Altered food selection in gastric bypass patients

As a consequence of an ongoing epidemic of obesity various methods of bariatric surgery are becoming popular. Many patients are recommended gastric bypass (GB), which seems to be the most commonly performed type of procedure and, at the same time, the most effective one. Despite the strong position of gastric bypass as the gold standard in weight loss surgery, mechanisms by which a maintained weight loss is achieved are not exactly understood. According to the new study (1) performed by Imperial College London scientists patients after gastric bypass choose low fat food products, not just because they are recommended to go on a diet, but as a result of an actual change in their food preferences. They simply stop eating high fat food.

According to WHO in 2008 about 200 million men and nearly 300 million women worldwide were suffering from obesity (2). Very often methods such as diet or physical exercise regimen do not provide adequate weight loss. Bariatric surgery should be offered to every patient presenting with Body Mass Index (BMI) higher than 40 kg/m2. In case of a coexisting medical condition, BMI suitable for operation drops to 35-40 kg/m2. Lately it has been suggested, that operations should be available even for patients with lower BMI (3). Considering the fact that obesity is a life threatening disorder, bariatric operations offer far greater benefits than just cosmetic loss of body mass. According to some data long-term mortality rate of gastric bypass patients is reduced by up to 40%(4).

The most frequently performed operation is Roux-en-Y gastric bypass (RYGB) which involves reducing the size of a stomach by well over 90%. Surgeon staples the upper part of the stomach to create a small thumb-sized pouch. Then it is being connected directly to the small intestine, bypassing the rest of the stomach and the duodenum. RYGB is considered both restrictive and malabsorptive procedure. Total food intake declines owing to the painful stretching of the walls of the pouch when ingesting large portions of food.

Associated with it vomiting and discomfort also limit the food intake. Nerve stimulation after eating just a small amount of food induces early satiety. Absorption rate also seems to be compromised. But why does GB overperform other types of bariatric surgery?

Recently the answer to this question has become a major topic for the scientists. In one randomised controlled trial comparing gastric bypass and vertical-banded gastroplasty, a reduced intake of high fat foods was confirmed one year after GB (5). During the recent study researchers from Imperial College London investigated how gastric bypass affects the intake of high fat meals and preference for this kind of food. They tested rats in an experimental study and humans within a trial setting. Again scientists managed to establish that proportion of dietary fat in gastric bypass patients was significantly lower six years after the surgery comparing to the patients after vertical-banded gastroplasty. Moreover, they found out that in rats gastric bypass reduced total fat and caloric intake and, more importantly, increased low fat products consumption.

Scientists suggest that the mechanism of this advantageous effect of GB on food preference may be connected with gut hormones. In this study, as expected, the postprandial level of Glucagon-like peptide-1 (GLP-1) was increased in patients after GB surgery. According to other studies (6) the concentration of peptide YY (PYY) rises along with GLP-1. Both peptides are thought to be satiety hormones. What seems to support this theory is that in mice PYY and GLP-1 administration activates neurons in brainstem that mediate effects of various types of aversive stimuli. Both hormones have been shown to be successful at conditioning taste aversions (7). Therefore the mechanism of how the food preference of patients after gastric bypass is changed may be rather complicated, but it definitely seems to be linked with hormonal balance.

The significance of this study outgrows the urge to understand how the gold standard gastric bypass really works. Considering the fact that for every obese patient surgery poses a large threat, less aggressive, non-surgical methods of reaching low BMI are needed. Perhaps when scientists finally fully understand the biochemistry of hunger and satiety, new treatment options for those in need will be invented.

Written by: Natalia Neumann

1.Am J Physiol Regul Integr Comp Physiol (July 6, 2011). doi:10.1152/ajpregu.00139.2011 Gastric bypass reduces fat intake and preference. Carel W le Roux et al.
3.MedTube Tribune. Being slim is really worth it. Karolina Kłoda
4.N Engl J Med. 2007 Aug 23;357(8):753-61.Long-term mortality after gastric bypass surgery. Adams TD, Gress RE, Smith SC, Halverson RC, Simper SC, Rosamond WD, Lamonte MJ, Stroup AM, Hunt SC
5.Ann Surg 244:18 715-722, 2006. Body composition, dietary intake, and energy expenditure after laparoscopic Roux-en-Y gastric bypass and laparoscopic vertical banded gastroplasty: a randomized clinical trial. Olbers T et al.
6.J Clin Endocrinol Metab 90: 359-365, 2005. Effects of Roux-en-Y gastric bypass surgery on fasting and postprandial concentrations of plasma ghrelin, peptide YY, and insulin. Korner J et al.
7.Am J Physiol 272: R726-R730, 1997 Infusion of GLP-1, but not leptin, produces conditioned taste aversions in rats. Thiele TE et al.

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