Introduction
The metabolic syndrome (MetS) represents a cluster of metabolic risk factors that co-occur to a greater degree than predicted by chance. While it is clear that MetS is associated with cardiovascular disease and diabetes [
1], it is still controversial whether MetS adds predictive value that goes beyond that of its single constituents [
1,
2]. Insulin resistance was initially the main focus of MetS, but adiposity, sedentary lifestyle, dietary and genetic factors have also received much attention when considering the pathogenesis of MetS [
3]. High prevalence of MetS has often been documented in developed countries and increasingly so in developing countries [
4‐
17], although data in developing countries and particularly in the Sub-Saharan region remain scarce [
18].
Over the past decade, several definitions of MetS have been coined. In this paper, we consider three major definitions: i) the definition of the World Health Organization (WHO), issued in 1998 [
19]; ii) the definition of the National Cholesterol Education Program Adult Treatment Panel III (ATP), issued in 2001 and updated in 2004 and 2005 [
20], and the definition of the International Diabetes Federation (IDF), introduced in 2005 [
21]. These definitions agree that the core criteria of MetS include: i) blood glucose impairment (hyperglycemia and/or insulin resistance), ii) excess abdominal/body fat (increased waist and/or obesity), iii) dyslipidemia (low HDL-cholesterol and/or high triglycerides), and iv) elevated blood pressure. However, criteria and cut-off values differ between these definitions, implying that different definitions may identify different people, as documented in the few studies that have addressed this question [
4,
7,
8,
22].
There is some controversy over whether identification of MetS should exclude people with diabetes since diabetes alone is sufficient to define high cardiovascular risk and MetS has been used as a tool to predict diabetes [
2,
23].
This analysis had four main objectives. First, we evaluated the prevalence of MetS according to three MetS definitions in the Seychelles, a rapidly developing country of the African region. Second, we examined the distribution of the MetS criteria according to the different MetS definitions. Third, we compared the level of agreement between the different MetS definitions and their ability to identify the same subjects. Fourth, we examined how the prevalence of MetS and the other end points differed upon restricting assessment of the metabolic syndrome to the non-diabetic population.
Discussion
The main findings of the study are as follows: i) the prevalence of MetS was high in this population of East Africa regardless of which MetS definition was used; ii) the prevalence of MetS decreased by approximately one third upon exclusion of persons with diabetes; and iii) agreement between different MetS definitions was limited and consequently, the similar prevalence of MetS according to either MetS definition actually identified, to a substantial extent, different subjects as having MetS.
In our study, the prevalence of MetS at the age of 25–64 years ranged between 25% and 30%. This is much higher than the 8% reported in Cameroon [
18], which is, to the best of our knowledge, the only other population based published assessment of the prevalence of MetS in Sub-Saharan Africa. A previous study in the Seychelles [
34] found that plasma aldosterone, but not plasma renin activity, was associated with MetS. However this study included participants from families with hypertension and was therefore not intended to assess the prevalence of MetS in the general population. In order to compare the prevalence in Seychelles with that in other countries, we compiled findings of selected population-based studies that had assessed MetS according to at least two MetS definitions, and included participants of 35–64 years (Table
5). The prevalence of MetS was similar in Seychelles as in many Western countries [
9‐
11,
13] and in urban India [
15]. The prevalence of MetS in Seychelles was lower than in certain countries, e.g. USA [
5], Portugal [
8], Samoa [
13], Turkey [
7] and Tunisia [
4] but higher than in Mexico [
6] and several Asian countries e.g. Korea [
14], Japan [
13] and China [
17]. The high prevalence of MetS in Seychelles is consistent with high prevalence of several MetS criteria, particularly overweight, hypertension, dyslipidemia and diabetes [
25‐
27].
