Turns out, you can be too fat for a weight loss surgery.

A large new study from the University of Michigan, Wayne State University and Henry Ford Health System showed that only one in three patients who have the operation succeed in getting their body-mass index (BMI) below 30, the cut-off for obesity, in the first year.But the odds of achieving this goal were much higher for those who had their weight-loss operation while they were still below the "morbid obesity" BMI level of 40. On the other end of the spectrum, less than 9 percent of those who went into surgery with a BMI above 50 got down to 30 or below in the first year.Getting down to a BMI under 30 also brought patients much better odds of escaping weight-related health risks. Those who hit this level were much more likely to report that they'd stopped taking medication to control high blood pressure, blood sugar and cholesterol, and achieved remission of an obesity-linked breathing condition called sleep apnea.

In all, 92 percent of those who hit a BMI under 30 in the first year said they were highly satisfied with their decision to have surgery, compared with 78 percent of those who didn't get to that level.The findings could help surgical teams counsel prospective patients about realistic expectations and the best timing for surgery.The authors also say their results also have implications for how insurers cover weight-loss surgery, including requirements that patients fail to achieve a specific BMI under medically supervised non-surgical options before plans will approve coverage for surgery."Despite its proven safety and efficacy, bariatric surgery remains highly regulated and can be misunderstood by referring physicians and patients alike," said lead author Oliver Varban. "This study provides additional data to help counsel patients appropriately about weight-loss expectations after bariatric surgery."

The study included about half of all patients who had bariatric surgery under the direction of one of 70 surgeons based at the 38 participating programs in hospitals of different sizes and types. Only those who had at least one year's worth of weight data and did not undergo a second operation to revise their first were included. Many also took a survey about health-related issues.On average, the patients had a BMI of 48 before their operation, and got down to 33 by the end of the first year. Just over 9,700 patients, or 36 percent, achieved a BMI under 30.The type of bariatric surgery patients had mattered too. Those whose surgeons chose to perform a sleeve gastrectomy, gastric bypass or duodenal switch operation were more likely to achieve a BMI under 30 than those who had adjustable gastric bands placed around their stomachs. The two groups didn't differ in terms of overall surgical complication rates, or rates of serious complications.

The study is published in JAMA Surgery.