Criteria for metabolic syndrome were developed to improve understanding of links between insulin resistance and vascular disease; however, their clinical role, which was designed to predict people at risk of events of cardiovascular disease or diabetes, remains contentious. Although metabolic syndrome is associated with increased risk for incident vascular events in middle-aged populations with or without existing coronary heart disease,1—3
the amount of association is modest. A recent meta-analysis4
showed around a 50% higher risk for coronary heart disease in patients with metabolic syndrome compared with those without, after adjustment for traditional risk factors of coronary heart disease. This hazard ratio (HR) is, at best, modest and thus when tested with receiver operating characteristic (ROC) analyses, criteria for metabolic syndrome in middle-aged populations do less well than do traditional risk algorithms—eg, Framingham risk score—and do not enhance risk prediction.3
Furthermore, the criteria for metabolic syndrome do not offer more than the sum of its parts.6
These observations have led several investigators to question the value of a formal diagnosis of metabolic syndrome in clinical practice8—11
for determination of vascular risk.
Yet, despite such findings, research using criteria for metabolic syndrome as an entity to establish vascular risk continues unabated,12—14
with conclusions that are often potentially misplaced or overstated. Moreover, few studies have simultaneously linked metabolic syndrome to risk for both incident diabetes and coronary heart disease, to establish whether prediction of both endpoints can be usefully achieved by one set of criteria.
We addressed these issues in two prospective studies in elderly populations—namely, the Prospective Study of Pravastatin in the Elderly at Risk (PROSPER)15
and the British Regional Heart Study (BRHS).16
BRHS helped us to corroborate and generalise our findings in PROSPER. In both prospective studies, all components of the metabolic syndrome were available at baseline (body-mass index [BMI] replacing waist circumference in PROSPER, as previously validated2
), and importantly, both studies had ascertained events of incident cardiovascular disease and type 2 diabetes, which is a fairly uncommon feature in prospective studies. Therefore, we were able to establish to what extent metabolic syndrome and its individual components were related to risk for cardiovascular events (fatal and non-fatal) and to risk for incident type 2 diabetes in elderly people. We hypothesised that metabolic syndrome would be weakly associated with risk of incident cardiovascular disease despite being strongly associated with risk of incident diabetes in both cohorts.
The Prospective Study of Pravastatin in the Elderly at Risk (PROSPER)
The protocol of PROSPER17
and the methods and outcomes of the main trial have been published.15
Between Dec 15, 1997, and May 7, 1999, we screened 23 770 individuals and enrolled 5804 from Scotland, Ireland, and the Netherlands. Men and women aged 70—82 years were recruited if they had either pre-existing vascular disease (coronary, cerebral, or peripheral) or raised risk of such disease because of smoking, hypertension, or diabetes. Their plasma total cholesterol had to be between 4·0—9·0 mmol/L and their fasting triglyceride concentrations less than 6·0 mmol/L. The institutional ethics review boards of all centres approved the protocol, and all participants gave written informed consent. The protocol was consistent with the Declaration of Helsinki.
Participants were randomly assigned to receive either pravastatin 40 mg daily or matching placebo.15
We reviewed participants every 3 months, over a mean follow-up of 3·2 (range 2·8—4·0) years. All data were processed and analysed at The Robertson Centre for Biostatistics in Glasgow (UK). The primary outcome of the original trial was the combined endpoint of definite or suspected death from coronary heart disease, non-fatal myocardial infarction, and fatal or non-fatal stroke, which were all labelled under the term cardiovascular disease. All events were adjudicated by an independent review board.
Participants with known or newly diagnosed diabetes at baseline were excluded from the present analyses. Diabetes at baseline was defined by self-reported history, a fasting blood glucose concentration of 7·0 mmol/L or greater or a blood glucose measurement of 11·1 mmol/L or greater when fasting status was uncertain, or self-reported use of antidiabetic drugs (any oral hypoglycaemic agent or insulin).
Incident diabetes during the trial was defined by the same criteria; the incidence date was assigned as the date of the study visit at which the criteria were satisfied. Glucose measurements throughout the study were fasting in more than 95% of participants and thus most diagnoses were based on criteria for fasting glucose.
Baseline characteristics were compared between participants who did or did not experience the two endpoints (cardiovascular disease or diabetes) during follow-up. Participants were classified according to five metabolic criteria, defined on the basis of criteria from the Third Report of the National Cholesterol Education Program (NCEP),18
but with the cutoff for BMI suggested as an alternative to waist circumference in European criteria.19
Moreover, in a recent meta-analysis4
of 43 studies, investigators recorded no heterogeneity (p=0·71) in relation to vascular risk dependent on whether the studies used waist, waist-hip ratio, or BMI in criteria for metabolic syndrome. Therefore use of BMI, at least in a dichotomous way, gives similar results to those for waist circumference.
Thus participants had metabolic syndrome if they fulfilled three or more of the following: BMI of 30 kg/m2 or more; triglyceride of 1·69 mmol/L or more; HDL cholesterol less than 1·04 mmol/L (men) or less than 1·29 mmol/L (women); fasting glucose of 6·1 mmol/L or more; or blood pressure of 130/85 mm Hg or more, or receiving treatment for hypertension.
