Reference intervals of bioelectrical impedance analysis-derived phase angles of community-dwelling older adults in Japan.

BACKGROUND & AIMS: The phase angle may serve as a clinically relevant indicator of skeletal muscle health beyond conventional muscle mass assessments; however, reference intervals for community-dwelling older adults remain scarce. This study aimed to establish sex- and age-specific reference intervals for bioelectrical impedance analysis-derived phase angles in older adults in Japan. METHODS: This cross-sectional study used pooled data from four community-based cohorts within the Tokyo Longitudinal Study on Aging, with available bioelectrical impedance analysis measurements. Data were collected between 2022 and 2024. The phase angle was measured using an InBody 770. Individuals without functional impairments were included, whereas those undergoing cancer treatment were excluded. After stratification by sex and 5-year age groups, the percentile values (at quartiles as well as at 2.5%, 10%, 90%, and 97.5%) for the phase angles were calculated. Phase-angle distributions were parameterized using a generalized additive model for the location, scale, and shape algorithms. The Jonckheere-Terpstra test was used to examine whether the phase angle decreases monotonically with increasing age. RESULTS: A total of 4,484 adults aged 65-96 years (1,885 men, 2,599 women; mean age 76.2 years) were included. According to Schwarz's criteria, the phase angle follows a normal distribution in both sexes. For whole-body phase angles, the median ranged from 5.2 to 4.2 in men and from 4.4 to 3.9 in women across age groups from 65-69 and 85-89 years, showing a consistent decline with age. CONCLUSIONS: This study established sex- and age-specific reference intervals for phase angles among community-dwelling older adults in Japan, including those aged ≥80 years. These intervals may facilitate clinical interpretation of the phase angle and support the assessment of skeletal muscle health in older populations.