After a restrictive procedure the size of the stomach is significantly reduced. Eating a meal in the same size as before the operation is impossible. Therefore, a specific dietary change is essential. A typical example for a pure restrictive procedure is the »adjustable gastric band. Pure restrictive procedures have only a moderate effect on hungry feeling and appetite. Therefore, for some patients the change in diet can be very difficult or the change is not permanent. Complications, as overexpansion of the esophagus or an insufficient weight loss, can be the consequence. In this relation patients often also develop an errating eating behaviour trying to compensate the change in nutrition. Especially sweet eating (increased intake of sweets) or fat eating (increased intake of fats) can occur.

In case of malabsorption a part of the nutrients included in the aliments is absorbed by the body while the rest is excreted. A typical example for a mainly malabsorptive procedure is the Scopinaro »bileopancreatic diversion (BPD).

In such a surgical procedure the anatomy of the gastrointestinal tract is altered. The digestive juices and the aliments are brought together only in the lower part of the bowel leading to a significantly shortened distance available for digestion (e.g. 50 cm instead of normally 5-8m). The disadvantages of purely malabsorptive procedures are often diarrohea and deficiencies of vitamins and micronutrients (iron, zinc etc). To avoid deficiencies, additional supplements are required.

The advantage of malabsorptive procedures is the effective and long-term weight loss that is often better than in any other procedure. Additionally, many metabolic problems as increased cholesterol or diabetes improve dramatically after the operation.

Due to the existing disadvantages of purely restrictive or malabsorptive procedures often a combination is applied as for example the »gastric bypass. In such a procedure the stomach is functionally reduced to achieve a restriction while at the same time the distance of digestion is shortened in order to achieve a defined malabsorption. Depending on the bowel length measured for the absorption the malabsorption can be varied, and hence, resulting in either a proximal gastric bypass (short) or distal gastric bypass (long distance). The combination of procedures avoids problems and disadvantages of each of them while achieving very good results in regard to a long-term weight loss.

Current scientific research undermines the relevance of the metabolic system as a success factor for bariatric surgery. The change in hormone secretion like reduced ghrelin levels (responsible for hungry feelings) and increased GLP-1 and PYY levels (responsible for sense of satiety) leads to change in hunger and appetite already shortly after the operation. This fact facilitates the dietary change for patients significantly. Additionally, these hormonal changes have a very positive effect on metabolic processes. Within a few days and even before weight loss occurs, metabolic problems, such as diabetes, can be solved.