Sleep apnea and metabolic syndrome: a strong association, without evidence of causality

A synthesis of 68 studies across 24 countries

The researchers pooled 68 studies involving 24,663 adults with obstructive sleep apnea and 11,423 adults without apnea. Their aim was to estimate the strength of the association between apnea and the presence of metabolic syndrome.

Confirmation of apnea by polysomnography was an important shared criterion. However, the diagnostic thresholds used varied between studies, as did the criteria defining metabolic syndrome. The available abstract does not specify follow-up durations.

3.33 times the odds, not a measure of individual risk

The pooled estimate gives an odds ratio of 3.33, with a 95% confidence interval of 2.86 to 3.87. In other words, the odds of metabolic syndrome are substantially higher in the group with apnea.

This does not mean that every person with apnea is 3.33 times as likely to develop this syndrome. An odds ratio is not a relative risk. The abstract does not provide the absolute frequencies needed to translate this association into a difference in probability between the groups.

The synthesis examines the association between the two conditions. It shows neither that apnea necessarily precedes metabolic syndrome nor that treating apnea reduces the occurrence of the syndrome.

Weight and differences between studies complicate interpretation

The results show substantial heterogeneity, with an I² statistic of 73.8%. The strength of the association varies notably by study type, geographic region, diagnostic criteria and publication year. The average estimate therefore does not describe all populations uniformly.

A larger difference in body mass index between the groups was associated with a stronger association between apnea and metabolic syndrome. This observation makes it harder to interpret any independent role of apnea, without allowing researchers to determine how much of the association is related to weight.

The authors recommend early, systematic metabolic screening in adults with apnea. This recommendation should be distinguished from the measured finding: the meta-analysis does not directly assess the benefits of a screening strategy. The authors also call for longitudinal and genetic data to explore causality.

What we can take away

Obstructive sleep apnea is associated with metabolic syndrome in this international synthesis. The association deserves attention, but it is not enough to establish a cause or promise a metabolic benefit from treating apnea.