It is now indisputable that many metabolic syndromes such as diabetes, hypertension, hyperlipidemia, polycystic ovary syndrome, and sleep apnea can be quickly relieved or cured after weight loss surgery. Since metabolic syndrome is often relieved without sufficient weight loss, it is not reasonable to explain the treatment effect of metabolic syndrome by weight loss, so it is speculated that weight loss surgery causes hormonal regulation changes and eventually normalizes the abnormal metabolism. Multiple hypotheses have been debated. Studies have found that a series of gastrointestinal hormonal changes occur after weight loss surgery, which in turn lead to the alleviation of insulin resistance and normalization of blood glucose. What causes these “downstream” hormonal changes? Hypotheses include reduced perioperative caloric intake, anterior and posterior intestinal theory, and altered bile acids and bacterial flora. But so far, there is still much controversy. Glycemic control usually occurs a few days after bariatric surgery, before significant weight loss. The first hypothesis that emerged was the increased hepatic insulin sensitivity due to reduced perioperative caloric intake. If this hypothesis holds true, the three bariatric procedures, gastric bypass (RYGB), sleeve gastrectomy (SG) and adjustable gastric banding (LAGB), should be equally effective in controlling blood glucose, however, the highest rate of diabetes remission is actually seen after RYGB surgery. Moreover, other GI surgeries also result in reduced caloric intake in the perioperative period without glycemic control, but instead result in elevated blood glucose due to the stress response. The foregut and hindgut doctrine is the classic hypothesis explaining the treatment of metabolic syndrome by bariatric surgery, i.e., food does not pass through the foregut after gastric diversion and reaches the hindgut earlier, thus causing a series of gastrointestinal hormonal changes that lead to rapid remission of metabolic syndrome. However, the anterior and hindgut doctrines do not plausibly explain why the other two weight loss surgical modalities, LAGB and SG, also cure metabolic syndrome. In both cases, the continuity of the digestive tract is not interrupted and there is no problem of food stimulating the hindgut earlier or bypassing the foregut. It has also been speculated that alterations in the intestinal flora are responsible for this phenomenon. Compared to non-obese individuals, obese individuals have an increased proportion of thick-walled bacteria/bacteroid bacteria and a reduced diversity of bacteria. This difference disappears after either surgery or diet control resulting in weight loss. It is evident that intestinal flora plays an important role in the extraction of energy from ingested food. One curious observation is that any surgery involving the stomach has some effect on the metabolic syndrome, whereas abdominal surgery such as gallbladder or colorectal resection does not result in a decrease in blood glucose. Among them, the remission rate of diabetes was higher in those who underwent total gastrectomy than in those who underwent major gastrectomy and endoscopic local resection. The effect of glucose lowering also varied among different gastrointestinal reconstruction modalities. The glucose-lowering effect of Bi-II or Roux-Y anastomosis was significantly better than that of Bi-I anastomosis. Thus, the author proposed the gastric center hypothesis, that the stomach is a mysterious endocrine organ with many known and unknown endocrine functions, the key link of which is in the greater curvature of the stomach. It is likely that there are some special cells in the gastric greater curvature that produce special, as yet unknown, factors involved in regulating important metabolic processes. The greater curvature of the stomach is removed after gastric sleeve resection; food bypasses the stomach and goes directly to the small intestine after gastric diversion; and the banding effect of gastric fasciculation reduces the amount and slows the rate of food entering the stomach. All three of these surgical procedures reduce the irritation of food to the stomach. It is likely that this leads to a reduction in the secretion of specific factors, which in turn allows insulin resistance to be relieved and sympathetic tone to be reduced, leading to remission of metabolic diseases such as diabetes, hypertension, and polycystic ovary syndrome. It is the removal or absence of this “upstream” part of the stomach that leads to changes in the “downstream” gastrointestinal hormones.