Bariatric surgery really works by remodelling gut hormones: it removes or isolates the fundus, main source of the hunger hormone ghrelin, and a smaller stomach is only the visible half.What bariatric surgery really works through is a remodelled gut-hormone system — the key difference from dieting. The fundus, the upper part of the stomach, is the main source of ghrelin, the hormone that makes you feel hungry: sleeve gastrectomy removes it and gastric bypass separates it from the path food takes. A smaller stomach is only the visible half.
2 · The hormone list
In a small study, ghrelin fell sharply after gastric bypass and its pre-meal rise vanished, and the lower small intestine releases more satiety signals such as and PYY.Ghrelin falls sharply: in a small study (Cummings 2002), people who had had gastric bypass had ghrelin 72% lower than obese controls of similar weight, and the rise before meals disappeared — hunger really does weaken; it is not willpower.
At the same time, food reaches the lower small intestine faster, and L cells there release more satiety signals such as and PYY; bile acids and the gut microbiome shift too. None of these were measured in the Cummings paper, which measured ghrelin only.
3 · Diabetes remits before weight loss
Within days after RYGB, before visible weight loss, glucose can improve sharply, which is why the field calls it metabolic surgery.Within days after RYGB, before visible weight loss, glucose can improve sharply; some stop insulin before discharge. That is why the field says metabolic surgery. of surgery plus medical therapy vs medical therapy alone found higher remission, but the surgical arms also lost considerably more weight — not a matched-weight contrast. SOS 2007 is a matched cohort on all-cause mortality (adjusted 0.71), not a diabetes-remission RCT.
4 · Surgery vs diet: ghrelin goes opposite ways
Weight loss by dieting makes ghrelin rise and leptin fall, switching on a hunger defence, while weight loss by surgery makes ghrelin fall and stay low.Weight loss by dieting → ghrelin rises (in the same Cummings 2002 study, people who lost 17% of body weight by dieting saw ghrelin rise 24%) and leptin falls → the body switches on its hunger defence. In Sumithran 2011, 62 weeks after very-low-calorie-diet weight loss, several appetite-related hormones still differed from their pre-diet levels; that study did not measure energy expenditure.
Weight loss by surgery → ghrelin falls and stays low, while and PYY rise. One explanation is that the body's weight set point is moved down — the set point is a model, not something measured directly.
GLP-1 drugs partly mimic this route, but weight often returns after they stop.