| It is not clear at present whether lysine requirement is different between genders. The primary objective was to investigate whether lysine requirement varied between genders and was affected by phases of the menstrual cycle. Lysine requirement was determined by the indicator amino acid oxidation (IAAO) method using an oral tracer protocol. L[1-13C]phenylalanine was used as the indicator amino acid (IAA). Because the less invasive oral tracer protocol has never been compared to the conventional intravenous (IV) tracer protocol in humans, the secondary objective was to investigate whether splanchnic uptake of the IAA during the fed state would alter the estimate of lysine requirement. Finally, a new statistical approach, resampling technique (bootstrap procedure), was applied to define EAR and RDA of lysine for the population.; Five healthy men were studied at graded levels of lysine intake, with either an oral or IV tracer protocol in a randomized crossover design. The absolute rates of phenylalanine oxidation were higher when the tracer was given orally (p < 0.001) due to the splanchnic uptake of the IAA. However, lysine requirements did not differ between oral and IV tracer administrations, as both yielded a similar mean requirement of 36.6 mg·kg−1 ·d−1. This protocol was then used to determine lysine requirements in five women, in both the follicular and luteal phases. The lysine requirement of men (36.6 mg·kg−1·d −1) as expressed per unit of body weight was significantly (p = 0.001) higher than that of women in the follicular phase (30.9 mg·kg −1·d−1), but was not different in the luteal phase (38.8 mg·kg−1·d −1). When requirement was expressed per unit of LBM, the lysine requirement of men (40.5 mg·kg LBM−1·d −1) and women in the follicular phase (43.1 mg·kg LBM −1·d−1) were not different, but these levels were significantly (p < 0.0001) lower than that of women in the luteal phase (62.7 mg·kg−1·d −1).; Because the lysine requirements for men and for women in the luteal phase are similar when expressed per unit of body weight, it is proposed that the EAR and the RDA of lysine for both men and women should be the same, at 36–39 and 49–51 mg·kg−1·d−1 , respectively. |
