Bariatric surgery for type 2 diabetes: Weighing the impact for obese patients

HOW DOES BARIATRIC SURGERY IMPROVE TYPE 2 DIABETES?

Three major mechanisms have been proposed to explain how bariatric surgery reverses diabetes. ^24,25 Table 2 summarizes the effects of the different procedures on factors involved.

Hypothesis 1: Weight loss increases insulin sensitivity

The enforced caloric restriction, negative energy balance, and weight loss after bariatric surgery reduce insulin resistance. Consequently, the beta cells can rest because they don’t need to produce as much insulin. These effects have been observed after both gastric restrictive procedures and gastric bypass procedures.

Hypothesis 2: Less lipotoxicity, inflammation

Another theory is that bariatric surgery lessens insulin resistance by reducing “lipotoxicity,” a condition related to dysregulated fatty acid flux, lipid metabolites in tissues, and direct and indirect effects of hormones secreted by adipocytes.

The strongest evidence for this theory comes from Bikman et al,²⁶ who found that insulin sensitivity increased after Roux-en-Y surgery more than expected from weight loss alone. One year after surgery, even though they remained anthropometrically obese (BMI > 30 kg/m²), the patients had insulin sensitivity levels similar to those in a control group of lean people (BMI < 25 kg/m²).

Insulin sensitivity begins to improve within 1 week of intestinal bypass procedures,^15,27 suggesting that these procedures are doing something more than simply forcing weight loss via caloric restriction, as gastric restrictive procedures do.

Hypothesis 3: An effect on gut hormones

The third theory is likely the most relevant and relates to various hormones secreted by the gut in response to food (Figure 1). Surgical exclusion of the duodenum in the Roux-en-Y procedure and exclusion of the duodenum and jejunum in biliopancreatic diversion result in altered sites—or at least altered relative distribution— of carbohydrate and fat absorption.

The “hindgut hypothesis” raised by Cummings et al²⁴ suggests that accelerated transit of concentrated nutrients (particularly glucose) to the distal intestine results in increased production of insulinotropic and appetite-controlling substances, which account for the reversal of hyperglycemia and obesity.

In contrast, the “foregut hypothesis” raised by Rubino et al²⁸ suggests that nutrient interactions in the duodenum are diabetogenic and, hence, bypassing the duodenum would reverse this defect. Their conclusions come from experiments in rodents that underwent jejunoileal bypass and subsequent refeeding through the bypassed intestine.

GUT HORMONES AND OTHER PEPTIDES ALTERED BY BARIATRIC SURGERY

Incretin hormones: GLP-1, GIP

Gastrointestinal hormones that increase insulin release after a meal are known as incretins. Of interest, they have this effect only when glucose is ingested orally—not when it is infused intravenously.^29,30

Glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP) account for 50% to 60% of nutrient-related insulin secretion. In addition to stimulating insulin, GLP-1 suppresses glucagon and slows gastric emptying, which delays digestion and reduces postprandial glycemia. GLP-1 also acts on the hypothalamus to induce satiety.

Laferrère et al³¹ and others^32,33 documented robust increases in postprandial levels of GLP-1 within 4 weeks after Roux-en-Y surgery. GLP-1 levels did not increase with comparable weight loss induced by diet.

Rubino et al^28,34 documented similar findings that occurred prior to marked weight loss, suggesting that the benefit of Roux-en-Y surgery on remission of diabetes may not be completely attributable to reduced caloric intake and weight loss. Insulin secretion is generally reduced after gastric restrictive procedures (eg, laparoscopic adjustable gastric banding) and biliopancreatic diversion,³⁵ and is increased after Roux-en-Y gastric bypass.^32,33,36

Noninsulinotropic peptides: Ghrelin, peptide YY

Noninsulinotropic gut peptides that are altered after Roux-en-Y surgery include ghrelin and peptide YY.

Ghrelin, a hormone derived from the gastric fundus, stimulates appetite. Ghrelin concentrations are lower after Roux-en-Y surgery, indicating that suppression of hunger signals helps sustain weight loss. In contrast, ghrelin levels increase with diet-induced weight loss.³⁷ However, the data on ghrelin levels at various times after bariatric surgical procedures are not consistent.^33,38

Peptide YY, like GLP-1, is secreted by L cells of the distal small intestine and is responsible for increasing satiety and delaying gastric emptying after meals. Numerous studies have consistently documented increases in postprandial peptide YY and GLP-1 levels after gastric bypass.^{32,33,39–41}

ACUTE EFFECTS OF BARIATRIC SURGERY ON INSULIN SECRETION, SENSITIVITY

Bariatric surgery alters both insulin secretion and insulin sensitivity, thus improving glucose regulation.

The relationship between insulin secretion and sensitivity is a hyperbolic curve, so that any change in insulin sensitivity is balanced by a reciprocal and proportionate change in insulin secretion. The development of type 2 diabetes is characterized by a reduction in insulin secretion (decompensation) relative to the severity of insulin resistance.

In the first 6 weeks after Roux-en-Y gastric bypass or biliopancreatic diversion, insulin sensitivity improves while insulin secretion increases disproportionately, associated with a robust increase in GLP-1, and resulting in normal glucose homeostasis.^16,31,42

In contrast, patients who lose weight by dieting or undergoing gastric restrictive procedures show a modest increase in insulin sensitivity and a compensatory reduction in insulin secretion, termed “beta-cell rest.”^16,31,42