Gastric bypass surgery is an older operation method in comparison to other bariatric surgery methods. Therefore, it may be deemed as the most used bariatric surgery among others. In the first step of gastric bypass surgery, the stomach is reduced to 30 millilitres. The gastric tissue close to oesophagus and stomach junction is used. The stomach is connected to small intestine. There are two types of gastric bypass surgery. In the first type, the small intestine is formed in a thin circle and is connected to stomach. This is called as “Mini-gastric bypass”. In the second type of gastric bypass surgery, the small intestine is bypassed and one end is connected to stomach, while the other end is connected to small intestine. This method is known as “Roux en Y gastric bypass”. Except of small technical differences, the two methods have similar surgical results. The mechanism in gastric bypass surgery is restrictive as in gastric sleeve surgery, i.e. patient may intake limited amount of food and bile salts and pancreas enzymes found in duodenum meet the nutrients in a distal part of small intestine. Therefore, the breakdown and absorption of nutrients are rendered difficult and the absorption mechanism is changed. In other words, patient eats less and gets less benefits from nutrients. The change in gastrointestinal hormones is more significant than in gastric sleeve surgery. Patient feels less hungry, even though eating less and blood glucose is regulated. Losing 60-80% of body weight is one of its advantages. The gastric bypass surgery is a food intake restrictive method and it has a more significant effect on gastrointestinal hormones. Also it is a reversible procedure in comparison with gastric sleeve surgery.