Marked improvement in blood sugar control occurs in patients with type 2 diabetes mellitus shortly after Roux–en–Y gastric bypass surgery (RYGB) and before there is major weight loss.

A study determined whether the magnitude of this change is primarily due to caloric restriction or is unique to the surgical procedure. Eleven hospitalized subjects who underwent RYGB and 14 subjects mean–matched for BMI, HbA1c, and diabetes duration were given a very low–calorie diet of 500 kcal/day with a macronutrient content similar to that consumed by patients after RYGB.

Frequently sampled intravenous glucose tolerance tests were performed before and after interventions.

Both groups lost an equivalent amount of weight over a mean study period of 21 days. Insulin sensitivity, acute insulin secretion after intravenous glucose administration, and β–cell function as determined by disposition index improved to a similar extent in both groups.

Likewise, changes in fasting glucose and fructosamine levels were similar. Based on these data, VLCD improves insulin sensitivity and β–cell function just as well as RYGB in the short term.