The discovery came about by accident more than a decade ago: Weight-loss surgery often led to dramatic improvement in the control of Type 2 diabetes, often before patients left the hospital.
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.
"We thought diabetes was an incurable, progressive disease," says Dr. Walter J. Pories, a professor of surgery at East Carolina University and a leading researcher on weight-loss surgery. "It . . . is a major cause of amputations, renal failure and blindness. This operation takes about an hour, and two days in the hospital, and these people go off their diabetes medication. It's unbelievable."
As many as 86 percent 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. 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 have in treating the illness.
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.
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.
The effect on diabetes can depend on the type of weight-loss surgery performed, with the highest rates of diabetes remission seen in people who have gastric bypass — about 83 percent.
But diabetes also tends to resolve or improve in 50 percent to 80 percent of people who have lap-band surgery, in which a band is placed around the top of the stomach to make it smaller, Pories says.
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.
It's not without risk
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 several studies are under way, or will soon begin, to examine the benefits of surgery in people with Type 2 diabetes and a BMI of less than 35.
But weight-loss surgery carries risk. The death rate is about 1 per 200 operations, and severe complications can occur, including blood clots, infection related to surgery, and the need for subsequent corrective surgery.
Other complications include vitamin and mineral deficiencies, dehydration, gallstones, kidney stones, hernia and low blood sugar.
However, a risk-benefit analysis published in April in the Journal of the American Medical Association by Purnell and a colleague suggests that if the number of gastric bypass operations performed on diabetic patients increased to 1 million per year, as many as 14,310 diabetes-related deaths might be prevented over five years.