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

Abstract

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.

Keywords: amino acid requirement; carbon oxidation method; elderly; indicator amino acid oxidation; indispensable amino acid; leucine requirement; stable isotope.

FIGURE 1

CONSORT flow chart of the study determining leucine requirement in (A) older males, and (B) older females, >60 y old, using the IAAO method. IAAO, indicator amino acid oxidation.

FIGURE 2

Effect of leucine intake on production of ¹³CO₂ from phenylalanine oxidation (F¹³CO₂) by IAAO in older adults aged >60 y, n = 16 subjects, 93 IAAO studies. Biphasic linear regression analysis of the F¹³CO₂ data identified a breakpoint of 77.8 mg ^. kg^{−1 .} d⁻¹, R² 0.45, P < 0.0001, for the males and 78.2 mg ^. kg^{−1 .} d⁻¹, R² 0.47, P < 0.0001, for the females (A), which represents the estimated mean leucine requirement. The 95% CIs of the estimates were 74.7, 81.0, and 74.5, 82.0 mg ^. kg^{−1 .} d⁻¹ for males and females, respectively. The breakpoint and the 95% CIs of the breakpoints for males and females combined (B) were determined to be 78.5 mg ^. kg^{−1 .} d⁻¹, and 76.1, 81.0 mg ^. kg^{−1 .} d⁻¹, R² 0.456, P < 0.0001. IAAO, indicator amino acid oxidation.

FIGURE 3

Leucine requirement per kg fat-free mass in older males and females who completed the study to determine leucine requirements in adults aged >60 y. Different superscripts show significant differences, P = 0.005, in leucine requirement per kg lean body mass (LBM) between males and females, compared by t-test.