The main finding
In one sentence: In a large Swedish study that followed people for 10 years, those who had bariatric surgery weighed 16.1% less than when they started, while people who received conventional care weighed 1.6% more.
Group averages, not an individual prediction. A trial result describes what happened to a defined group over a defined time.
On this page
Why this study matters
Most weight-loss studies last one or two years. This report from the Swedish Obese Subjects (SOS) study asked a harder question: do the benefits of bariatric surgery last a full decade? Bariatric surgery means an operation on the stomach or gut to help with weight loss. Canadian readers thinking about long-term options, or supporting someone who is, may want to know what happens many years after surgery, not just in the first months.
Who was in the study
The SOS study enrolled adults living with severe obesity. Their average age was 48 years, and their average body mass index (BMI) was 41. BMI is a number that compares weight with height. A total of 4047 people had been in the study for at least 2 years when this analysis was done, and 1703 had been in it for at least 10 years. The study was run in Sweden. Not everyone came back for laboratory testing: 86.6% did at 2 years and 74.5% did at 10 years.
What the researchers did
This was a prospective, controlled cohort study, not a randomized trial. "Randomized" means a computer assigns each person to a treatment by chance. That did not happen here. Instead, people who chose to have gastric surgery were matched, at the same time, with similar people who received conventional (non-surgical) care. Researchers then followed both groups forward in time and measured weight, eating, physical activity, and several health risk factors. The published summary does not describe the conventional care in detail.
What they found
The main finding was about weight. After 2 years, the surgery group had lost 23.4% of their starting weight, while the control group had gained 0.1%. After 10 years, the surgery group was still 16.1% below their starting weight, and the control group was 1.6% above it (R32-C95). The summary reports that both differences were statistically significant (P less than 0.001), which means they were very unlikely to be due to chance.
| Time point | Surgery group | Conventional care group |
|---|---|---|
| 2 years | 23.4% weight loss | 0.1% weight gain |
| 10 years | 16.1% weight loss | 1.6% weight gain |
The surgery group also ate fewer calories and had a higher share of physically active people throughout the study.
Several health risk factors improved more in the surgery group. At both 2 and 10 years, more people in the surgery group recovered from diabetes, high triglycerides (a type of blood fat), low HDL ("good") cholesterol, high blood pressure, and high uric acid. Fewer people in the surgery group developed new diabetes, high triglycerides, or high uric acid.
Two results did not favour surgery. Recovery from high total cholesterol was no different between the groups. The study could not detect a difference in how many people developed new high cholesterol or new high blood pressure.
Side effects and people who stopped
The published summary we reviewed does not report surgical complications or side effects for this analysis. It does report that laboratory follow-up dropped to 74.5% at 10 years, so a sizeable share of eligible people had missing lab data at that point.
What this study does not tell you
- It cannot prove that surgery caused the differences. Because people were not randomized, those who chose surgery may have differed in ways the matching did not capture.
- Only 74.5% of eligible people had lab results at 10 years. Missing data can shift the results in either direction.
- The operations used in this study were done decades ago and differ from the procedures most often used today.
- The summary does not describe what conventional care included, so this is not a comparison with modern weight-management medicines.
- The study did not report deaths in this paper. A separate SOS report covers that question.
- The summary we reviewed does not state who funded the study.
What it means in Canada
This study is one of the longest running comparisons of surgery and non-surgical care, and it is often cited in Canadian guidelines. It does not tell you whether surgery is right for any one person, and it does not compare surgery with the medicines available today. If you want to know how weight-management medicines are used in Canada, see semaglutide in Canada and tirzepatide in Canada. For questions about what provincial plans pay for, see public drug coverage by province.
Common questions
How much weight did people keep off 10 years after bariatric surgery in the Swedish Obese Subjects study?
At 10 years, the surgery group weighed 16.1% less than at the start. The conventional care group weighed 1.6% more than at the start.
Was the Swedish Obese Subjects study a randomized trial?
No. People chose surgery or not, and each surgery patient was matched to a similar person receiving conventional care. That makes the results an association, not proof of cause.
Did weight-loss surgery improve cholesterol in the Swedish Obese Subjects study?
Recovery from high total cholesterol did not differ between the surgery and control groups, even though several other risk factors improved more with surgery.
Related reading
Other studies in this library that bear on the same question.
Bariatric surgery and death rates in the Swedish Obese Subjects study
Bariatric surgery and heart events: the Swedish Obese Subjects study
Life expectancy after surgery: the Swedish Obese Subjects study
Bariatric surgery and diabetes prevention in the Swedish Obese Subjects study
STAMPEDE: surgery versus medical therapy for type 2 diabetes
Source
Original paper: Lars Sjöström, Swedish Obese Subjects 10-year metabolic outcomes, The New England journal of medicine, 2004. PubMed: https://pubmed.ncbi.nlm.nih.gov/15616203/. DOI: https://doi.org/10.1056/NEJMoa035622. Funding: The summary we reviewed does not state the funding source. How we summarized it: abstract and extracted data.
This page explains a published study. It is not medical advice and does not describe whether a medicine is right for you. Talk to your doctor, nurse practitioner or pharmacist about your own situation. Reviewed and signed by two pharmacists registered in British Columbia, 2026-09-18.
Evidence records behind this page
Each record is a row in the clinical evidence file, taken from the published paper named above. The caution column is the limit the researcher recorded for that number.
This table scrolls sideways.
| Record | What the paper reports | Type | Population | Caution | Source |
|---|---|---|---|---|---|
| R32-C95 | At 10 years in the nonrandomized Swedish Obese Subjects cohort, mean weight was 16.1% below baseline after surgery and 1.6% above baseline with conventional care. | direct evidence | Adults with severe obesity; mean age 48 years and BMI 41 | Nonrandomized;10-year laboratory attrition; no causal interpretation. Related mortality 17715408/33053284 and cardiovascular 22215166. | PubMed 15616203 |