Bethesda, Md. (July 27, 2011)—Roux-en-Y gastric bypass, the most common type of bariatric surgery in the United States, is currently considered the most effective therapy for morbid obesity. Patients who undergo this procedure, in which the stomach is reduced to a small pouch and connected to the middle of the small intestine, often lose massive amounts of weight. However, the reasons behind this surgery's success have been unclear. Shedding more light on why this procedure prompts such dramatic weight loss, a team of researchers has found, in a study using both humans and rats, that Roux-en-Y appears to lead patients to significantly reduce their intake of dietary fat. This effect, which was present for both solid and liquid dietary fat, lingered for up to 200 days after surgery in the animals. Further experiments suggest that this fat avoidance is triggered through digestive consequences, rather than just altered taste, and may be the result of an excess of hormones previously linked to food avoidance.
The study is entitled "Gastric Bypass Reduces Fat Intake and Preference." It appears in the Articles in PresS section of the American Journal of Physiology – Regulatory, Integrative, and Comparative Physiology, published by the American Physiological Society.
The researchers used data from a study in obese people comparing gastric bypass to vertical-banded gastroplasty. At 1 and 6 years after surgery, these patients were asked to fill out questionnaires that included a series of questions to determine whether they avoided certain foods.
The researchers also performed either Roux-en-Y gastric bypass or sham operations on rats. At time points ranging from 10 to 200 days after surgery, these animals were subjected to various food preference experiments.
In one experiment, the rats were offered either high- or low-fat chow, with the researchers comparing the animals' intake of both foods before and after surgery. Similarly, the rats were presented with bottles containing either water or various concentrations of a fat emulsions, with the researchers recording intake of each liquid. To determine whether taste could be responsible for preference, the researchers recorded how many licks the animals took at each bottle when it was offered for only a brief amount of time.
To determine whether digestive effects instead might be responsible for aversions, rather than taste, the researchers offered the rats water flavored with saccharine, which they typically prefer to unflavored water. They then placed a feeding tube directly into the stomachs of the animals and infused some with corn oil and others with saline solution. Other animals received an injection that causes unpleasant sensations. Previous studies have shown that animals can be conditioned to avoid foods they associate with illness—for example, by avoiding saccharine-flavored water after becoming ill from lithium chloride. Consequently, the researchers reasoned that if corn oil caused unpleasant digestive effects, the rats fed this liquid might also avoid the saccharine-flavored water. Additionally, the researchers tested the levels of GLP-1 and PYY, hormones previously linked with food avoidance, in both the Roux-en-Y and sham-operated animals.
Questionnaires from human patients revealed that those who had gastric bypass were significantly more likely to report a reduction in dietary fat intake compared to those who had gastroplasty instead.
The researchers found that rats who underwent Roux-en-Y surgery consumed significantly lower proportions of high-fat chow and higher proportions of low-fat chow compared to the sham-operated animals. Additionally, those animals who underwent the bypass surgery showed a lower preference for high concentrations of the fat emulsions compared to those who received the sham operation. The Roux-en-Y animals had about the same preference for all concentrations of the fat emulsion when exposed to these liquids for only brief amounts of time. However, those that had a small amount of corn oil infused into their stomach were more likely to avoid the saccharine-flavored water than those infused with saline, much like those given the lithium chloride injections. Levels of GLP-1 were significantly higher in the Roux-en-Y rats compared to the non-bypass animals.
Importance of the Findings
The results suggest that people and animals who undergo Roux-en-Y gastric bypass decrease their consumption of both solid and liquid dietary fat, possibly helping people stick to a healthier diet. This avoidance does not appear to be triggered by taste alone, but can be influenced by the effects after ingestion. The hormone GLP-1 might be partly responsible for these effects.
"These findings suggest that changes in fat preference may contribute to long-term maintained weight loss after gastric bypass," the authors wrote. "By elucidating the mechanisms by which obesity surgery reduces the consumption of high fat foods, new surgical and non-surgical therapies could be developed that mimic these mechanisms to offer safe and effective weight loss."