AIMS/HYPOTHESIS: Short-term dietary studies suggest that high-protein diets can enhance weight loss and improve glycaemic control in people with type 2 diabetes. However, the long-term effects of such diets are unknown. The aim of this study was to determine whether high-protein diets are superior to high-carbohydrate diets for improving glycaemic control in individuals with type 2 diabetes. METHODS: Overweight/obese individuals (BMI 27-40 kg/m(2)) with type 2 diabetes (HbA(1c) 6.5-10%) were recruited for a 12 month, parallel design, dietary intervention trial conducted at a diabetes specialist clinic (Melbourne, VIC, Australia). Of the 108 initially randomised, 99 received advice to follow low-fat (30% total energy) diets that were either high in protein (30% total energy, n = 53) or high in carbohydrate (55% total energy, n = 46). Dietary assignment was done by a third party using computer-generated random numbers. The primary endpoint was change in HbA(1c). Secondary endpoints included changes in weight, lipids, blood pressure, renal function and calcium loss. Study endpoints were assessed blinded to the diet group, but the statistical analysis was performed unblinded. This study used an intention-to-treat model for all participants who received dietary advice. Follow-up visits were encouraged regardless of dietary adherence and last measurements were carried forward for study non-completers. RESULTS: Ninety-nine individuals were included in the analysis (53 in high protein group, 46 in high carbohydrate group). HbA(1c) decreased in both groups over time, with no significant difference between groups (mean difference of the change at 12 months; 0.04 [95% CI -0.37, 0.46]; p = 0.44). Both groups also demonstrated decreases over time in weight, serum triacylglycerol and total cholesterol, and increases in HDL-cholesterol. No differences in blood pressure, renal function or calcium loss were seen. CONCLUSIONS/INTERPRETATION: These results suggest that there is no superior long-term metabolic benefit of a high-protein diet over a high-carbohydrate in the management of type 2 diabetes. TRIAL REGISTRATION: ACTRN12605000063617 ( www.anzctr.org.au ). FUNDING: This study was funded by a nutritional research grant from Meat and Livestock Australia (MLA). J.E. Shaw is supported by NHMRC Fellowship 586623.