Dietary Tryptophan Requirements in Adults Aged >60 Years Is Not Affected by Sex or Decades of Age

Current dietary tryptophan recommendation of 4 mg/ kg/d, is based on a study in young females. However, evidence suggests that amino acid requirements may differ between young and old adults.

The objective of this study was to determine the tryptophan requirement in healthy males and females aged ≥60 y.

Tryptophan requirement was determined using the indicator amino acid oxidation method with L-[1-¹³C] phenylalanine as the indicator. A total of 23 older adults, 11 healthy older males (n = 6, 60-69 and n = 5, 70-79 y) and 12 healthy older females (n = 6, 60-69 and n = 6, 70-79 y), were studied at 4-7 randomly assigned tryptophan intake levels (0.5, 2, 3, 5, 7, 10, and 14 mg/ kg/d). Mean tryptophan requirements were determined from the oxidation of L-[1-¹³C] phenylalanine using a 2-phase linear mixed-effects model stratified by sex and age, with 95% confidence interval (CI) calculated using parametric bootstrap. The overlap of the 95% CIs between the age and sex groups was used to compare the requirement estimates.

A total of 144 studies were analyzed, with participants stratified by sex and decade group. The estimated tryptophan requirement was 4.3 mg/ kg/d for the 60-69-y group (R²_m = 0.23, R²_c = 0.86, 95% CI: 3.2 mg/kg/d, 5.4 mg/kg/d) and 3.6 mg/ kg/d (R²_m = 0.44, R²_c = 0.82, 95% CI: 3.2 mg/kg/d, 4 mg/kg/d) for 70-79-y 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/d (R²_m = 0.30, R²_c = 0.84, 95% CI: 3.3 mg/kg/d, 4.5 mg/kg/d).

To our knowledge, this is the first study to establish the tryptophan requirement in older adults using the isotopic method. These findings will guide dietary recommendations and support precision nutrition for healthy aging. The trial was registered at clinicaltrials.gov as NCT06283706 (https://clinicaltrials.gov/study/NCT06283706).