FRIDAY, Jan. 23 (HealthDay News) -- Three conditions often linked to obesity have also been tied independently to sleep apnea, new studies show.

Insulin resistance, the progression of liver disease, and living a less-than-active life were all found to be associated with the common breathing disorder, regardless of people's weight, according to reports published in the February issue of the American Journal of Respiratory and Critical Care Medicine.

A John Hopkins University study found a strong tie between insulin resistance -- the body's inability to metabolize glucose -- and sleep-disordered breathing (SDB), pauses or other abnormalities in breathing while sleeping.

"What our research tells us is that SDB is characterized by multiple physiological deficits that increase the predisposition for type 2 diabetes mellitus," study leader Dr. Naresh Punjabi, an associate professor of medicine and epidemiology at Johns Hopkins University School of Medicine, said in an American Thoracic Society news release.

Another Hopkins study found that obese people with chronic intermittent hypoxia, the lack of oxygen that occurs during obstructive sleep apnea (OSA), showed liver issues in proportion to the severity of the sleep disorder.

"We hypothesize that severe obesity, per se, acts as a first hit in the progression of liver disease, inducing hepatic steatosis, whereas the presence of the chronic intermittent hypoxemia that often characterizes OSA acts as a second hit," lead researcher Dr. Vsevolod Y. Polotsky, of the Johns Hopkins Asthma and Allergy Center, said in the same news release. "The hypoxic stress of OSA may induce oxidative stress in the livers of patients with severe obesity, leading to further inflammation."

Enzyme levels and other findings, though, suggest that obesity and sleep apnea are not completely tied to each other, he said, meaning that each condition must be dealt with separately to also address the complications of both.

"Our data suggest that patients with OSA and severe nocturnal hypoxemia should be screened for liver disease, and, conversely, patients with liver disease should be screened for OSA," Polotsky said.

The third study found that excessive sitting or standing during the day causes a fluid shift in the legs during sleep that may have a role in the development of sleep apnea.

When people lie down to sleep, fluid that has been retained in the legs during the day gets redistributed to the upper body, Dr. T. Douglas Bradley, professor of medicine and director of the Centre for Sleep Medicine and Circadian Biology at the University of Toronto, explained in the news release. "It is, therefore, plausible that some of the displaced fluid might reach the neck and predispose one to upper airway constriction," he said.

The researchers found these changes in people who were sedentary but not obese and who they suspected had obstructive sleep apnea -- a discovery that might help explain why 40 percent of people with the breathing disorder are not obese and why exercise without weight loss appears to reduce sleep apnea issues in some people.

"An important implication of our observations is that sedentary living may predispose to OSA, not only by promoting obesity but also by causing dependent fluid accumulation in the legs, which can shift rostrally to the neck overnight," Bradley said.

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Related MedlinePlus Pages:

• Liver Diseases
• Metabolic Syndrome
• Sleep Disorders