Metabolic and Orexin-A Responses to Ketogenic Diet and Intermittent Fasting: A 12-Month Randomized Trial in Adults with Obesity

Background/Objectives: Intermittent fasting and ketogenic dietary approaches are increasingly investigated for their potential metabolic benefits in obesity. However, their long-term neuroendocrine effects—particularly those involving Orexin-A, a peptide implicated in energy regulation—remain poorly understood. The objective of this study was to compare the long-term metabolic, inflammatory, and orexinergic responses to different dietary strategies in adults with obesity. Methods: In this 12-month randomized, three-arm trial, 30 adults with obesity (BMI ≥ 30 kg/m2) were randomly assigned (1:1:1) to a hypocaloric ketogenic diet (KD), a 16:8 time-restricted eating regimen (TRF16:8), or a 5:2 intermittent fasting protocol (ADF5:2). Anthropometric parameters, body composition, fasting glucose, lipid profile, inflammatory cytokines (CRP, IL-6, TNF-α, IL-10), and plasma Orexin-A levels were assessed at baseline and every 3 months. Dietary adherence was monitored through structured logs and monthly assessments. Statistical analyses included repeated-measures models with sensitivity analyses adjusted for age and sex. Results: All participants completed the intervention. The ketogenic diet produced the largest sustained reductions in BMI, fat mass, fasting glucose, and total cholesterol over 12 months. TRF16:8 elicited more rapid early metabolic improvements and showed the most consistent longitudinal increase in Orexin-A levels. The ADF5:2 protocol resulted in moderate improvements across outcomes. In all groups, increases in Orexin-A were associated with markers of improved metabolic flexibility and reduced inflammation; however, mediation analyses were exploratory and non-causal. Between-group differences remained significant for fat mass, glucose, and Orexin-A trajectories after correction for multiple comparisons. Conclusions: The ketogenic diet was associated with the most pronounced long-term metabolic improvements, whereas 16:8 time-restricted eating yielded faster early responses and the most stable enhancement in Orexin-A levels. These findings indicate distinct metabolic and neuroendocrine adaptation profiles across dietary strategies. Given the small sample size, results should be interpreted cautiously, and larger trials are warranted to clarify the role of Orexin-A as a potential biomarker of dietary response in obesity.