Type 2 Diabetes and Metabolic Surgery: How Does Surgery Affect Blood Sugar?

For a long time bariatric surgery was seen purely as a method of losing weight. That view changed when it was noticed that blood sugar in operated patients began to improve before any significant weight had been lost. Today these operations are assessed separately, under the heading of metabolic surgery, in selected patients with type 2 diabetes. This article explains what that distinction means.

The Difference Between Metabolic and Bariatric Surgery

The operations performed are largely the same. The difference lies in the aim and in patient selection. In bariatric surgery the primary target is the loss of excess weight; in metabolic surgery the primary target is correcting metabolic disorders such as blood sugar, blood lipids and blood pressure. Weight loss in that case is not the goal itself but one of the outcomes.

The practical consequence of this distinction is that metabolic surgery may also be considered in patients whose body mass index falls below the classical thresholds for bariatric surgery but whose diabetes cannot be brought under control with medication.

How Does Surgery Improve Blood Sugar?

The clearest observation showing that the effect does not come from weight loss alone is this: after operations that alter the intestinal route in particular, blood sugar begins to improve within days, while the patient is still in hospital. At that point no meaningful weight has been lost. The mechanisms under discussion are:

• Changes in gut hormones: food reaching the lower parts of the small intestine earlier strengthens the response of hormones such as GLP-1 that increase insulin secretion. Several medicines widely used in diabetes act on this same hormonal pathway.
• Food bypassing the duodenum: there are findings suggesting that taking the first part of the small intestine out of the route reduces insulin resistance.
• Changes in the circulation of bile acids and the resulting effects on glucose metabolism.
• Changes in the gut microbiota.
• Reduced calorie intake and weight loss: a component that affects insulin resistance but is not sufficient on its own.

These mechanisms remain an area of ongoing research; the extent to which each contributes is not fully established.

Who Is Assessed as a Candidate?

The criteria emphasised in international guidelines are:

• Patients with type 2 diabetes and a body mass index of 35 or above.
• Patients with a body mass index of 30 to 34.9 in whom blood sugar targets are not reached with medical treatment.
• In people of South and East Asian origin the thresholds are lowered by about two and a half points, because metabolic risk is higher at the same body mass index.

Type 1 diabetes is not within the scope of metabolic surgery. In type 1 diabetes the problem is not insulin resistance but the loss of insulin producing cells, and in that setting surgery does not have the corrective effect on blood sugar that is expected.

You can work out your own body mass index on the BMI calculation page. BMI is not a decision criterion on its own; assessment is made together with history, tests and examination.

Factors That Influence the Outcome

The same operation does not produce the same result in every patient. The main factors that help predict the outcome beforehand are:

• Duration of diabetes: the more recent the disease, the more marked the improvement in blood sugar. In diabetes of many years' standing the pancreas may have lost capacity to produce insulin.
• Insulin use: the likelihood of complete improvement is lower in patients using insulin than in those taking tablets only.
• Remaining pancreatic capacity: a C-peptide measurement gives an indication of this.
• The level of blood sugar control before surgery.
• Age and accompanying conditions.

Which Operation?

Two methods stand out:

Sleeve gastrectomy. An operation in which a large part of the stomach is removed and the intestinal route is left unchanged. It is technically simpler and affects vitamin absorption less. It has a marked beneficial effect on blood sugar.

Gastric bypass. A small stomach pouch is created and part of the small intestine is taken out of the route. Because the intestinal route changes, the hormonal effect is more pronounced, and the effect on diabetes has generally been found stronger than with sleeve gastrectomy in published studies. On the other hand absorption is affected more and vitamin and mineral monitoring is carried out more closely. It is a separately evaluated option in patients who also have reflux.

The choice is made by considering the characteristics of the diabetes, body mass index, the presence of reflux, previous abdominal surgery and the patient's ability to adhere to follow up. The comparison between methods is covered in detail in the article on choosing a method.

What Does Remission Mean, and Does It Last?

Remission means that blood sugar values remain within the target range without diabetes medication. After metabolic surgery the need for medication disappears or decreases in a substantial proportion of patients; the reported rate varies over a wide range depending on the definition used in the study, the type of operation and the length of follow up.

Two points need to be clear here. First, remission is not the same as cure; the diabetes is under control, not eliminated. Second, raised blood sugar can return over the years in some patients. Recurrence is usually related to weight being regained and to how long the diabetes had been present before surgery.

Follow up is therefore an inseparable part of the operation. Monitoring of blood sugar and HbA1c together with vitamin and mineral levels continues for life. How the first year after surgery unfolds is covered in a separate article.

What Happens to Diabetes Medication?

Medication is adjusted from immediately after the operation. Because calorie intake falls and the hormonal response changes, the need for insulin and glucose lowering drugs changes quickly; continuing the medication unchanged during this period can cause blood sugar to fall too low. Dose adjustment is therefore carried out jointly by the surgical team and endocrinology, not by the patient independently.

How Is the Decision to Operate Made?

Metabolic surgery is not a first line treatment. Dietary management, physical activity and medical treatment form the foundation of care and continue after surgery as well. Surgery is an option evaluated in patients who do not reach their targets through these steps and who meet the criteria.

The decision is taken through a process in which general surgery, endocrinology, nutrition and, where needed, psychiatric assessment are brought together.

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