(A) Body weight (left panel), percentages of whole-body fat mass (middle panel) and lean tissue mass (right panel) as assessed by low-field nuclear magnetic resonance imaging. (B) Ratio of subcutaneous to visceral fat calculated from abdominal 7-Tesla MRI scanning. (C) In vivo microCT analysis of the tibia was carried out to measure cortical thickness, trabecular bone mineral density (BMD), as well as fat and lean mass around the tibia expressed as percentage of the total body weight. (D) Metabolic parameters derived from the metabolic treadmill as a function of age. Energetic cost (left panel); heat generation (second panel); RER, respiratory exchange ratio (third panel); and Work (right panel). Energetic cost was determined from VO2 per unit of body weight and gait speed while work was expressed as mJoules. (E–G) Multiple regression analyses with frailty index (E, F) and gait velocity (G) as response variables. The modeling approach consisted of fitting a series of four regressions for each of the response variables. The explanatory variables included in the multiple regression modeling are depicted in Figure 2—source data 1. (E) left panel, Effect of gait velocity on the relationship between frailty and age; right panel, Impact of age on the interaction between frailty and gait velocity. (F) left panel, Effect of bone cortical thickness on the relationship between frailty and age; right panel, Impact of age on the interaction between frailty and bone cortical thickness. (G) left panel, Effect of energetic costs on the relationship between gait velocity and age; right panel, Impact of age on the interaction between gait velocity and energetic costs. Although this is a cross-sectional study, the points were joined to help the reader see the various associations without suggesting that there is causal association; for example, that animals with higher age tend to have higher frailty within the same gait velocity (E, left panel). (H–K) Mice at different ages were placed into metabolic cages for 72 hr to measure VO2, VCO2, RER, heat production, and ambulatory activity counts. The values associated with the first 12 hr acclimatization phase (L1) were discarded. (H) Body weight as mice entered the metabolic cage. (I) Averaged hourly RER trajectories were captured during two dark/light cycles in young (n = 8), adult (n = 22) and old (n = 18) mice. ZT, zeitgeber time is defined as the 12:12 hr light/dark cycle that synchronizes organismal biological rhythms. Each point represents mean ± SEM. (J) Scatter plot depicting RER values averaged over 48 hr for individual mice in the three age groups. (K) Relationship between RER and age. Data in (H) and (J) were analyzed using one-way ANOVA with Dunnett’s post-hoc analysis. *, p<0.05; **, p<0.01; ****, p<0.0001. (L) Diagram depicting the impact of age on body composition and energetic parameters in AL-fed male mice.