Dietary leucine requirement of older men and women is higher than current recommendations.

Background: Current national (34 mg . kg-1 . d-1) and international (39 mg kg-1 . d-1) recommendations for leucine in older adults are based on data from young adults. Evidence suggests that the leucine requirements of older adults are higher than those of young adults.

Objective: The objective of the current study was to directly determine the leucine requirements in healthy older adult male and female study participants aged >60 y.

Methods: Leucine requirement was determined using the indicator amino acid oxidation method (IAAO) with l-[1-13C]phenylalanine as the indicator. Sixteen older adults (n = 7 male and n = 9 female participants) were randomly assigned to receive 3 to 7 leucine intakes from 20 to 120 mg . kg-1 . d-1. The rate of release of 13CO2 from l-[1-13C]phenylalanine oxidation was measured, and breakpoint analysis was used to estimate the leucine requirement. The 95% CI was calculated using the parametric bootstrap method.

Results: The mean leucine requirement for male participants was 77.8 mg . kg-1 . d-1 (upper 95% CI: 81.0) and for female participants, it was 78.2 mg . kg-1 . d-1 (upper 95% CI: 82.0) with no sex effect based on body weight. The data were therefore combined to yield a mean of 78.5 mg . kg-1 d-1 (upper 95% CI: 81.0 mg . kg-1 . d-1 ) for both sexes. On the basis of fat-free mass, the mean ± SEM leucine requirements were 115 ± 3.2 and 127 ± 2.4 mg . kg-1 . d-1 for male and female participants, respectively (P < 0.005).

Conclusions: The estimated leucine requirement of older adults is more than double the amount in current recommendations. These data suggest that leucine could be a limiting amino acid in the diet of older adults consuming the current RDA for protein and those consuming a plant-based diet. In view of the functional and structural role of leucine, especially its importance in muscle protein synthesis, current leucine recommendations of older adults should be revised. This trial was registered at clinicaltrials.gov as NCT03506126.