NEW YORK (Reuters Health) - Canadian researchers have identified a new risk factor for obstructive sleep apnea (OSA) in normal-weight men: too much sitting.

The more time a man spends seated during the day, Dr. Stefania Redolfi of the Toronto Rehabilitation Institute and her colleagues found, the greater is the volume of fluid that shifts from his legs to his neck overnight; and the greater the fluid shift, the worse the OSA.

When people are active, calf muscle contractions help prevent fluid from collecting in the legs, the researchers explain. But many people's work requires them to sit for long periods of time, causing fluid to accumulate.

Obesity is known to increase one's likelihood of OSA, which is gauged by the apnea-hypopnea index, or the number of times a person's breathing stops or slows per hour of sleep. As many as 60 percent of people who have the sleep disorder aren't obese, Redolfi and her team add.

People with swelling due to kidney or heart failure are known to be more prone to OSA, the researchers note, possibly due to fluid retained around the upper airway. She and her colleagues previously showed that displacing fluid from healthy, non-obese people's legs boosted their neck circumference, narrowed their windpipes, and made the upper airway more collapsible.

The researchers recently tested their hypothesis again with 23 non-obese men with suspected sleep apnea overnight.

They found that the amount of fluid that redistributed from the legs to the upper body corresponded with apnea-hypopnea index scores. This was also strongly linked to overnight changes in neck circumference and the amount of time a man had spent sitting the previous day. And amount of time spent sitting was itself linked to a greater fluid shift.

"An important implication of our observations is that sedentary living may predispose to OSA not only by promoting obesity," but also by causing accumulation of fluid in the legs, colleagues write in the American Journal of Respiratory and Critical Care Medicine.

Further research is needed, they conclude, to determine if preventing fluid from collecting in the legs during the day-or from shifting to the neck overnight-will help reduce OSA severity.

SOURCE: American Journal of Respiratory and Critical Care Medicine, January 26, 2009.

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