Table 5
Prevalence of the metabolic syndrome in the Seychelles and in other populations, according to different definitions of the metabolic syndrome
Africa
|
Seychelles | Nation wide | 25–64 | 1218 | 24 | 32 | 28 | 25 | 25 | 25 | 25 | 35 | 30 |
| City of Tunis | ≥ 40 | 863 | 15 | 31 | 24 | 26 | 31 | 29 | 30 | 56 | 46 |
| City of Yaoundé and three rural villages | 24–74 | 1573 | < 0.5 | < 0.2 | - | < 8 | < 6 | - | < 2 | < 2 | - |
Americas
|
| NHANES 1999–2002 | ≥ 20 | 3601 | 34 | 35 | 35 | - | - | - | 41 | 37 | 39 |
| Nation wide | 20–69 | 2158 | 29 | 25 | 27 | 13 | 14 | 14 | - | - | - |
Europe
|
| Istanbul (urban) and Kayseri (rural) | > 20 | 1568 | 41 | 43 | 38 | 23 | 18 | 19 | 46 | 48 | 42 |
| City of Porto | 18–92 | 1433 | 32 | 40 | 37 | 30 | 24 | 26 | 38 | 44 | 42 |
| Nation wide | > 30 | 1030 | 28 | 29 | 28 | 33 | 24 | 28 | - | - | - |
| North-Trondelag Health Study (HUNT 2) | 20–89 | 10,206 | 27 | 25 | 26 | - | - | - | 29 | 30 | 30 |
| PROCAM study | 16–65 | 7131 | 25 | 18 | - | - | - | - | 32 | 23 | - |
| Representative sample of Greek adults | > 18 | 9,669 | 25 | 24 | 25 | - | - | - | - | - | 43 |
Asia
| | | | | | | | | | | | |
| Population-based (DETECT 2) | > 35 | 1344 | 39 | 57 | - | 22 | 26 | - | 45 | 60 | - |
| Population-based (DETECT 2) | > 35 | 9409 | 36 | 28 | - | 26 | 18 | - | 42 | 33 | - |
| Korean National Health and Nutrition Survey | ≥ 20 | 6601 | 18 | 21 | 19 | - | - | - | 15 | 24 | 20 |
| City of Chennai | ≥ 20 | 2350 | 17 | 19 | 18 | 27 | 20 | 23 | 23 | 28 | 26 |
| Taiwan National Nutrition and Health Survey | ≥ 19 | 2608 | 12 | 17 | - | - | - | - | 6 | 13 | - |
| Population-based (DETECT 2) | > 35 | 2016 | 8 | 10 | - | 3 | 3 | - | 8 | 11 | - |
| Two agricultural counties | 25–64 | 18,630 | 5 | 12 | 8 | - | - | - | 4 | 11 | 7 |
The prevalence of MetS in Seychelles did not differ markedly according to the three different MetS definitions, consistent with observations in several populations [
5,
9,
10,
14,
17], but not in others [
4‐
7,
12,
13]. However, the prevalence of MetS in Seychelles was moderately higher according to IDF than ATP or WHO, a finding reported in most studies that had assessed this issue [
4,
5,
7,
8,
10‐
13,
15], except for a few [
16,
17]. It has frequently been reported that the prevalence of MetS according to the WHO definition is generally greater among men than women [
35] but we did not observe such a gender difference in Seychelles.
The level of agreement between the different definitions of MetS was not optimal in our study. Less than half of the individuals labeled as having MetS according to any of the three considered definitions were classified as MetS-holders according to all three definitions. We found that agreement between the MetS definitions was better for IDF-ATP than for WHO-IDF and WHO-ATP. This finding is consistent with data in several populations on several continents [
7,
8,
10,
13,
15]. This difference is expected since MetS is based on the same criteria according to IDF and ATP, except for the adiposity criterion. Agreement between the MetS definitions was generally better among women than men in Seychelles, consistent with previous reports [
5,
8,
14]. This may relate to the fact, at least in Seychelles, that the prevalence of the adiposity criterion was virtually identical and close to 100% across the three MetS definitions among women with MetS, but was lower and largely different between MetS definitions in men.
