrogen, follicular phase, cyclists, triathletes, energy intake, nitrogen turnover
- MeSH terms
- Adult, Female, Humans, Nitrogen, Physical Endurance, Sports
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