Effect of high-protein feeding on performance and nitrogen balance in female cyclists
Journal Title (Medline/Pubmed accepted abbreviation): Med Sci Sports Exerc
Year: 2010
Volume: 43
Number: 1
Page numbers: 44–53
doi: 10.1249/MSS.0b013e3181e93316

Summary of Background and Research Design

Background:Research into the effect of nutritional supplementation to improve exercise performance has been conducted mostly in men. Although it appears that female athletes require ~15%-25% less dietary protein than male athletes, no specific recommendation for protein supplementation of female athletes has been proposed. Furthermore, the effect of protein consumption on exercise recovery in women is unknown.

Hypothesis/purpose of study:To determine the protein requirements during intensive training and the effect of a high- or low-protein recovery diet on performance in well-trained female cyclists.

Subjects:The study included 12 well-trained female cyclists (mean [SD]: age, 30 [7] yr; height, 166.7
[6] cm; body mass, 60.8 [3.4] kg; estimated fat-free mass [FFM], 49.5 [3.1] kg; body fat mass, 19% [3%] of total; maximal oxygen consumption (VO2max), 3.4 [0.4] L/min; peak power output [Wmax], 260 [26] W.

Experimental design:Double-blind, randomized, crossover design.

Treatments and protocol: Two experimental periods, spaced 28 days apart and beginning 3-7 days after the first day of the menstrual cycle were used to reduce the potential effect of menstruation on outcomes. Participants completed 3 high-intensity rides: 2.5 hours of interval training on day 1 and repeat-sprint performance tests on days 2 and 4. Days 1 and 2 included a 4-hour recovery following the high-intensity exercise period during which athletes ingested a recovery drink consisting of either a low-carbohydrate (CHO)/protein/fat drink (1.4/0.7/0.26 g/kg/hr) or a high-CHO/protein/fat drink (2.1/0.1/0.26 g/kg/hr) every 30 minutes for 3 hours. The last drink was taken any time within hours 3 and 4. Day 3 was a rest day. During other times, cyclists consumed an isoenergetic high-CHO diet. Blood samples were collected during the recovery periods before food, every 30 minutes for 2 hours after food ingestion (day 1) or 30, 60, and 90 minutes post-food ingestion (day 2), and at 3 hours post-food ingestion (day 1 only) for lactate, glucose, and creatine kinase levels. Urine samples were also collected from immediately following exercise on day 1 to pre-exercise on day 2, during day 2 exercise, and from the end of day 2 exercise to day 4 pre-exercise meal for assessment of nitrogen balance and estimation of protein requirement. Perceptual rating of fatigue and exertion were performed.

Summary of research findings:
  • No change in mean power during repeat sprint tests was noted on day 2 (high-protein vs control:
    • 1.1%; 95% confidence limits [CI]: ± 4.6%) or on day 4 (1.7%; 95% CI: ± 4.6%)
    • Effect of high-protein supplementation on fatigue was unclear between groups (day 2 = 1.4%; 95% CI: ± 4.9%, day 4 = 0.5%; 95% CI: ± 4.9%)
  • In the high-protein condition:
    • Leg tiredness and soreness increased; leg strength was reduced.
    • Plasma lactate concentrations were lower during the sprint exercise.
    • Plasma glucose concentrations were lower during recovery
  • The high-protein condition experienced positive net nitrogen (N) balance (mean ± SD = 177 ± 140 mg of N/kg FFM), whereas nitrogen balance was negative in controls (–81 ± 73 mg of N/kg FFM)
  • Based on these observations, estimated protein requirement for highly trained female cyclists was 1.28 g/kg/d (95% CI: 0.54, 2.67 g/kg/d).

Interpretation of findings/Key practice applications:

This study observed no benefit of increased protein recovery drink on exercise performance in female cyclists. Female cyclists undergoing an intense training regimen require ~1.6 times the recommended daily allowance of protein required for typical adults, but only 0.65 times the protein intake required by similarly trained male cyclists. One primary study limitation is that estimated, rather than measured, nitrogen losses from sweat, feces, and miscellaneous (eg, sloughed skin cells, hair loss) was used for calculations. Furthermore, few studies have been conducted to accurately measure nitrogen loss in female athletes; therefore, estimations of nitrogen loss included in this study are based on data from male athletes. It should also be noted that estimates of protein requirements in athletes are not very precise and that individual protein demand can be highly variable. A difficulty in conducting this type of study is that in the effort to keep the 2 treatments isocaloric (a scientifically valid consideration), both the carbohydrate and protein intake levels of the treatment varied. Thus, it is not clear whether the increased leg tiredness and soreness observed in the high protein condition was due to the higher protein content or the lower carbohydrate content of this treatment vs the control. This could have affected the accuracy of the nitrogen balance determination as well, because sufficient carbohydrate is important to spare protein from being oxidized for energy. This is a difficult problem to resolve, as it is also important to maintain the isocaloric nature of these treatments. One potential solution would have been to vary the fat content instead of the carbohydrate content of the diets.
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