Hence, we found a similar prevalence of MetS in Seychelles according to the three different MetS definitions but the different MetS definitions actually identified, to a substantial extent, different individuals. Poor agreement between MetS definitions has several clinical and epidemiological implications [
16,
22]. First, it is questionable to directly compare the burden of MetS between populations based on different definitions. Second, it remains unclear whether cardiometabolic outcomes differ if MetS is defined according to one, two or three MetS definitions. Few studies have assessed the predictive value of MetS when MetS is based on more than one definition [
36]. The study of Benetos et al [
36] showed that the prevalence of MetS was markedly higher when MetS was based on either the IDF or ATP (2005 version) definitions as compared to the 2001 ATP definition alone, but only individuals with MetS according to the 2001 ATP definition had a higher risk of all-cause and cardiovascular mortality.
The prevalence of MetS restricted to the non-diabetic population (e.g. according to the EGIR definition, which is a definition of MetS that explicitly excludes diabetes [
33]) was similar in Seychelles as in several countries in Europe [
37‐
39] and higher than in Japan and Korea [
13]. In the Seychelles, about one third of the adults who had MetS also had diabetes and, inversely, most adults who had diabetes also had MetS. Hence, the prevalence of MetS, according to either the ATP, WHO and IDF definitions, decreased markedly (a relative decrease of approximately one third) upon exclusion of persons with diabetes. This difference is larger than reported in other studies [
6,
13]. We also observed that this decrease in the prevalence of MetS upon exclusion of diabetes was larger according to the WHO definition compared to the ATP and IDF definitions, which has also been observed in Samoa [
13] and Mexico [
6].
The issue of whether MetS should be defined upon exclusion of diabetes is relevant to both clinical practice and epidemiology. It has been argued that MetS status does not add incremental information for cardiovascular management of diabetic individuals [
23], since diabetes alone defines high cardiovascular risk. For the same reason, it has been suggested that the definition of MetS should exclude individuals with known cardiovascular disease [
23]. The question of whether MetS should exclude diabetes and/or cardiovascular disease is clearly dependent on the expected purpose of MetS, i.e. whether MetS is used to predict cardiovascular diseases, diabetes, insulin resistance, or a combination of these conditions independent of conventional risk factors. While our cross-sectional study emphasizes that different definitions of MetS tend to classify different individuals as having MetS, the most important question remains to determine, based on longitudinal data, whether MetS predicts cardiovascular events above the risk factors that constitute the syndrome.
Our study has some limitations. First, we cannot exclude a healthy volunteer bias related to voluntary participation to the study, which could tend to underestimate the actual prevalence of MetS in the population. Second, we assessed insulin resistance based on HOMA-IR, which is only partially correlated with the gold standard (hyperinsulinemic euglycemic clamp), yet a valid proxy for insulin resistance [
30,
31]. Third, OGTT was only performed on participants unaware of having diabetes and who had FBG between 5.6 and 7.0 mmol/l, hence a source of slight underestimation of diabetes. On the other hand, strengths of the study include the population-based design of the study, a fairly large sample size, and a broad panel of investigations that allowed us to directly compare the different MetS definitions.
In conclusion, this study contributes to mapping the prevalence of MetS worldwide, particularly with regards to the African region. The study also further contributes to the longstanding debate regarding the significance of MetS. The substantially different prevalence of MetS upon exclusion of individuals with diabetes underlies the need to clarify whether MetS is a tool for predicting cardiovascular disease, diabetes, insulin resistance or any other risk condition. These issues are further prompted by the finding, in our study, that the different considered MetS definitions identified, to a substantial extent, different individuals. Not withstanding a much needed unified definition of MetS, our findings in Seychelles emphasize the growing burden of lifestyle-related non-communicable diseases in countries in epidemiological transition including in the African region, consistent with the ongoing epidemic of obesity worldwide and in the Seychelles in particular.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
CK led the analysis of data and the write up of the manuscript; JW had a coordinating role in the conduct of the study and reviewed the manuscript; WR performed most of the blood analyses and reviewed the manuscript; FP participated in the study design and reviewed the manuscript; PB was the principal investigator of the study and actively participated in the data analysis and the write up of the manuscript.