Fibromyalgia is a painful condition to live with. To add insult upon injury, many fibromyalgia sufferers also struggle with unwanted weight gain of 25 pounds or more.
Some medications can make people gain excess weight. And because fibromyalgia sufferers have a lot of pain, this makes them less active. Less calories burned will produce unwanted weight gain.
Metabolism often slows in fibromyalgia. Hormone deficiencies and imbalances are common. Slower metabolism results in fewer calories burned, making people gain excess weight.
The hormone insulin may not be doing proper job, so excess glucose ends up in the muscles. The muscles aren't up to storing all this glucose, converting the extra glucose into fat.
"The body is forced to go into a fat-storing mode where it converts this extra glucose into fatty tissue. Contrary to the popular myth that obesity is a result of eating too much fatty foods, obesity is usually the result of eating too many carbohydrates. A carbohydrate rich diet causes weight gain by converting the extra glucose into fat and, if Fibromyalgia causes more insulin activity and sensitivity, then the weight gain can be even greater."
http://www.prohealth.com/fibromyalgia/library/showArticle.cfm?libid=12301&B1