Neck circumference as a measure of central obesity: Associations with metabolic syndrome and obstructive sleep apnea syndrome beyond waist circumference

Summary

Background & aims

To investigate the relationship of neck circumference (NC) to metabolic syndrome (MetS) and obstructive sleep apnea syndrome (OSAS) and whether it adds information to that provided by waist circumference.

Methods

Cross-sectional analysis of a population sample of 1912 men and women, aged 55.1 ±12 years, representative of Turkish adults. MetS was identified based on modified criteria of the ATP-III, OSAS when habitual snoring and episodes of apnea were combined with another relevant symptom.

Results

NC measured 36.7 (±3.5) cm in the total sample. It was significantly correlated with numerous risk factors, above all body mass index and waist girth (r ≥ 0.6), homeostatic model-assessed insulin resistance, blood pressure and, inversely, with smoking status and sex hormone-binding globulin. Sex- and age-adjusted NC was associated significantly with MetS, at a 2–3-fold increased likelihood for 1 standard deviation (SD) increment. After further adjustment for waist circumference and smoking status, a significant residual odds ratio (OR, 1.13 [95% CI 1.08; 1.19]) persisted, corresponding to ORs of 1.53 and 1.27 in males and females, respectively, for 1 SD increment. Even when adjusted for all MetS components, a residual OR (1.08 [95% CI 1.000; 1.17]) remained. Sex- and age-adjusted NC was associated significantly also with OSAS in genders combined, independent of waist girth, yielding an added OR of 1.3 for 1 SD increment.

Conclusions

NC contributes to MetS likelihood beyond waist circumference and the MetS components. Regarding association with OSAS, NC is of greater value than WC among Turkish men, not women.

Introduction

It is accepted that metabolic syndrome (MetS) increases the relative risk of cardiovascular disease, though it is still debated whether MetS adds to global cardiovascular disease risk assessed by traditional risk factors. It is believed that visceral adiposity lies at the root of the cardiometabolic risk with the consequent syndrome of central obesity/insulin resistance. Clinical definitions of MetS by National Cholesterol Education Program-ATP-III¹ or International Diabetes Federation² have been of enormous value in the diagnosis, management of and research on the cluster of metabolic risk factors. Yet, there is increasing recognition that other atherogenic, pro-thrombotic and inflammatory aspects of this syndrome are not captured by these practical clinical definitions which warrant further investigation, particularly for valuable clinical markers.³

Even central obesity (the hallmark of MetS, for which abdominal obesity is used as surrogate) is considered to encompass further ectopic dysfunctional adipose tissue such as thoracic visceral adiposity and hepatosteatosis.⁴ Upper-body fat distribution has long been recognized as related to increased cardiovascular disease risk, and neck skinfold⁵ or neck circumference (NC)6, 7 has been used as an index for such an adverse risk profile. Free fatty acid release from upper-body subcutaneous fat was reported to be larger than that from lower-body subcutaneous fat.⁸

Epidemiological population-based studies on the clinical significance of NC in regard to MetS are lacking, and correlative studies exist more so in relation to the obstructive sleep apnea syndrome (OSAS). Disorders in lipid or glucose metabolism and fasting hyperinsulinemia were found to prevail highest in the highest quintile of NC in a study from Finland.⁹ NC was proposed as a screening measure for identifying overweight and obese individuals in 979 Israeli adults visiting a family medicine clinic¹⁰; sex-specific optimal cut-off points were proposed. In a subset of the mentioned group, higher NC was found correlated positively with the factors of the MetS.¹¹ NC was also a good correlate of raised androgens in 107 severely obese premenopausal women.¹²

Obstructive sleep apnea has a major impact in public health and cardiovascular health since it is closely associated with several coronary disease risk factors like hypertension, arrhythmia, left ventricular dysfunction, as well as with coronary heart disease, stroke and pulmonary hypertension.¹³ Sleep apnea is associated also with mortality stemming both from accidents and from a higher rate of cardiovascular diseases. In a study on 670 patients with suspected sleep apnea, obese patients with sleep apnea had fatter necks than equally obese nonapneic snorers, though abdominal circumferences were similar.¹⁴ It was also reported that even relatively nonobese patients with sleep apnea/hypopnea syndrome had excess fat deposition, especially anterolateral to the upper airway compared with control subjects with similar NC.¹⁵

By studying the distribution and covariates of NC, as well as its likelihoods for MetS and OSAS in a population-based cohort in whom MetS is highly prevalent,¹⁶ we aimed to delineate whether 1) NC is significantly associated with the stated syndromes, 2) NC adds to the information with respect to central obesity provided by waist circumference, 3) such information may vary depending on sex or the specific syndrome. The final survey of the Turkish Adult Risk Factor Study is analyzed cross-sectionally for this purpose.