I read with interest the recent two articles on metabolic syndrome published in Diabetes Care, one by Ford (1) on the prevalence of metabolic syndrome defined by the International Diabetes Federation (IDF) among adults in the U.S. and the other by Kahn et al. (2) that critically appraised the definitions of metabolic syndrome.
Ford reported a higher prevalence estimate of the metabolic syndrome than the estimate based on the National Cholesterol Education Program (NCEP) definition (unadjusted prevalence 39.0 ± 1.1 vs. 34.5 ± 0.9%, respectively) in U.S. adults (1). The lower IDF criteria as compared with NCEP criteria for defining central obesity (men ≥94 vs. >102 cm and women ≥80 vs. >88 cm) appeared to account for much of this difference (3,4). The NCEP criteria suggest selecting three of five parameters, whereas IDF criteria have one fixed component (central obesity) and then select two of the other four parameters (3,4). With high-school mathematics, we can calculate the number of possible combinations in selecting X of Y items by the following equation: Y!/[(X!)(Y − X)!], where Y! reads as Y factorial = Y × (Y − 1) × (Y − 2)… × 3 × 2 × 1. The number of combinations in selecting three parameters out of five (the NCEP criteria) = 5!/(3! × 2!) = (5 × 4 × 3 × 2 × 1)/(3 × 2 ×1 × 2 ×1) = 10, whereas IDF criteria has 4!/(2! × 2!) = 6 combinations. The prevalence of metabolic syndrome will depend on the percentage of individual combinations. In other words, if all five parameters of NCEP or IDF criteria are the same, the IDF criteria will always diagnose less (instead of more) subjects with metabolic syndrome as compared with NCEP criteria.
Following this mathematical exercise, we can also see that the prevalence of the metabolic syndrome defined by IDF criteria is highly dependent on and will be always less than the prevalence of central obesity of that particular population, since central obesity is regarded by IDF as a prerequisite and those centrally obese subjects having two additional parameters with be defined as having metabolic syndrome (5). Kahn et al. (2) have commented that the metabolic syndrome has been imprecisely defined with a lack of certainty regarding its pathogenesis. Whereas the IDF criteria have identified central obesity as the only essential component of metabolic syndrome, Kahn et al. reminded us that the attempt to define the metabolic syndrome as the result of a single (or even major) unifying pathophysiological process, e.g., insulin resistance, is problematic.
The ultimate importance of metabolic syndrome is that it helps identify individuals at high risk of cardiovascular disease (CVD) (maybe type 2 diabetes as well) (3,6,7). While central obesity is a strong CVD risk factor, it is unlikely to be the only pathogenetic cause for metabolic syndrome (i.e., high CVD risk). The new IDF criteria for metabolic syndrome has evolved to identifying a selected or complicated subgroup of subjects with visceral obesity who are centrally obese and have already developed at least two complications such as hypertension or dyslipidemia (3). In that case, it is more appropriate to call the new IDF criteria for metabolic syndrome the “central obesity syndrome.”
But then what about metabolic syndrome? Metabolic syndrome should be a concept instead of a strict definition. Subjects with more than one well-established CVD risk factor are at increased risk for developing CVD. Other CVD risk factors tend to cluster in this group of patients as well. If we all agree that the metabolic syndrome is implying a cluster of CVD risk factors, then it can be labeled to all individuals with two, three, or more well-established CVD risk factors such as hypertension, dysglycemia, dyslipidemia (not only high triglyceride and low HDL cholesterol but also high total cholesterol), obesity (both general and central), strong family history of CVD, ageing, and so on. It should be a “loose” but serious term, just like “high CVD risk.”
