This is largely based on the observation that in many patients with diabetes, glycemic control rapidly improves within the first days of surgery (often even before significant weight loss sets in). Proponents of this “specific” benefit of surgery generally invoke the profound changes in gut hormones following bariatric surgery for these effects.
However, most bariatric clinicians will have observed similar “instantaneous” improvements in glycemic control in diabetic patients, who simply go on a fast or otherwise drastically restrict their caloric intake.
Now, a study by Mirjam Lips and colleagues from the Leiden University Medical Center in The Netherlands, published in Clinical Endocrinology suggests that most (if not all) of the (early) beneficial effect of bariatric surgery on glycemic control may be explained by the caloric restriction induced by this procedure.
In their study, Lips and colleagues, matched diabetic patients undergoing Roux-en-Y gastric bypass surgery with individuals undergoing the same degree of weight loss on a 600 KCal liquid very-low-calorie diet (VLCD). Both groups were exposed to a standardised meal challenge three weeks post-sugery or into the VLCD.
In both groups, baseline glycemic parameters as well as post-prandial glucose responses were similarly improved (essentially normalised in both groups).
Thus, caloric restriction was as effective in restoring early glycemic control in diabetic patients as was Roux-en-Y gastric surgery, suggesting that most of this effect was related to caloric restriction (and some weight loss) rather than any intrinsic mechanisms particular to surgery.
On the other hand, there were clear differences in the impact of surgery on incretins including increased levels of GLP-1 and PYY responses as well as greater suppression of ghrelin.
Based on these findings it appears that while the early beneficial effect of bariatric surgery on glycemic control may well be largely mediated by caloric restriction, the changes in gut hormones may well explain why patients with surgery are more successful in the long term in their ability to continue adhering to lower caloric intake and keeping the weight off.
Lips MA, de Groot GH, van Klinken J, Aarts E, Berends FJ, Janssen IM, Van Ramshorst B, Van Wagensveld BA, Swank DJ, Van Dielen F, van Dijk KW, & Pijl H (2013). Calorie restriction is a major determinant of the short-term metabolic effects of gastric bypass surgery in obese type 2 diabetic patients. Clinical endocrinology PMID: 23711328