Obesity is a major health problem in cats and a risk factor for diabetes. It has been postulated that cats are always gluconeogenic, and that the rise in obesity might be related to high dietary carbohydrates. We examined the effect of a high carbohydrate/low protein (HC) and a high protein/low carbohydrate diet (HP) on glucose and fat metabolism during euglycemic hyperinsulinemic clamps, adipocytokines, and fat distribution in 12 lean cats (LEAN) and 16 obese cats (OBESE) before and after weight loss. Feeding HP led to greater heat production in LEAN but not in OBESE. Regardless of diet, OBESE had markedly decreased glucose effectiveness and insulin resistance, yet, greater suppression of non-esterified fatty acid during the euglycemic hyperinsulinemic clamp was seen in OBESE on HC compared to LEAN on either diet or OBESE on HP. In contrast to humans, OBESE had abdominal fat equally distributed subcutaneously and intra-abdominally. Weight loss normalized insulin sensitivity; however, increased non-esterified fatty acid suppression was maintained and fat loss was less in cats on HC. Adiponectin was negatively and leptin positively correlated with fat mass. LEAN, and cats during weight loss, but not OBESE, adapted to the varying dietary carbohydrate/protein content with changes in substrate oxidation. We conclude that HP is beneficial through maintenance of normal insulin sensitivity of fat metabolism in OBESE, facilitating the loss of fat during weight loss, and increasing heat production in LEAN. These data also show that insulin sensitivity of glucose and fat metabolism can be differentially regulated in cats.