Not only is obstructive sleep apnea linked to insulin resistance and liver disease independent of obesity, but at least one risk factor is also common to obesity and obstructive sleep apnea: prolonged daytime sitting or standing. Even when the sedentary lifestyle does not lead to obesity, it may still lead to obstructive sleep apnea and its concomitant health risks, according to another research article in the first issue for February 2009 of the American Journal of Respiratory & Critical Care Medicine.
“Overnight fluid displacement from legs, related to prolonged sitting, may play a previously unrecognized role in the pathogenesis of obstructive sleep apnea,” wrote principle investigator, T. Douglass Bradley, M.D., professor of medicine & director of the Centre for Sleep Medicine and Circadian Biology at the University of Toronto.
The researchers also found the volume of fluid shift was directly linked to the hours in a day that the subject reported sitting or standing and was independent of the excess weight that often accompanies sedentary lifestyles.
“In more recent years, the introduction of modern technologies into the workplace has greatly reduced the need for physical activity and increased the number of jobs requiring prolonged sitting, during which absence of the contraction of calf muscles leads to dependent fluid accumulation in the legs that is proportional to the time spent in that position,” explained Bradley.
“When assuming the recumbent position at bedtime, the fluid retained in the legs during the day in redistributed to the upper body. It is therefore plausible that some of the displaced fluid might reach the neck and predispose to upper airway constriction,” Bradley continued.
To determine whether that, in fact, was the case, the researchers recruited 23 non-obese subjects who were being evaluated for suspected obstructive sleep apnea and performed standard sleep studies that assessed each subject for sleep stages and number of arousals, as well as oxygen saturation of the blood. The circumferences of their calves and necks were also measured at bedtime and upon awakening, before they got up.
Indeed, they found that the only significant correlating factor with respect to severity of obstructive sleep apnea was the overnight change of fluid volume in the leg, which explained 67 percent of the variance in obstructive sleep apnea severity. The change in fluid in the leg was, in turn, directly related to the amount of time the subject reported sitting each day.
“An important implication of our observations is that sedentary living may predispose to obstructive sleep apnea not only by promoting obesity, but also by causing dependent fluid accumulation in the legs, which can shift rostrally to the neck overnight,” said Bradley.
This finding may also help explain why 40 percent of patients with obstructive sleep apnea are not obese, and why a reduction in obstructive sleep apnea has been described when subjects begin exercise programs, even in the absence of weight loss.
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