Obesity is a risk factor for cancer. Intentional weight loss in the obese might protect against malignancy, but evidence is limited. To our knowledge, the Swedish Obese Subjects (SOS) study is the first intervention trial in the obese population to provide prospective, controlled cancer-incidence data.

The SOS study started in 1987 and involved 2010 obese patients (body-mass index [BMI] ≥34 kg/m² in men, and ≥38 kg/m² in women) who underwent bariatric surgery and 2037 contemporaneously matched obese controls, who received conventional treatment. While the main endpoint of SOS was overall mortality, the main outcome of this exploratory report was cancer incidence until Dec 31, 2005. Cancer follow-up rate was 99·9% and the median follow-up time was 10·9 years (range 0–18·1 years).

Bariatric surgery resulted in a sustained mean weight reduction of 19·9 kg (SD 15·6 kg) over 10 years, whereas the mean weight change in controls was a gain of 1·3 kg (SD 13·7 kg). The number of first-time cancers after inclusion was lower in the surgery group (n=117) than in the control group (n=169; HR 0·67, 95% CI 0·53–0·85, p=0·0009). The sex–treatment interaction p value was 0·054. In women, the number of first-time cancers after inclusion was lower in the surgery group (n=79) than in the control group (n=130; HR 0·58, 0·44–0·77; p=0·0001), whereas there was no effect of surgery in men (38 in the surgery group vs 39 in the control group; HR 0·97, 0·62–1·52; p=0·90). Similar results were obtained after exclusion of all cancer cases during the first 3 years of the intervention.

Bariatric surgery was associated with reduced cancer incidence in obese women but not in obese men.

Swedish Research Council, Swedish Foundation for Strategic Research, Swedish Federal Government under the LUA/ALF agreement, Hoffmann La Roche, Cederoths, AstraZeneca, Sanofi-Aventis, Ethicon Endosurgery.