A low-carbohydrate, high-protein (LCHP) diet is often recommended for the prevention and management of diabetes in cats; however, the effect of macronutrient composition on insulin sensitivity and energetic efficiency for weight gain is not known. The present study compared the effect in adult cats (n 32) of feeding a LCHP (23 and 47 % metabolisable energy (ME)) and a high-carbohydrate, low-protein (HCLP) diet (51 and 21 % ME) on fasting and postprandial glucose and insulin concentrations, and on insulin sensitivity. Tests were done in the 4th week of maintenance feeding and after 8 weeks of ad libitum feeding, when weight gain and energetic efficiency of each diet were also measured. When fed at maintenance energy, the HCLP diet resulted in higher postprandial glucose and insulin concentrations. When fed ad libitum, the LCHP diet resulted in greater weight gain (P,0·01), and was associated with higher energetic efficiency. Overweight cats eating the LCHP diet had similar postprandial glucose concentrations to lean cats eating the HCLP diet. Insulin sensitivity was not different between the diets when cats were lean or overweight, but glucose effectiveness was higher after weight gain in cats fed the HCLP diet. According to the present results, LCHP diets fed at maintenance requirements might benefit cats with multiple risk factors for developing diabetes. However, ad libitum feeding of LCHP diets is not recommended as they have higher energetic efficiency and result in greater weight gain.Key words: Diet effect: Cats: Glucose and insulin: Weight gainThere are currently a large number of diets available commercially for cats, which vary greatly in nutrient composition. Cats are strict carnivores and do not metabolise carbohydrates efficiently (1 -5) . The typical prey diet of cats is low in carbohydrate (,10 % metabolisable energy (ME)) (6) ; however, most commercially available dry cat foods are moderate to high in carbohydrate content (. 25 -55 % ME), partly because of the difficulty in formulating extruded, dry diets that are low in carbohydrate, and partly because cereal is a relatively inexpensive ingredient, and there is demand for lower-cost diets in the pet food market.Low-carbohydrate, high-protein (LCHP) diets have been reported in research abstracts and in a recent publication to significantly reduce the postprandial increase in blood glucose concentration compared with high-carbohydrate, low-protein (HCLP) diets (Farrow HA, Rand JS and Sunvold GD (2002); Singh R, Rand J and Morton J (2006), unpublished results) (7) , possibly indicating a reduced demand on the b-cells to produce insulin. A LCHP diet has also been shown to reduce the need for exogenous insulin administration in diabetic cats (8) . Diabetes in cats is similar to human type 2 diabetes, characterised by decreased insulin secretion and insulin resistance (9,10) , and it has been hypothesised that a high-carbohydrate diet fed long term potentially might be a risk factor for the development of diabetes mellitus in cats (11) .Increa...