By Jennifer Couzin-Frankel
Usually, science starts in the lab and then moves to patients. Gastric bypass surgery has taken the opposite path.
Originally offered as a radical treatment for severe obesity, the surgery's effects on the digestive system and metabolism have turned out to be far more mysterious and fascinating than anyone expected. Now, a new study probes another of the surgery's effects: its impact on microbes in the gut and how changing these microscopic communities might drive weight loss.
The most popular type of gastric bypass surgery is called Roux-en-Y. Surgeons make a small pouch from the top of the stomach and separate it from the rest of the organ, then connect that directly to the middle of the small intestine. Originally, doctors believed that patients who underwent gastric bypass lost weight for a simple reason: Their stomach couldn't hold as much food, and they couldn't absorb as many nutrients.
But quickly, the picture got more complicated. In many people with type 2 diabetes, the disease vanishes almost immediately after surgery, too quickly to be explained by the gradual weight loss that happens later. Patients also describe not being as hungry, or craving foods like salad that they hadn't liked much before.
"Food doesn't call out to them anymore," says Lee Kaplan, a molecular biologist and gastroenterologist at Harvard Medical School and Massachusetts General Hospital, both in Boston.
Likely many mechanisms are at work. Some may stem directly from how the altered digestive system works - secreting different levels of hormones, for example - or changes in nerve cells that communicate with the gut. Kaplan and Harvard University microbiologist Peter Turnbaugh, who had been studying gut microbes in obese and lean animals, were intrigued by other work suggesting that in both humans and rats, the microbial balance in fecal samples changed after gastric bypass. Along with a postdoc in Kaplan's lab, Alice Liou, they decided to test whether the surgery itself caused the changes in the population of gut microbes, known as the microbiota.