Excess weight is known to aggravate fibromyalgia symptoms – such as pain sensitivity and sleep disturbances – and reduce quality of life among patients. New research now shows, the more severe the obesity, the more severe the fibromyalgia symptoms, and those who have a body mass index, or BMI, of 35 or higher experience the worst symptoms.

“I’m not surprised,” says study co-author Terry H. Oh, MD, assistant professor of physical medicine and rehabilitation at the Mayo Clinic, in Rochester, Minn. She says that earlier studies have shown that obesity increases fibromyalgia symptoms, but there hadn’t been concrete information before this study about whether different levels of obesity affected fibromyalgia symptoms differently.

In other words, is all obesity the same when it comes to fibromyalgia? The answer is no: Bigger definitely is worse, leading to significantly more severe symptoms. “The severely obese seemed to have the most problems,” says Dr. Oh.

Her study, which was published online in the journal Arthritis Care & Research, divided 888 adults with fibromyalgia into four groups: non-obese (BMI less than 25), overweight (BMI of 25 to 29.9), moderately obese (BMI of 30 to 34.9) and severely obese (BMI equal to or greater than 35). Patients were tracked for three years, underwent a physical evaluation and were asked to fill out health surveys and the Fibromyalgia Impact Questionnaire (a questionnaire used to assess fibromyalgia’s impact on a patient).

The researchers found – for most symptoms, but not all – that as the BMI category increased, so did the severity of the symptoms. Patients in the highest BMI group, for example, missed work more often because of fibromyalgia symptoms and reported more pain, stiffness and fatigue compared with those who were not obese or less obese. Also, the study found that the severely obese patients had more tender points on their bodies and worse physical functioning than the non-obese and less obese.