A Continuous Metabolic Syndrome Risk Score: Utility for epidemiological analyses

This study was designed to validate a continuous metabolic syndrome risk score (cMSy) using the International Diabetes Federation risk factors (1). Increasing evidence supports using a cMSy instead of a binary definition for epidemiological analyses: 1) dichotomizing continuous outcome variables reduces statistical power (2); 2) cardiovascular risk is a progressive function of several metabolic syndrome (MSy) risk factors, eliminating the need to dichotomize these factors (3); and 3) cardiovascular and diabetes risk increase progressively with increasing numbers of MSy risk factors, eliminating the need to dichotomize MSy (3–4).

The National Institute of Statistics randomly selected a community sample of 18- to 75-year-old Flemish adults. In total, 571 men (aged 46.7 ± 11.2 years) and 449 women (aged 45.8 ± 10.8 years) without cardiovascular disease and diabetes, tested between October 2002 and April 2004, were included.

Calculation of cMSy involved two steps. First, principal component (PC) analysis (varimax rotation) was applied to the normalized risk factors to derive PCs representing large fractions of MSy variance, revealing two PCs (eigenvalue ≥1.0). In men, PC1 and PC2 explained 33 and 28% of variance, respectively (loadings PC1 [PC2]: waist circumference 0.51 [0.55], triglycerides 0.82 [0.16], HDL cholesterol −0.85 [0.09], blood pressure 0.12 [0.72], and glucose −0.08 [0.73]). In women, PC1 and PC2 explained 33 and 27% of variance, respectively (loadings PC1 [PC2]: waist circumference 0.61 [0.54], triglycerides 0.42 [0.49], HDL cholesterol 0.12 [−0.90], blood pressure 0.83 [0.01], and glucose 0.62 [0.04]). Second, cMSy was computed by summing both individual PC scores, each weighted for the relative contribution of PC1 and PC2 in the explained variance. Resulting cMSy was 0 ± 1.42 in men and 0 ± 1.41 in women.

cMSy was higher (P < 0.001) in subjects with MSy as defined by the International Diabetes Federation (1) (men [12.8%]: 2.03 ± 1.00, women [8.5%]: 2.63 ± 1.28) versus subjects without (men: −0.30 ± 1.21, women: −0.24 ± 1.16). Moreover, cMSy increased progressively (Tukey’s honestly significant difference comparison, P < 0.001) with increasing numbers of risk factors in men (0 [30.1%]: −1.21 ± 0.96; 1 [33.8%]: −0.26 ± 0.87; 2 [21.2%]: 0.67 ± 0.84; 3 [11.2%]: 1.76 ± 0.73; and ≥4 [3.7%]: 3.04 ± 0.94) and women (0 [47.7%]: −0.96 ± 0.79; 1 [28.3%]: 0.16 ± 0.82; 2 [16.3%]: 1.21 ± 0.82; 3 [5.3%]: 2.17 ± 0.81; and ≥4 [2.4%]: 4.09 ± 0.99).

cMSy is a more appropriate (2,3,4) and valid alternative for epidemiological analyses, although the binary definition (1) remains useful for clinical practice.