Today, evidence of the connection is so solid that some doctors say surgery should be considered as a treatment for diabetes, regardless of a person's weight or desire to lose weight.
As many as 86% of obese people with Type 2 diabetes find their diabetes is gone or much easier to control within days of having weight-loss surgery, according to a meta-analysis of 19 studies published earlier this year in the American Journal of Medicine (78% of patients with a remission of diabetes and 86.6% with remission or improvement). But experts still aren't sure why obesity surgery helps resolve Type 2 diabetes or how long the effect might last. And they disagree on how big a role surgery should take in treating the illness.
"We are going from seeing the results to understanding why it happens," said Dr. Santiago Horgan, director of the Center for the Treatment of Obesity at UC San Diego.
This much is clear: Patients who have weight-loss surgery begin to lose weight rapidly, which by itself improves Type 2 diabetes, allowing diabetics to more easily control their blood glucose levels. But something else appears to be occurring as well.
There is strong evidence that surgery -- especially gastric bypass surgery, which makes the stomach smaller and allows food to bypass part of the small intestine -- causes chemical changes in the intestine, says Dr. Jonathan Q. Purnell, director of the Bionutrition Unit at Oregon Health & Science University. The small intestine has been thought of simply as the place where digestion occurs.
But researchers now suspect it has other functions related to metabolism. Surgery somehow alters the secretion of hormones in the gut that play a role in appetite and help process sugar normally.
Multiple studies in humans and animals indicate that surgery triggers reductions in ghrelin, the hormone that stimulates hunger, and elevates levels of peptide YY and glucagon-like peptide-1, both of which act as appetite suppressants. Another theory is that surgery might alter the expression of genes that regulate glucose and fatty-acid metabolism.
"There are these known components that improve glucose metabolism," Purnell says. "But there are very likely other things happening as well."
The effect on diabetes can depend on the type of weight-loss surgery that is performed, says Pories, past president of the American Society for Metabolic and Bariatric Surgery. The highest rates of diabetes remission are seen in people who have gastric bypass -- about 83%.
But diabetes also tends to resolve or improve in 50% to 80% of people who have lap-band surgery, in which a band is placed around the top of the stomach to make it smaller, he says. And there is some evidence that the effect occurs a newer type of weight-loss surgery called gastric sleeve, in which a portion of the stomach is removed so that it takes the shape of a tube or sleeve.
Evidence suggests the effect on diabetes can last for an extended period or even indefinitely, particularly if people don't regain a lot of weight.
"There is durability, but we also know that some people do get the disease back again," Purnell says. "Weight rebound is probably one factor. We also know that diabetes is a progressive disease. It may depend on how long you've been diagnosed with diabetes. If it's early on, I think the durability may be better."
It's not clear yet why people have different responses.
"There is some evidence that African Americans don't respond as well as Caucasians, and men don't respond as well as women," Pories says.
Despite the unknowns, the evidence that a majority of people experience long-term improvement in blood glucose control suggests the surgery could eventually play a greater role in the treatment of obese people with Type 2 diabetes. The majority of American adults with Type 2 diabetes are overweight.
Traditional medical guidelines, which insurers follow, state that weight-loss surgery should be restricted to patients with a body mass index of 35 or greater who have related health problems. But some diabetes and nutrition experts think those recommendations don't go far enough. Several studies are underway, or will soon begin, to examine the benefits of surgery in people with Type 2 diabetes and a BMI of less than 35.