Global obesity rates are increasing more and more. Short-chain fatty acids, the end products of fermentation of indigestible carbohydrates by intestinal bacteria, can influence metabolism and therefore play a role in the development of obesity and diabetes. The studies described in this dissertation show that the administration of short-chain fatty acids to the colon of overweight men increases energy expenditure and fat metabolism and reduces appetite and inflammation. The increase in the number of short-chain fatty acids in the colon by food intake can vitally contribute to the treatment or prevention of obesity and diabetes.