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Weight loss bacteria in gut and gastric by-pass surgery

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An article by Carol Torgan, Ph.D.posted in the National Institute of Health Research Matters that reported on the findings of research conducted on both humans and mice, suggests that one of the reasons people may lose weight after Roux-en-Y gastric by-pass (RYGB) surgery is that there is a shift in the balance of bacteria in the digestive tract ... the microbiota of the gut... resulting in an increase of beneficial bacteria. This popular type of gastric by-pass surgery results in significant weight loss as well as improvements in associated excessive weight such as type 2 diabetes.

Research from earlier studies published in Science Translational Medicine, had shown a significant change in the microbiota gut bacteria of an obese person after the gastric by-pass surgery. When compared with someone who was thin, the type of bacteria was similar.

As pointed out in a article reviewing details of the studies; "People who have the surgery generally lose 65 percent to 75 percent of their excess weight, but scientists have not fully understood why." Decreased calories resulting from reduced caloric intake cannot fully account for the weight loss. There was no clear answer as to whether the change in bacteria was the direct result of gastric by-pass surgery or the weight loss that followed the surgery or if the weight loss was influenced by the change in bacteria. The article went on to say "now, the researchers are saying that bacterial changes may account for 20 percent of the weight loss."

Dr. Alice P. Liou from Massachusetts General Hospital, Dr. Lee M. Kaplan director of the obesity, metabolism and nutrition institute at the Massachusetts General Hospital, and Dr. Peter J. Turnbaugh from Harvard University led the team of researchers that conducted experiments on mice to determine what the connection was between the microbiota gut bacteria changes in gastric by-pass surgery and weigh loss.

The researchers performed RYGB surgery on the mice in the study. They then collected samples of gut microbes from mice that had undergone gastric by-pass surgery, mice that had undergone sham surgery, (surgery without actually performing the gastric by-pass surgery) and from mice that had sham surgery plus a restricted caloric diet. The microbiota samples were then put into the stomachs of lean mice that were microbiota free (they had no preexisting gut bacteria). A comparison of the type of gut bacteria in the test animals and weight changes was conducted .

Dr. Kapland was reported as saying "his group’s experiments were the first to try to find out if microbial changes could account for some of the weight loss after gastric by-pass."

Results included:

  • Mice with RYGB surgery lost 29% of their body weight within 3 weeks ... despite no change in net food intake.
  • Mice with RYGB surgery also had a marked change in the composition of microbes in the gut, with changes seen as early as a week after surgery.
  • Changes seen throughout the entire digestive tract of the mice that underwent RYGB were similar to changes previously reported in humans and rats.
  • The mice that received microbes from the RYGB surgery mice lost weight and had less fat mass than mice those who received microbes from either group of sham surgery mice.

This research demonstrated that the beneficial effects of RYGB surgery are due in part to changes in the gut microbial community. "Exactly how the altered intestinal bacteria might cause weight loss is not yet known," the researchers said. The microbes seem to influence energy balance and step up metabolism so that the animals burn off more energy.

“Our findings emphasize the importance of accounting for the influence of the trillions of microbes that inhabit our body when we consider obesity and other complex diseases,” said Dr. Peter J. Turnbaugh from Harvard University, one of the researchers,

Changing the gut bacteria may not be a replacement for all bariatric procedures , but the findings are promising. "There is a need for other therapies,” Dr. Kaplan said. “In no way is manipulating the microbiota going to mimic all the myriad effects of gastric bypass. But if this could produce 20 percent of the effects of surgery, it will still be valuable.” These findings could eventually lead to adjusting the microbe levels, or “microbiota,” in the gut as a weight loss treatment.

According the Centers for Disease Control and Prevention (CDC), Adult Obesity Facts report and based on a NCHS Data Brief, more than one-third of U.S. adults age 20 and over (35.7%) were obese in 2010, and that number is growing each year. Obesity is defined as a BMI equal to or greater than 30. That amounted to 78 million (41 million women and 37 million men). It is estimated that over 70% now fall into either the overweight or obesity range. According to a Health US News article, 18 million people in the United States suffer from severe obesity, the second-leading cause of preventable death in the United States. Severe obesity, defined as a weight that is at least 100 pounds more than the ideal for age and height is rising sharply.

According to the American Society for Bariatric Surgery, the number of weight loss procedures has increased from approximately 16,000 annually in the early 90's to over 200,000 annually. Roux-en-Y gastric bypass surgery is one of many bariatric surgical procedures used to treat severe obesity by surgically rerouting the digestive tract so that food actually by-passes most of the stomach and the upper portion of the intestine, where some of the food's calories are normally absorbed.

This information is not intended to replace a one-on-one relationship with a qualified health care professional and is not intended as medical/nutritional/fitness advice. Information presented is subject to change as additional discoveries are made or additional research is published.

Additional information: American Society of Metabolic and Bariatric Surgery

Sources: CDC/NCHS, National Health and Nutrition Examination Survey, 2009–2010,,,,,,,,,,



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