Bariatric Surgery

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

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.

After sleeve gastrectomy surgery, food intake decreases, resulting in weight loss.  Sleeve gastrectomy greatly reduces the volume of the stomach, and this may cause earlier satiety (the sense of feeling "full"), so that the individual eats less.  However, the modern view is that there are a large number of neural and endocrine changes that probably contribute to weight loss following sleeve gastrectomy (see below).

Roux-en-Y Gastric Bypass (RYGB)

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 cause weight loss by reducing absorption of nutrients, as well as limiting intake due to the small size of the stomach pouch.  The mechanism of weight loss does not seem to be related to malabsorption.  Similar to patients who have undergone sleeve gastrectomy, patients who have had RYGB surgery eat less.  Complex endocrine and neural changes may underlie the reduced food intake and altered metabolism that promote weight loss.

Because of the bypass of the stomach and duodenum, RYGB creates a situation that can cause the 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.

Endocrine Changes Following Bariatric Surgery

Both types of bariatric surgery, in addition to producing profound weight loss, are effective in causing remission of type 2 diabetes mellitus (T2DM). T2DM is a disorder that is characterized by hyperglycemia (high blood glucose) and insulin resistance, which is a decreased sensitivity to insulin.  Patients with T2DM also usually have defects in insulin secretion.  The majority of people with T2DM are overweight or obese.

Remission means a reduction in blood glucose to normal levels without the need for diabetes medication. Profound weight loss probably 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.

Effects on three key hormones are thought to play a major role in weight loss and remission of T2DM that occurs in patients who have had bariatric surgery.

• One effect is decreased secretion of ghrelin. Ghrelin is a hormone that is primarily secreted by cells in the stomach epithelium.  Ghrelin secretion is normally highest right before eating.  It is thought that ghrelin functions to promote appetite and increase food intake.  In addition to its affects to promote appetite, ghrelin may have effects on insulin secretion and insulin sensitivity.
• A second effect is that bariatric surgery alters the GI tract so that there is more rapid delivery of food to the ileum to stimulate hormone release.  The ileum is the location of L cells, endocrine cells in the epithelium of the GI tract that release the hormones GLP-1 and PYY.  GLP-1 is an incretin hormone, a hormone that acts on the endocrine cells in the pancreas to enhance glucose-dependent insulin secretion.    PYY is a hormone that acts in the brain to promote satiety.
• A third possibility is that the effects on glucose homeostasis are caused by an as yet unidentified hormone released from the upper GI tract. The secretion of this hormone changes when the upper GI tract is bypassed, and this change leads to the improvement in insulin secretion and sensitivity.  RYGB is the most effective surgery for causing remission of T2DM, providing support for this as a key endocrine mechanism.
  

The regulation of satiety and nutrient homeostasis is complicated and other factors are likely to be involved.

Comparison Between the Two Procedures

Two randomized trials published in January 2018 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.

A larger trial published in 2020 that focused on diabetes mellitus outcomes found that RYGB was slightly better than sleeve gastrectomy for achieving remission of T2DM.

If you are interested, there are links to the articles reporting on these studies in the darker blue "Optional" section below the quick quiz.