Background/Objectives: Sleeve gastrectomy induces substantial changes in body composition, energy metabolism, and systemic inflammation, but their temporal patterns during early postoperative follow-up remain incompletely characterized. This pilot study primarily characterized longitudinal changes in body composition, resting energy metabolism, and inflammatory biomarkers during the first nine months following sleeve gastrectomy. A secondary exploratory objective examined associations between the dietary omega-6/omega-3 ratio and these outcomes. Methods: Thirty-seven adults (67.6% female; mean age 35.1 ± 11.3 years) undergoing sleeve gastrectomy were prospectively assessed at baseline and 3, 6, and 9 months postoperatively. Dietary intake was assessed using three-day food records. Body composition, resting metabolic rate (RMR), respiratory quotient, and inflammatory biomarkers (CRP, IL-6, and IL-10) were evaluated longitudinally. Linear mixed models adjusted for age, sex, and comorbidity were used to assess temporal changes and dietary associations. Results: Distinct postoperative trajectories were observed across outcomes. BMI declined progressively from 43.76 ± 6.44 to 28.49 ± 3.77 kg/m² and total body fat from 52.32 ± 5.76% to 39.62 ± 5.71% (both p < 0.001 for time). In contrast, RMR showed a marked early decline by 3 months, followed by relative stabilization (p < 0.001). CRP progressively decreased from 12.84 ± 12.60 to 2.96 ± 2.95 mg/L (p = 0.001), whereas IL-6 and IL-10 showed no significant temporal changes. Notably, no consistent associations were observed between the dietary PUFA measures and the evaluated outcomes. Conclusions: Postoperative adaptation following sleeve gastrectomy differed across physiological domains, with progressive reductions in adiposity and CRP, an early decline followed by relative stabilization of RMR, and no detectable longitudinal changes in IL-6 or IL-10. Dietary PUFA measures showed no significant associations with postoperative outcomes.