Dietary Tryptophan Requirements in Adults Aged Over 60 Years is not Affected by Sex or Decades of Age.

BACKGROUND: Current dietary tryptophan recommendation of 4 mg. kg-1.d-1, is based on a study in young females. However, evidence suggests amino acid requirements may differ between young and old adults. OBJECTIVES: The objective of this study was to determine the tryptophan requirement in healthy males and females aged ≥ 60 y. METHODS: Tryptophan requirement was determined using the indicator amino acid oxidation method (IAAO) with L-[1-13C] phenylalanine as the indicator. 23 older adults: 11 healthy older males (n=6, 60-69 and n=5, 70-79 years) and 12 healthy older females (n=6, 60-69 and n=6, 70-79 years) were studied at 4 to 7 randomly assigned tryptophan intake levels (0.5, 2, 3, 5, 7, 10, 14 mg. kg-1.d-1). Mean tryptophan requirements were determined from the oxidation of L-[1-13C] phenylalanine using a two-phase linear mixed effect model stratified by sex and age, with 95% CI calculated using parametric bootstrap. The overlap of the 95% confidence intervals between the age and sex groups were used to compare the requirement estimates. RESULTS: A total of 144 studies were analyzed, with participants stratified by sex and decade group. The estimated tryptophan requirement was 4.3 mg. kg-1.d-1 for the 60-69-year group (R2m= 0.23, R2c= 0.86, 95% CI [3.2, 5.4]) and 3.6 mg. kg-1.d-1 (R2m= 0.44, R2c= 0.82, 95% CI [3.2, 4]) for 70-79-year group, with no significant sex differences observed. As no significant differences were observed between age groups, pooled analysis estimated an overall tryptophan requirement of 3.9 mg·kg-1·d-1 (R2m = 0.30, R2c = 0.84, 95% CI: 3.3-4.5 mg·kg-1·d-1). CONCLUSIONS: This is the first study to establish the tryptophan requirement in older adults using isotopic method. These findings will guide dietary recommendations and support precision nutrition for healthy aging. THE TRIAL WAS REGISTERED AT CLINICAL TRIALS REGISTRY: # NCT06283706, https://clinicaltrials.gov/study/NCT06283706.