Researchers found that people who suffer from obstructive sleep apnea - a condition where breathing stops and starts during sleep - are also likely to have a lower peak oxygen uptake during aerobic activity.
This was the conclusion of a study led by the University of California San Diego (UCSD) and published in the Journal of Clinical Sleep Medicine.
Obstructive sleep apnea - or more commonly just sleep apnea - is a condition where breathing starts and stops during sleep. A common feature is gasping or snorting noises during sleep - these are the moments when sleep is interrupted.
People who suffer from sleep apnea may also feel sleepy during the day because due to frequent interruption, night-time sleep is not sufficiently restorative.
Researchers believe that an early marker of higher risk for stroke and heart attack is a measure of exercise capacity known as VO2 max or peak VO2 - the maximum oxygen a person burns during strenuous exercise.
VO2 max is commonly measured during cardiopulmonary exercise testing (CPET) where a patient's heart and lung function is assessed during aerobic activity such as riding a bicycle.
The authors note that there is increasing interest in using CPET as a way to categorize sleep apnea patients in terms of heart risk.
However, lead author Jeremy Beitler, assistant clinical professor in pulmonary and critical care medicine at UCSD, and colleagues suspect that using CPET and VO2 max in this way with sleep apnea patients may not be as straightforward as it might seem.
The relationship between sleep apnea and exercise capacity is not clear, they note in their study background. So they set out to test the idea that sleep apnea itself might be linked to impaired exercise capacity.
Obesity is not a reason apnea is linked to impaired exercise capacity
And what they found confirmed their suspicions; people with moderate to severe obstructive sleep apnea appear to have a lower peak oxygen uptake during aerobic activity compared to people who do not suffer from the sleep disorder.
One explanation might be that apnea sufferers are also more likely to be obese, and would be expected to be less fit anyway. But the team found patients with sleep apnea had reduced aerobic fitness even when compared with people of similar body mass index.
Prof. Beitler says that encouraging patients with sleep apnea to exercise more might be part of the answer, but it is not the whole answer:
"We believe the sleep apnea itself causes structural changes in muscle that contributes to their difficulty exercising."
In their investigation, the team evaluated men and women with a range of apnea symptoms. They assessed the severity of each patient's condition and also screened them for other sleep disorders that might interfere with the study findings.
Eventually, 15 men and women with moderate to severe apnea and 19 with mild or no apnea (the controls) took part in the CPET aerobic fitness assessment. After undergoing resting state assessments, the participants were asked to pedal an exercise bike, gradually increasing resistance until reaching the point of exhaustion.
Moderate to severe sleep apnea linked to 14% lower VO2 max
After adjusting for baseline differences, the results showed that the participants with moderate to severe apnea had on average 14% lower VO2 max than the controls.
Using the data from the sleep evaluation together with the CPET results, the researchers also found a link between the severity of apnea - as measured by the number of times breathing stops for 10 seconds or more per hour of sleep - and reduced peak VO2.
The team also found that this measure of apnea severity - known as Apnea Hypopnea Index - could predict 16% of the variability seen in the group's peak VO2, a result that Prof. Beitler sees as "a big discrepancy."
The researchers conclude that sleep apnea is linked to impaired exercise capacity and say further research is needed to evaluate whether CPET can contribute usefully to the prognosis of patients with sleep apnea.
In September 2014, Medical News Today reported a study that showed the reason sleep apnea causes brain damage is because of weaker blood flow in the brain.