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Satiety, not restriction

Research finds unexpected gastric band mechanism

Adjustable gastric bands work by invoking satiety earlier, rather than restricting body's access to food, as previously suspected
Increased stomach activity to move food across gastric band induces feeling of satiety

The laparoscopic adjustable gastric band (LAGB) is ideally placed on the cardia of the stomach, just below the oesophagogastric junction. In the past it was assumed that the presence of a band in this position caused a meal to accumulate in the pouch of stomach proximal to it, before gradually being released into the remainder of the gut. Thus, the band was thought to work by restricting the volume of food ingested to that able to be accommodated in the proximal pouch.

This small volume of food was thought to stretch the stomach and cause early satiety. Gradual emptying of the proximal pouch into the infra-band stomach was though responsible for prolonged inter-meal satiation.

Recent studies by our group, lead by Dr Paul Burton, confirm that mechanism of action of LAGB is the induction of early and prolonged satiety, however, the intraluminal events that lead to this are far more complex than simple retention of food in the proximal pouch.

By combining high resolution video manometry1-4 with nuclear studies of gastric emptying5, 6, we have demonstrated that the expected physiology of a LAGB at its optimal volume does not cause a food bolus to rest above the band in the proximal pouch. Rather, the bolus will transit across the band in stages over a period of 45-60 seconds due to 4-6 repeated contractions of the lower oesophagus. The infra-band stomach subsequently empties normally (figure 1)7.

The increased activity of the lower oesophagus and upper stomach appears to be critical to the sensation of satiety achieved with a small meal. Animal studies modelling the human situation performed by Dr Brian Oldfield’s 8 group suggest that the vagus is an important mediator of this sensation, and the flow of food past an optimally activated band may be triggering these afferents.

If a band is over-adjusted, or if patients engage in inappropriate eating behaviour, eating too quickly or too big a volume, food will accumulate within the proximal pouch. The patient will experience significant adverse symptoms, including discomfort and regurgitation. These symptoms are caused by vigorous peristaltic contractions that hyper-pressurise the proximal pouch in an attempt to transit the bolus across the band. Over time, this increase in pressure can lead to pathological proximal pouch formation9. If a band is underfilled, there is no limitation to transit of food across the LAGB and therefore the proximal stomach and lower oesophagus are not stimulated. The patient will not be satiated a small meal and be able to eat bigger volumes. Unsurprisingly, weight loss is then difficult to achieve and maintain.

Dr Burton’s work has helped us to understand the optimal intraluminal pressure milieu in the presence of a band (figure 2) 1. Ideally we would use these data to objectively calculate an optimal adjustment for an individual patient, but as yet we lack technology to permit this.

Understanding how an AGB affects the intraluminal pressures and flow of food with eating has allowed us to improve our patient education. We work closely with our patients, explaining the critical importance of eating small volumes of food slowly. We also aim to adjust the band only to control hunger. We hope that this improved patient education along with better band adjustments will help improve our outcomes, both in terms of weight loss and complication rate.

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