A lot of people think that gastric surgeries work by making you eat less. True — but that’s only part of the story.
With about 35 percent of adult Americans considered obese, many people are looking for long-term fixes with rapid results. Approximately 140,000 people in the United States get bariatric surgery each year. These weight-loss surgeries are accompanied by significant health improvements, including a reduction in cholesterol levels and blood glucose numbers. In fact, for people who are obese with type 2 diabetes, bariatric surgery is becoming a go-to for treatment.
“There are multiple mechanisms by which people lose weight from these surgeries,” said Allison Goldfine, M.D., head of the section on Clinical, Behavioral and Outcomes Research at Joslin Diabetes Center. “Breaking apart the different mechanisms and understanding the combinations that are important are active areas of investigation for many scientists around the world.“
Bariatric surgeries come in three general varieties—an adjustable gastric band, a gastric sleeve, and a gastric bypass. All surgeries carry some risk, and long term follow-up of bariatric surgical patients and late complications also differ based on the procedure and must be considered in context of non-surgical approaches which can also be effective for many patients. Mechanistically, the procedures differ to the degree they cause restrictive, malabsorptive, and hormonal changes.
All three surgeries produce a small pouch at the top of stomach, and thus have component of restriction. Restrictive surgeries reduce the size of the stomach, thus reduce the amount of food that can fit in the smaller pouch. Gastric banding, one type of restrictive surgery, involves placing an adjustable band around the top part of the stomach. The band can be tightened (or loosened) non-invasively by injecting saline (salt water) solution into a balloon that sits under the skin. This usually happens every four to six weeks during the first year after the surgery and on an as-needed basis as time goes on.
The gastric band can help the patient improve their serving sizes long-term. “There is probably also a bit of a ‘Pavlovian effect’, so you learn that you get uncomfortably full after a big portion, and thereby relearn or train yourself as to what is an appropriate portion size. Years later, even if the band is loose, many people have learned how to eat better and can maintain weight loss, and for others the band can be adjusted.“ said Dr. Goldfine.
A sleeve gastrectomy, a relatively new technique, also creates a smaller stomach by stapling the stomach vertically and removing the section that is no longer in use. “The sleeve is gaining in enthusiasm because it’s relatively simple to do. However, the long term effectiveness of the sleeve has been least studied of the three more common surgical approaches,” said Dr. Goldfine. Short-term results show that people who get this surgery lose an average of 50 to 60 percent of their excess body weight, slightly more than with the gastric banding which achieves between an average of 45 to 50 percent excess weight loss.
Gastric bypass surgeries are what they sound like; the stomach and intestines are rerouted so that food bypasses a large part of the digestive system known as the early small intestine. The most popular and effective version of the gastric bypass is the Roux-en-Y (after everything is rearranged, the innards vaguely resemble a “Y” shape). A small stomach pouch is formed, so there is also an element of restriction with this procedure. Roux-en-Y connects this pouch to a lower part of the intestine so there is also an element of malabsorption of nutrients. People who get Roux-en-Y surgeries lose, on average, 60 to 70 percent of their excess weight.
The Hunger Hormones
There are also hormonal changes that occur to varying degrees after these procedures which decrease hunger, increase satiety and improve insulin secretion and action. The greatest hormonal changes occur after the gastric bypass. For example, people who have had the Roux-en-Y and sleeve gastrectomy surgeries show reduced levels of ghrelin, a hormone that stimulates appetite.
“Typically, ghrelin goes up before you eat, signaling hunger to your brain. Then you eat, and ghrelin gets suppressed, letting you know you are no longer hungry. Ghrelin is part of our signal to initiates a meal,” said Dr. Goldfine. “People who lose weight on a diet have higher ghrelin levels, driving appetite and weight gain. In contrast ghrelin levels are lower in people who have had bypass, despite weight loss, contributing to the ability to maintain their new lower weight. This may be part of the reason why it’s so hard to lose weight with diet because you get more stimulation of these hormones and you eat more”.
Bariatric surgeries have benefits for patients with type 2 diabetes, as well. Many studies demonstrate 78 percent of gastric surgery patients with diabetes may have a remission of the disease that may last many years; eighty-six percent showed improvements although continue to need medications. The weight loss in itself can improve insulin sensitivity, but studies show variations in different hormones may also play a role in the improvements in blood sugars.
People who have either Roux-en-Y surgeries or sleeve gastrectomies show an increase in the level of a hormone called GLP-1 (or glucagon-like peptide-1). This hormone helps the body make insulin in response to a meal. GLP-1 can also be found in type 2 diabetes medications, where it increases the release of insulin from beta cells. If the body can increase its production of this hormone by itself, that could help in the disease’s remission. It also helps to tell the body that it’s full after a meal. More of this hormone means quicker satisfaction after eating, making going back for seconds less appealing. Another hormone that promotes insulin sensitivity, called adiponectin, also increases after any gastric surgery.
How it Helps
Some researchers suggest that bariatric surgery improves blood glucose control because of where food hits—or doesn’t hit—in the digestive system. The hindgut hypothesis says that because food gets to the intestines faster, it’s able to better trigger the secretion of hormones that affect blood glucose control. The foregut hypothesis proposes that keeping certain parts of the system from coming in contact with food prevents the body from sending out certain insulin resistance signals.
Microbes could also play a role. “There are probably changes in the microbiome. There are bacteria that live in our gut and help process the food that we eat. We think that the bacteria that we host, that we cohabitate with, are also important for nutrient regulation,“ said Dr. Goldfine. The colonies of microscopic organisms that live in everyone’s guts are altered by gastric surgery, affecting the body’s metabolism for the better.
It seems that the more intensive the surgery, the greater the results. Roux-en-Y, the most invasive of the three most commonly performed surgeries, has the highest weight loss and diabetes remission success rate. Sleeve gastrectomy comes in second place on both counts, slightly ahead of gastric banding. But these surgeries also have greater risk.
“When considering which surgery to recommend, occasionally there are reasons which clearly make one procedure better than other for an individual patient, but in general doctors work with the patient to understand what are the patient’s own risk tolerances, how adherent the patient would be in the different follow up that is needed for the different procedures to help ensure health, and what are their own goals. So it’s very individualized,” she said. The risk and potential success of each surgery depends on the patient’s individual situation, and all options should be discussed in full with health care providers before deciding on surgery.
“It’s a very interesting area of research, and we have many lessons to learn,” said Dr. Goldfine.