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SOS study: bariatric surgery guidelines need updating

Guideline revision should give more importance to metabolic variables and less to BMI

The latest review of data from the Swedish Obese Subjects (SOS) trial has called for current guidelines for bariatric surgery to be updated, to improve access for those patients who are most likely to benefit from surgery. The paper, which was published in the Journal of Internal Medicine, also states that a revision of the guidelines should give more importance to metabolic variables and less to BMI.

Current guidelines focus on BMI and co-morbidities as determining a patient’s eligibility for surgery. This paper states that this is not the correct approach, arguing that glucose and insulin status are more relevant to the long-term health of patients.

“The findings of the SOS study have clearly demonstrated that, in contrast to baseline glucose and insulin, baseline BMI does not predict the surgical treatment effect on outcomes,” the authors write. “Thus, current guidelines for bariatric surgery need to be updated.”

The paper argues that the currently available non-surgical alternatives do not result in long-term weight loss or a reduction in the cardiovascular and metabolic disease risk factors reported in surgical patients. Surgery also leads to reductions in mortality and, the researchers claim, should be considered as a preventative treatment for diabetes in certain patients.

For example, antiobesity drugs are reported to result in 7%–10% weight reduction over two to four years, compared to 4%–6% in placebo groups or those treated with lifestyle modification. However, almost all patients relapse and regain the weight within a few years.

In the three surgery subgroups in the SOS trial, weight losses were significantly higher and more durable: 25± 11% in the bypass group, 16±11% in the gastroplasty group and 14±14% in the banding group, after ten years. After 15 years, the eight losses were 27±12%, 18±11% and 13± 4%, respectively.


The SOS study reported that bariatric surgery reduces overall mortality. During the follow-up period, 129 subjects died in the control group and 101 in the surgery group. Mortality in the surgery group includes postoperative deaths occurring within the first 90 days after surgery. Overall, surgery was associated with an unadjusted hazard ratio of 0.76 relative to the control subjects (p=0.04]. After multivariable adjustments for baseline conditions, the risk reduction was approximately 30%.


After 2 years of follow-up, 72% of SOS surgical patients with type 2 diabetes before surgery were in remission.

“The one- to two-year remission rates after bariatric surgery are extremely high compared with those seen after usual care in the SOS control group and after lifestyle interventions, exercise alone, weight loss medication or antidiabetic drug treatment,” the authors write.

Bariatric surgery also reduced the incidence of new cases of type 2 diabetes mellitus in non-diabetic subjects by at least 75% at two and ten years, according to the SOS data. In subjects without diabetes at baseline (1,771 controls and 1,658 in the surgery group), bariatric surgery reduced the risk of developing type 2 diabetes mellitus by 96%, 84% and 78% after two, ten and 15 years, respectively.

Impaired fasting glucose at baseline was associated with a more pronounced diabetes preventative effect of bariatric surgery, compared with normal fasting glucose (p=0.002). The number of patients needed to treat to prevent one diabetes case over ten years was only 1.3 in patients with Impaired fasting glucose, compared to 7.0 in patients with normal fasting glucose (p<0.05).

Importantly, baseline BMI did not predict the diabetes preventive effect of bariatric surgery (p=0.545).

Macrovascular disease

In addition, patients with  type 2 diabetes mellitus at baseline, the incidence of myocardial infarction was reduced in surgery, compared with control patients (p=0.025). The implication being that surgery has a long-term macrovascular benefit in type 2 diabetes mellitus patients in spite of a considerable ‘biochemical’ relapse rate after the initial two-year remission.


According to the authors, the value of BMI as a predictor of treatment effect does not seem to have been evaluated except in the SOS study. They report that BMI–treatment (surgery vs. control) with respect to mortality (p=0.60), cardiovascular disease events (p=0.59), cancer (p=0.90) and diabetes (p=0.55), did not predict the effect of surgery on any of these endpoints. However, insulin predicted the treatment effect with respect to mortality (p=0.013), cardiovascular events (p<0.001) and incidence of diabetes (p=0.007). There was also a strong impaired fasting glucose–treatment interaction with respect to diabetes incidence (p=0.002).

“These findings suggest that guidelines for bariatric surgery need to be modified. To select those patients who are most likely to benefit from surgery, more importance should be given to metabolic variables and less to BMI,” they write. “In contrast to baseline glucose and insulin, baseline BMI does not predict the surgical treatment effect on outcomes.”


The authors conclude that surgery is the only treatment for obesity resulting in an average of more than 15% documented weight loss over ten years and has dramatic positive effects on most, but not on all, cardiovascular disease risk factors.

The paper concludes that “Most countries do not have bariatric surgery capacity enough and with limited resources it has become very important to select those individuals who would benefit most from bariatric surgery.”

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