Train, Fast, Adapt: Effects of Intermittent Fasting and Ketogenic Diets on Resting Energy Expenditure (REE) in Trained Adults

Resting energy expenditure (REE) is the largest component of daily energy expenditure and a key determinant of metabolic health. Dietary strategies such as the ketogenic diet (KD) and intermittent fasting (IF) have been proposed to modulate REE and substrate utilization, but their effects remain unclear. This thesis aims to investigate the impact of a ketogenic diet and a 16/8 intermittent fasting protocol on REE, respiratory quotient (RQ), and body composition, through secondary analysis of data from three clinical studies. A total of 57 subjects were included in the analysis: 37 followed a ketogenic diet (KD), while 20 adhered to intermittent fasting (IF). Body composition was assessed using both dual-energy X-ray absorptiometry (DEXA) and bioelectrical impedance analysis (BIA), whereas resting metabolic rate was evaluated through indirect calorimetry. In the overall sample, a strong positive correlation was found between fat-free mass (FFM) and REE, confirming the central role of lean mass in determining resting metabolism. By contrast, no significant association was observed between energy intake (EI) and REE, and caloric intake remained stable across the study period. When analyzed separately, KD did not affect REE or FFM but induced a significant reduction in RQ, indicating a shift toward fat oxidation under carbohydrate restriction. This suggests that KD influences substrate utilization rather than overall energy metabolism, while preserving lean tissue. IF, on the other hand, produced no significant changes in REE, RQ, FFM, or body mass, highlighting a limited short-term effect in metabolically healthy, physically active adults when caloric intake is not restricted. In summary, KD enhanced lipid utilization without compromising lean mass, whereas IF showed minimal metabolic effects. Across both interventions, FFM emerged as a key determinant of REE, underlining the importance of lean mass preservation in sustaining energy balance. Further research with larger, more diverse samples and longer interventions is needed to clarify the long-term effects of these nutritional strategies.