|Abstract:||Neutering is an important risk factor for obesity, which reduces the quality and length of life in dogs and cats. Presently, dietary interventions have been recognized as one of the preventive and therapeutic options for obesity in human and companion animals. The objective of this study was to evaluate the effects of specially formulated diets on BW, body composition, and blood hormones and metabolites of adult female dogs after spay surgery. All procedures were approved by the University of Illinois Institutional Animal Care and Use Committee prior to experimentation. Twenty-eight healthy adult intact female beagles (3.02 ± 0.7 yr; 10.28 ± 0.8 kg; BCS: 4.98 ± 0.57) were used in a longitudinal spay study. Twenty-four dogs were spayed and randomly allotted to one of three experimental diets: 1) moderate-protein, moderate-fiber diet (control; COSP), 2) high-protein, high-fiber diet (HP-HF), or 3) high-protein, high-fiber diet plus omega-3 and medium-chain triglyceride fats (HP-HF-O). Four dogs were sham-operated and fed the control diet (COSH). During the study, food intake was recorded daily, body weight was recorded twice a week, and body condition score was evaluated weekly. Blood was collected every 4 wk for blood hormone and metabolite measurements. Body composition was measured via DEXA scans, and voluntary physical activity was measured using Actical® devices every 8 wk. All dogs were fed to maintain BW for 12 wk after spay surgery (restricted phase), then allowed to consume twice that amount for another 12 wk (ad libitum phase). All data were analyzed using the Mixed Models procedure of SAS. Change from baseline data were analyzed for treatment, time, and treatment*time effects. Several treatment*time and treatment effects were observed. Notably, during the first 12 wk, HP-HF and HP-HF-O dogs had lower (p<0.01) blood cholesterol concentrations than COSH or COSP dogs, and HP-HF-O dogs had serum ALP concentrations lower (p<0.05) than COSP dogs. During this time, HP-HF and HP-HF-O dogs had a greater (p<0.05) reduction in WBC and neutrophils than COSP dogs. During the second 12 wk, dogs fed HP-HF or HP-HF-O ate more (p<0.01) food (g/d) than for COSH dogs. Change in BCS of dogs fed COSP was greater (p<0.01) than COSH dogs from wk 21 to 24, but dogs fed HP-HF or HP-HF-O were not different. When comparing data by feeding regimen (restricted vs. ad libitum phase), several treatment*feeding regimen and treatment effects were observed. During the first 12 wk, HP-HF and HP-HF-O dogs had a greater reduction in serum cholesterol (p<0.001) and CALP concentrations (p<0.01) than COSH or COSP dogs. During this phase, HP-HF and HP-HF-O dogs also had a greater (p<0.01) reduction in total WBC and neutrophils (p<0.01) than COSP dogs. During the second 12 wk, dogs fed HP-HF or HP-HF-O ate more (p<0.01) food (g/d) than dogs fed COSP, with all spayed dogs consuming more (p<0.01) food than sham-operated dogs. COSH dogs and those fed HP-HF or HP-HF-O had a lower (p<0.001) increase in BCS than COSP dogs. Serum triglyceride concentrations were not different among COSH or COSP dogs across the entire study, but HP-HF and HP-HF-O dogs had a greater (p<0.001) reduction in serum triglyceride concentrations during the first 12 wk than the second 12 wk. HP-HF-O dogs had no difference (p>0.05) in serum leptin concentrations compared to COSH dogs during this time. COSP dogs had higher (p≤0.01) serum CRP concentrations than dogs fed HP-HF-O. Overall, body fat increase in COSP dogs was greater (p<0.05) than for COSH dogs at wk 24, but HP-HF and HP-HF-O dogs were intermediate. Our results indicate that a high-protein, high-fiber diet can limit weight gain and body fat increase and attenuate serum cholesterol, triglycerides, ALP, CALP and leptin concentrations in dogs after spay surgery.