Fat Mass Accounts for Insulin Resistance Impact on Functional Decline and Mortality in Nondiabetic Older Adults

Objective: To assess the potential role of body composition in the association of insulin resistance (IR) with functional decline and mortality in nondiabetic older persons. Design: Longitudinal population-based cohort of community-dwelling people from Toledo, Spain, aged 65 years or older. Setting and Participants: A total of 1114 nondiabetic persons from the Toledo Study of Healthy Aging cohort (mean age: 74.5, 56.10% female) with complete data at baseline were included. Only 914 participants had fully assessment of functional evaluation during the follow-up period. Methods: IR was determined by the homeostasis model assessment index (HOMA-IR) at baseline while frailty was assessed by the Frailty Trait Scale-5 (FTS-5) at baseline and after 2.99 years' median follow-up period. A total of 319 participants experienced functional decline (2.5-point reduction in the FTS-5 score). A total of 143 deaths were recorded (6.31 years median follow-up) from the Spanish National Death Index. Body compositions were determined using dual-energy x-ray absorptiometry. Multivariate regression models analyzed the effect of HOMA-IR on outcomes, with age, sex, Charlson index, and number of medications included in the basic adjustment model. Results: A 1-logaritmic unit increment in HOMA-IR increased the risk of functional decline after basic adjustment [odds ratio (95% confidence interval): 1.41 (1.09-1.83), P = .009]. This significant association was lost when further adjusted for total fat mass [1.14 (0.86-1.50)] and trunk fat mass [1.03 (0.77-1.37)], which accounted for 62.92% and 91.49% of the association. HOMA-IR was inversely associated with mortality risk [hazard ratio 0.66 (0.49-0.87), P = .0037], an association lost after adjustment for total fat mass [0.74 (0.55-1.01)] and trunk fat mass [0.80 (0.58-1.09)], accounting for 29.05% and 45.78% of the association. Adjustment by lean mass did not modify any of the associations. Conclusions and Implications: Body fat mass, especially in the trunk region, mediates the association of IR with functional decline and to a lesser extent with reduced risk of mortality in nondiabetic older subjects.