Dietary protein can vary in amount, source and form, and how this affects the development of diet-induced obesity is still not fully understood. Here we have used male C57BL/6J mice as an experimental model to further investigate the effect of dairy protein, in different background diets, on the development of diet-induced obesity. The effect of protein amount, using casein protein, was investigated both in a high-fat diet with sucrose as the main carbohydrate source and in a diet based on the contemporary western diet with respect to macronutrient composition. The effect of protein source, comparing casein and whey, was investigated in a high-fat diet, while the effect of protein form, comparing intact and hydrolyzed casein, was investigated in a diet based on the contemporary western diet with respect to macronutrient composition. The results show that while protein amount, source and form all can reduce the development of diet-induced obesity, the underlying mechanisms are different. Hydrolyzed casein seems to be able to increase white adipose tissue energy expenditure, possibly by effecting fed state insulin levels, while the difference between casein and whey seems to be related to the TCA-cycle and gluconeogenesis.