Obesity is an increasing health problem. Since drug treatments are limited, diets remain popular. High-protein diets (HPD) reduce body weight (BW) although the mechanisms are unclear. We investigated physiological mechanisms altered by switching rats from Western-type diet (WTD)-induced obesity (DIO) to HPD. Male rats were fed standard (SD) or WTD (45% calories from fat). After developing DIO (50% of rats), they were switched to SD or HPD (52% calories from protein) for 14 days. Food intake (FI), BW, body composition, glucose tolerance, insulin sensitivity and intestinal hormone plasma levels were monitored. Rats fed WTD showed an increased FI starting at 3 weeks and had a 25% greater BW gain after 9 weeks compared to SD (P<0.05). DIO rats switched from WTD to HPD reduced daily FI by 30% on day 1 which lasted to day 9 (-9%) and decreased BW during the 2-week period compared to SD/SD (P<0.05). During these 2 weeks, WTD/HPD rats lost 72% more fat mass than WTD/SD (P<0.05), whereas lean mass was unaltered. WTD/HPD rats had lower blood glucose than WTD/SD at 30 min post glucose gavage (P<0.05). The increase of gastric inhibitory polypeptide and pancreatic polypeptide during the 2-h dark phase feeding was higher and that of peptide YY lower in WTD/HPD compared to WTD/SD (P<0.05). These data indicate that HPD reduces BW in WTD rats, which may be related to decreased FI and the selective reduction of fat mass accompanied by improved glucose tolerance suggesting relevant benefits for HPD in the treatment of obesity.