Hyperinsulinaemic hypoglycemia after bariatric surgery. Overweight and obesity are an increasing health problem globally and in the Netherlands, about 15% of the population in the Netherlands is obese. Obesity is associated with increased risk of morbidity, such as cardiovascular disease and type 2 diabetes (T2D). Weight loss is the most important intervention in obese patients, reducing obesity related morbidity and increasing life expectancy. Non-invasive weight loss interventions such as diet, lifestyle or medication only have a moderate effect which is short lived. Weight reducing surgery, i.e. bariatric surgery, is the only intervention that leads to persistent weight loss and is superior above conventional treatment. The most performed and preferred bariatric surgery is Roux-en-Y Gastric Bypass (RYGB), which is a food restrictive and malabsorptive procedure. Besides weight loss, metabolic improvement in T2D patients is an additional result after RYGB.