Bariatric surgery and heart events: the Swedish Obese Subjects study

Why this study matters

Living with obesity raises the risk of heart attack and stroke. Losing weight might lower that risk, but when this study was published, solid evidence was lacking. The Swedish Obese Subjects (SOS) study is one of the longest running comparisons of bariatric surgery (an operation on the stomach or gut to help with weight loss) and usual care. This report looked at heart attacks, strokes, and deaths from cardiovascular causes. Canadian readers may see this study cited when clinicians talk about the long-term effects of surgery.

Who was in the study

The study included 2010 adults who had bariatric surgery and 2037 matched adults who received usual care. People were recruited between September 1, 1987 and January 31, 2001 at 25 public surgical departments and 480 primary health care centres in Sweden. To join, people had to be 37 to 60 years old with a body mass index (BMI) of at least 34 for men or at least 38 for women. BMI is a number that compares weight with height. Follow-up ran until December 31, 2009, with a median of 14.7 years (range 0 to 20 years). The median is the middle value.

What the researchers did

This was a nonrandomized, prospective, controlled study. "Prospective" means people were enrolled and then followed forward in time. "Controlled" means there was a comparison group. Because it was not randomized, a computer did not assign people to surgery or usual care; each person who chose surgery was matched with a similar person who did not. The exclusion rules were the same in both groups.

Among surgery patients, 13.2% had gastric bypass, 18.7% had banding, and 68.1% had vertical-banded gastroplasty. The usual care group received care through the Swedish primary health care system. Everyone had physical and laboratory examinations at planned intervals, and the researchers cross-checked national databases.

The main outcome of the overall SOS study was total deaths, which was published in 2007. Heart attack and stroke were secondary outcomes that the researchers had defined in advance. The 2012 report explains those secondary outcomes.

What they found

There were 28 cardiovascular deaths among the 2010 surgery patients and 49 among the 2037 usual care patients. After adjustment, the hazard ratio was 0.47 (95% CI 0.29 to 0.76; P=0.002). A hazard ratio compares how often an event happens in two groups over time; a value below 1 means it happened less often in the surgery group. The 95% confidence interval (CI) is the range in which the true value is likely to fall.

The number of first cardiovascular events, meaning a heart attack or stroke, fatal or not, whichever came first, was lower in the surgery group: 199 events among 2010 surgery patients compared with 234 among 2037 usual care patients (adjusted hazard ratio 0.67; 95% CI 0.54 to 0.83; P less than 0.001) (R32-C98).

Side effects and people who stopped

The published summary we reviewed does not report surgical complications, side effects, or how many people left the study.

What this study does not tell you

• It shows an association, not proof of cause. People who chose surgery may have differed from the usual care group in ways that matching and adjustment could not fully correct.
• Most surgeries (68.1%) were vertical-banded gastroplasty, a procedure that is rarely used today. Results may differ with current operations.
• The study only enrolled people aged 37 to 60 with severe obesity. It does not tell you about younger or older adults or people with lower BMI.
• This report, the 2007 mortality report, and the 2020 life expectancy report all use the same people. They are overlapping analyses, not separate confirmations.
• The summary does not describe what usual care included, and the study predates modern weight-management medicines.
• The summary we reviewed does not state who funded the study.

What it means in Canada

This study is often cited when Canadian clinicians discuss the long-term heart-related effects of bariatric surgery. It does not tell you whether surgery is right for you, and it does not compare surgery with medicines. Separate randomized trials have looked at heart outcomes with some weight-management medicines; see semaglutide in Canada and tirzepatide in Canada. For questions about what provincial plans pay for, see public drug coverage by province.

Related reading

Other studies in this library that bear on the same question.

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