AIMS: To determine the feasibility and efficacy of a high-protein diet compared with a standard diet aiming for weight maintenance in insulin treated type-2 diabetic patients on insulin requirement, body weight and metabolic parameters over 12 weeks. METHODS: In a randomized controlled trial we assigned 44 type-2 diabetic patients on insulin therapy either to high-protein or standard diet over 12 weeks. Parameters were evaluated at baseline and monthly. RESULTS: After 12 weeks, the high protein diet significantly decreased insulin requirement (9.4 ± 16.3 vs. +0.8 ± 4.8 IU, mean ± SD; p=0.007), fasting plasma glucose (41.7 ± 62.5 vs. 2.1 ± 39.0 mg dl(-1); p=0.02), body mass index (1.1 ± 0.8 vs. 0.3 ± 0.7 kg m(-2); p=0.003), fat-free (0.8 ± 0.5 vs. 0.2 ± 0.5 kg; p=0.001), fat mass (2.6 ± 1.7 vs. 0.8 ± 1.6 kg; p=0.001) and increased serum folate (4.2 ± 8.3 vs. − 0.8 ± 5.5 nmol l(-1); p=0.04) compared to the standard diet. These beneficial metabolic effects are most likely related to the achieved weight loss. No significant differences between groups in renal function were observed. CONCLUSIONS: In this study we demonstrate that a high protein diet with emphasis on plant source protein vs. a standard diet is feasible in insulin-treated type-2 diabetic patients and reduces insulin requirement and body weight and improves metabolic parameters up to 12 weeks. A high protein diet can thus be considered as an appropriate diet choice for type-2 diabetic patients.