"Dumping Syndrome" is usually divided into early and late phases- the two phases have separate physiologic causes and will be described separately. In practical fact, a patient usually experiences a combination of these events and there is no clear cut division between them.
Early dumping is caused by the high osmolarity of simple carbohydrates in the bowel. The various types of sugar all have small molecules, so that a gram of (example) sucrose has many more molecules than a gram of protein, creating a higher concentration (# of molecules per cc) from simple sugars than from other foods. This matters because, inside the body, fluid shifts will generally go toward the higher concentration of molecules. So if a patient consumes a bite of milk chocolate (lots of sugar), when it goes into the roux limb it will quickly " suck" a significant amount of fluid into the bowel. This rapid filling of the small bowel causes it to be stretched which in itself causes cramping. This also causes the activation of hormonal and nerve responses that cause the heart to race ( palpitate) and cause the individual to become clammy and sweaty. Vomitting and diarrhea may follow as the intestines try to quickly rid itself of this "irritant".
Late dumping has to do with the blood sugar levels. The small bowel is very effective in absorbing sugar, so the rapid absorption of all relatively small amount of sugar can cause the glucose level in the blood to "spike" upward. The pancreas responds to this glucose challange by "cranking up" its output of insulin. Unfortunately, the sugar that started the whole cycle was such a small amount that it does not sustain the increase in blood glucose, which tends to fall back down at about the time that the insulin surge really gets going. These factors combine to produce hypoglycemia ( low blood sugar) which causes the person to feel weak, possibly pass out, feel sleepy, and profoundly fatigued.
Late dumping is the mechanism by which sugar intake can create low blood sugar and is also a way for gastric bypass patients to get into a vicious cycle of eating . If the patient takes in sugar or a food that is closely related to sugar (simple carbs like rice, potaoes, and pasta) they will experience some degree of hypoglycemia in the hour or two after eating. The hypoglycemia stimulates appetite, and its easy to see where that is going...
The reason sugar does not cause dumping in non surgically altered people is that the stomach, pancreas, and liver work together to prepare the nutrients (or sugar) before they reach the small intestine for absorption. The stomach serves as a reservoir that releases food downstream only at a controlled rate, avoiding sudden large influxes of sugar that can occur after a Roux-En-Y. The released food is also mixed with stomach acid, bile, and pancreatic juices to control the chemical makeup of the stuff that goes downstream and avoid all the effects outlines above.
Obviously surgeons consider dumping syndrome to be a beneficial effect of gastric bypass- it seems to be important to provide quick and reliable negative feedback for intake of the "wrong" foods. in practise, most patients do NOT experience full-blown dumping more than once or twice. Most simply say that they have "lost their taste" for sweets. Here is some more valuable info:
Most patients do not have a good appetite for many months following gastric bypass and some never really recover a "regular" appetite. In the short term after surgery, this is a useful adjunct to weight loss. For the first month or so after GBS, as long as patients are taking in adequent fluids for hydration and experimenting with healthy food to learn what works, they don't need to worry too much about getting in "enough" food for nutrition. The body had adequate stores for the early time interval, just as it has the obvious fat stores that we are trying to eliminate.
Experience is that over time the stomach pouch and small bowel learn to work together, and capacity for food increases as well-as themonths go by protein intake increases to a point that the patient is abl to reliably take in adequate protein to maintain nutrition.
