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

A study evaluated whether the magnitude of this change is mainly a result of 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,received 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 prior to and following the interventions.

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

Changes in fasting glucose and fructosamine levels were also similar. The data therefore suggest that VLCD improves insulin sensitivity and β-cell function just as well as RYGB in the short-term.