Obesity is a serious health problem and currently bariatric surgery (weight loss surgery such as gastric bypass surgery) is the most effective long-term treatment. The general aim of the thesis is contribute to the optimization of the care after bariatric surgery and consists of two parts. In part I. Nutritional opportunities it is investigated how nutrition, in particular dietary protein and its building blocks the amino acids, could be useful to optimize the results of gastric bypass surgery. Among other things, the influence of amino acids in blood on the feeling of hunger/satiety and associated hormones is studied. Additionally, a meal test with two different proteins, varying in amino acid composition, is used to compare the effect of these proteins on hunger, satiety and associated hormones. Part II. Nutritional challenges investigates the occurrence and cause/contributing factors of postbariatric hypoglycemia, a complication after bariatric surgery in which patients have too low glucose concentrations 1-2 hours after a meal. In popular terms this is referred to as ‘hypo’, which in severe cases could lead to fainting or coma. These studies use a meal test, after which the blood glucose concentration and various hormones/bile acids are determined. It is also explored whether the test subjects have symptoms of this postbariatric hypoglycemia and whether the body can respond adequately to this with a stress response. An extra focus is placed on subjects with a cholecystectomy (gallbladder removal), to evaluate the influence this procedure on the ‘hypo’.