Syndrome Z is defined as the co-occurrence of obstructive sleep apnea (OSA) and metabolic syndrome. There is a paucity of information on the magnitude of syndrome Z in the community and the factors associated with it.

We conducted a two-stage, cross-sectional, community-based study in four different socioeconomic zones of the South Delhi district, India, from April 2005 through June 2007. In stage 1, a systematic random sample of subjects of either gender aged 30–65years were administered a questionnaire by door-to-door survey. Subjects that responded were classified as habitual and non-habitual snorers. In stage 2, all the habitual and 10% of randomly selected non-habitual snorers were invited for overnight polysomnography and evaluation for metabolic syndrome. The National Cholesterol Education Program–Adult Treatment Panel III (NCEP–ATPIII) criteria were used to define metabolic syndrome.

Of the 2860 subjects approached, 2505 (88%) completed stage 1; 452 (18%) were habitual snorers. In stage 2, OSA (defined as apnea–hypopnea index ⩾5) was observed in 94 (32.4%) of 290 habitual snorers and 3 (4%) of 75 non-habitual snorers. Seventy (77%) of the 91 habitual snorers with OSA also had metabolic syndrome; none of the non-habitual snorers with OSA had metabolic syndrome. The estimated population prevalence of metabolic syndrome was 43% [95% CI: (41.0–44.9%)] and syndrome Z was 4.5% (95% CI: 3.7–5.3). On multivariable analysis, age [OR: 1.05 (1.00–1.09)], male gender [OR: 5.64 (2.06–15.49)], percent body fat [OR: 1.08 (1.04–1.13)] and ΔSaO₂ (%) (defined as the difference between baseline and minimum SaO₂ during overnight sleep study) [OR: 5.80 (2.36–14.26), 17.70 (5.97–52.17) and 57.1 (19.12–170.40) for 10–20%, 20–30% and >30% reduction respectively as compared to <10% reduction] were independently associated with syndrome Z.

To the best of our knowledge, this is the first population-based study on the prevalence and risk factors of syndrome Z, and it reveals that a considerable proportion of community-dwelling northern Indian adults have syndrome Z. Age, male gender, percent body fat and severity of nocturnal desaturation were independent risk factors for syndrome Z.