There are three general classes of bariatric procedures.  These procedures function by either decreasing the amount a person is able to eat (restrictive procedures) or by causing food to be absorbed less effectively (malabsorptive procedures).

Restrictive procedures - These procedures involve creating a small gastric pouch with a restrictive (small diameter) outlet or a long narrow gastric tube (sleeve gastrectomy).  This makes the patient feel "full" after eating less.  If they try to eat large amounts, they get a very uncomfortable sensation.  Since no intestine is bypassed, there is no malabsorption.  "Sweet eaters" tend no to lose as much weight with these procedures because high calorie liquids (milkshakes, juices, pop), refined foods (cookies, cakes, pies) and soft foods pass easily through the pouch and restrictive outlet and get digested normally.  Average excess weight loss is 40 - 60%.  The most common restrictive procedure is the Lapband.

Malabsorptive procedures - These procedures produce malabsorption via re-routing the small intestines and result in a  greater amount of weight loss than other procedures.  There is an increased risk of vitamin, mineral and protein deficiencies compared to other procedures.  There is also the more common side effect of malodorous, frequent diarrhea depending on your diet and the length of intestine that is left in circuit to absorb food. 

Combination Procedures - These procedures utilize both mechanisms to induce weight loss.  In general terms, the stomach is divided and a small pouch is created.  A limb of intestine (roux limb) is then attached to the pouch.  The 60 - 85% average excess weight loss is better than with the restrictive procedures but less than the purely malabsorptive procedures.  There is also usually less difficulty with diarrhea and deficiencies of vitamins, minerals, and malnutrition.  "Sweet eaters" are usually steered toward a combination surgery.  This is because if they eat the foods mentioned previously (sweets)with a gastric bypass in place, they will usually experience the dumping syndrome. This is an unpleasant feeling of nausea, bloating, cramping followed later by sweating, increased heart rate and hypoglycemia.  This is caused by a large carbohydrate meal entering the small intestine all at once.  This feeling tends to condition "sweet-eaters" not to eat sweets.  The Roux-en-Y gastric Bypass is the most common bariatric procedure performed in the United States.  There are variations of this surgery with respect to limb length and pouch size, and the route of the limb to the pouch and banding of the pouch.