The purpose of this study is to investigate harmful effects of intentional weight loss. Intentional weight loss has been linked to increased risk of cardiovascular disease in overweight individuals, but the link between intentional weight loss and thrombotic disease is poorly understood. Postprandial coagulation activation, including thrombin generation, is a potential mechanism after high-fat meals. Thirty obese patients admitted to gastric bypass will be included in a randomized, cross-over clinical trial. All patients will consume a high-fat meal and a low-fat meal served in a randomized order on two study days (two to seven days apart). This cross-over study will be carried out before weight loss, during lifestyle-induced weight loss, and during weight loss 3-4 months after gastric bypass. Fasting and postprandial blood samples are collected on each study days, while one fecal sample is collected for each study period. The study will contribute to our understanding of mechanisms underlying harmful effects of weight loss, and future, dietary guidelines in relation to intentional weight loss programs must be modified.