Background: Temporal increases in energy intake over time have been implicated as the cause of the worsening epidemic of obesity but are difficult to examine due to limitations in energy intake assessments. In an ad libitum energy intake model, we examined longitudinal and seasonal differences in energy intake over 20 years. Methods: Participants (n=292; 59% Indigenous Americans; 61% male; BMI (mean ± SD) 31.6 ± 8 kg.m2) were admitted for a 10 day inpatient stay during which measurements such as body composition (by DXA), 24 hour energy expenditure including spontaneous physical activity (SPAVS(1 ) otherwise known as fidgeting, and ad libitum energy intake over 3 days using a validated vending machine paradigm were collected. General linear models (GLM) were used for secular and seasonal trends adjusting for sex, age, FFM-index, FMindex, and race. FFM-index and FM-index consider height to normalize body composition. SPA was also added to these models in later analysis. Results: Total energy intake (kcal) (B= -411), percent weight maintaining energy needs (%WMEN) (B= -15), and fat kcals (B= -197) were lower (p<0.05) in the summer compared to winter but these differences were attenuated after adjustment for SPA (all p>0.11). KJ (1 As expected FFM-index was positively associated with all food intake measures (all p<0.05) while FM-index was negatively associated (all p<0.05). Adjusted models of secular trends, expressed in years, indicated total kcals (B= -55), %WMEN (B= -2), protein kcals (B= -10), fat kcals (B= -27), and carb kcals (B= -22) all decreased over time. SPA was positively associated with total energy intake (partial R: 0.21, p<0.0036), %WMEN (partial R: 0.18, p<0.01), protein kcals (partial R: 0.21, p<0.0032), fat kcals (partial R: 0.016, p<0.021), and carb kcals (partial R: 0.0020, p<0.0043) when included in above models. Secular trends in body composition measures revealed decreased FFM-index (partial R: -0.39, p<0.0001), but increased FM-index (partial R: 0.34, p<0.0001) with no changes in weight, BMI, and percent body fat (all p>0.20). Conclusions: Ad libitum intake and FFM-index are decreasing with time. FFM-index is the major determinant of energy demand, so its decline may play a role in overall reduction in intake.