Very recently, the first empirical measurement of nitrogen
turnover was made in endurance trained female athletes (21). The
estimated daily protein requirement to maintain nitrogen balance
was 1.63 g protein/kg/day. This value is approximately 25–30%
higher than the ones previously computed with indirect methods
in women athletes (3). In fact, it is situated within the range recommended for men undertaking a comparable average weekly
training volume. The other noteworthy outcome of this study was
the variability of individual nitrogen balance that, according to the authors, could be due to the variation in dietary macronutrient
composition and meal timing, energy intake, sex-hormone levels,
and menstrual cycle phase. All these factors are possible modifying covariates and should be further investigated (21).