Surgical therapy for obesity is known as bariatric surgery, from the Greek baros- (meaning "weight") and iatreia (meaning "medical intervention"). The two most popular surgical approaches used in the United States are sleeve gastrectomy and Roux-en-Y gastric bypass (RYGB). Both of these procedures are often performed laparascopically, a minimally invasive surgical technique in which a small abdominal incision is made and the surgeon is guided by a laparoscope—a video camera used to visualize structures in the abdominal cavity.
Sleeve gastrectomy involves removing the fundus and the greater curvature of the stomach, so that the stomach becomes a narrow tube (see the figure at right). Initially sleeve gastrectomy was performed as the first stage in a more involved procedure to treat extreme obesity. However, it began being performed as a stand-alone surgery when it was found to cause significant weight loss by itself. It is now the most common bariatric procedure performed.
Sleeve gastrectomy greatly reduces the volume of the stomach, causing the person to “feel full” more rapidly so that food intake is reduced. Thus, it is considered a restrictive procedure in which weight loss occurs because of limited food intake. However, other factors probably contribute to weight loss after sleeve gastrectomy.
Importantly, sleeve gastrectomy greatly reduces the level of ghrelin, a hormone that is primarily secreted by cells in the stomach. Ghrelin secretion is normally highest right before eating. Ghrelin functions to promote appetite and increase food intake.
Another effect of sleeve gastrectomy is that is causes food to progress more quickly to the lower intestine to stimulate hormone release. The ileum is the location of endocrine cells called L cells, which secrete the incretin hormone GLP-1 and another hormone, PYY. As an incretin, GLP-1 increases insulin secretion by pancreatic beta cells. PYY and GLP-1 are also thought to work in the brain to promote satiety. (Optional: review the web page on Incretins)
In RYGB, there is a rearrangement of the digestive tract (see
figure at right). The stomach is divided so that a small gastric
pouch is created just below the entry of the esophagus into the
stomach. This is linked to a piece of the lower intestine, usually
the jejunum. This creates an alimentary channel that bypasses
the greater part of the stomach and the duodenum. The segment
consisting of the stomach and the upper small intestine is linked
to the alimentary channel at a lower point along the small
intestine. Important secretions from the liver (bile) and the
pancreas (digestive enzymes) are still able to enter the GI tract
via this biliopancreatic channel.
This configuration basically creates a Y. One branch of the Y is the alimentary channel containing food. The other branch of the Y is the upper small intestine containing the biliopancreatic secretions (biliopancreatic channel). Note that digestion and absorption cannot occur until these contents are mixed in the common channel (the stem of the Y).
Since the common channel is shorter than the small intestine before surgery, RYGB has the potential to be malabsorptive (causing weight loss due to reduced absorption), as well as restrictive (causing weight loss due to less food intake). The length of the common channel determines how much absorption can occur, and a surgeon may sometimes create a much shorter common channel in order to create a greater degree of malabsorption.
Because of the bypass of the stomach and duodenum, RYGB also causes malabsorption of micronutrients (vitamins and minerals). Post-surgery, patients should take vitamin and mineral supplements to avoid micronutrient deficiencies. A common micronutrient deficiency that occurs in people who have had RYGB surgery is iron deficiency.
Two randomized trials published in January 2018 (see below) comparing sleeve gastrectomy with RYGB found little difference in the weight loss achieved after five years. There were also similar rates for remission of type 2 diabetes mellitus between the two types of surgery, although the numbers were better for RYGB. One notable difference is that in patients with gastroesophageal reflux disease (GERD) there tended to be improvement following RYGB; however, GERD was found to worsen or newly arise in some patients following sleeve gastrectomy.
Both types of bariatric surgery, in addition to producing profound weight loss, are effective in causing remission of type 2 diabetes mellitus (T2DM). Remission means a reduction in glycated hemoglobin to normal levels without the need for diabetes medication. Certainly, profound weight loss plays a role in improving insulin sensitivity. However, many patients with T2DM begin to show improvements in insulin sensitivity within the first few weeks after surgery, well before they lose weight. This is particularly true for patients who have undergone RYGB. It is thought that the rearrangement of the digestive tract somehow causes endocrine changes that are beneficial.
There are several endocrine effects that may contribute to remission of improved glucose homeostasis.
For a recent review discussing how bariatric surgery causes remission of T2DM, see: Abbasi, J.(2017) Unveiling the "Magic" of Diabetes Remission After Weight-Loss Surgery JAMA 317(6): 571-4 (link) [URL--http://jamanetwork.com/journals/jama/fullarticle/2601499]
Here are the references to the two studies that compared sleeve gastrectomy and RYGB:
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