The British Regional Heart Study (BRHS)
On the basis of initial findings from PROSPER, we sought to corroborate data in a second prospective study that also ascertained events of both incident cardiovascular disease and diabetes. Furthermore, we wanted to include a study that used screening in the general population. BRHS is a prospective study of cardiovascular disease with 7735 men aged 40—59 years at baseline who were selected from the age-sex registers of one general practice in each of 24 UK towns, and who were screened between 1978 and 1980.16
The population studied was socioeconomically representative of British men but consisted almost entirely of white Europeans (>99%). In 1998—2000, all surviving men, now aged 60—79 years, were invited for a 20th year follow-up examination. The men completed a questionnaire that included questions about their medical history and lifestyle behaviour. They were requested to fast for a minimum of 6 h, during which time they were instructed to drink only water and to attend for measurement at a prespecified time between 0800 h and 1800 h. All men were asked to provide a blood sample, which was obtained with the Sarstedt Monovette system (Nuembrecht, Germany). 4252 men (77% of survivors) attended for examination. Men were asked to recall a doctor diagnosis of coronary heart disease (myocardial infarction or angina), stroke, diabetes, and other cardiovascular disorders.
Anthropometric measurements including body weight, height, and waist circumference were recorded and methods for lipids, blood pressure, and glucose have been described previously.20—22
All men have been followed up from initial examination (1978—80) to June, 2006, for all cause mortality and cardiovascular morbidity, and follow-up has been achieved for 99% people in the cohort.23
In the present analyses, all morbidity follow-up is based on follow-up from rescreening in 1998—2000 at a mean age 60—79 years, with a mean follow-up of 7 (range 6—8) years. Information about death was obtained through the established record linkage procedures provided by the UK National Health Service registers. Fatal stroke episodes were those coded on the death certificate to 9th Revision of the International Classification of Diseases (ICD) codes 430—438. Events of non-fatal stroke were those which produced a neurological deficit that was present for more than 24 h. Fatal events of coronary heart disease were defined as death with coronary heart disease (ICD codes 410—414) as the underlying code. A non-fatal myocardial infarction was diagnosed according to WHO criteria.24
Cardiovascular deaths included all those with ICD codes 401—459. Evidence for non-fatal coronary heart disease, angina, stroke, and diabetes was obtained by reports from family doctors and supplemented by reviews every 2 years of the patients' practice records (including hospital and clinic correspondence) that was undertaken through to the end of the study period, and from repeated personal questionnaires administered to surviving participants after initial examination.
In BRHS, both waist circumference and BMI were available, and so we used the original NCEP criteria with waist greater than 102 cm as one of the five cutoff points, with other endpoints the same as those in PROSPER.
In PROSPER, the prevalence of each metabolic factor, the distribution of the number of metabolic factors, and the prevalence of metabolic syndrome (three or more metabolic factors) are reported in subgroups that were defined by the presence or absence of vascular disease at baseline. Kaplan-Meier estimates of the cumulative event rate in participants with or without the metabolic syndrome at baseline were used to report the incidence of events of cardiovascular disease and diabetes over a mean of 3·2 (range 2·8—4·0) years follow-up. Multivariable Cox proportional hazards models, with adjustment for age, sex, country, and treatment allocation (pravastatin or placebo), were used to estimate the hazard ratio associated with the presence of the metabolic syndrome, and every constituent metabolic factor at baseline, for events of cardiovascular disease and the endpoint of incident diabetes. In BRHS, the prevalence of every metabolic factor, the distribution of the number of metabolic factors, and the prevalence of metabolic syndrome (three or more metabolic factors) are reported. Cox's proportional hazards model was used to assess the age-adjusted relative risks.
Role of the funding source
The sponsors of the study had no role in study design, data collection, data analysis, data interpretation, or writing of the report. The corresponding author had full access to all the data in the study and had final responsibility for the decision to submit for publication.
In PROSPER, we excluded a total of 760 participants with prevalent diabetes (626 with a known history of diabetes and 134 newly diagnosed patients with raised fasting glucose). An additional 232 participants had insufficient information to be categorised at baseline. Therefore, 4812 participants were included in this analysis. Of these participants, 287 developed new onset diabetes (209 of whom had no evidence of existing vascular disease and 78 had vascular disease at baseline).
shows that individuals in PROSPER who developed cardiovascular disease were older, more likely to be men, had a significantly lower HDL-cholesterol, and had greater prevalence of existing vascular disease than those who did not have disease, but BMI and fasting glucose or triglyceride concentrations did not differ. Individuals who subsequently developed diabetes had significantly higher baseline fasting glucose, BMI, triglyceride and systolic blood pressure, and lower HDL-cholesterol than did those who did not develop diabetes, but were not more likely to have vascular disease.
Comparison of baseline characteristics in participants who subsequently had an incident cardiovascular event or new-onset diabetes in the Prospective Study of Pravastatin in the Elderly at Risk (PROSPER)†
In BRHS, men with a doctor diagnosis of diabetes and those with a fasting glucose greater than 7 mmol/L were considered to have prevalent diabetes and were excluded (n=482). We further excluded men who self-reported a diagnosis of coronary heart disease (myocardial infarction or angina) or stroke (n=857) and those with missing components for the syndrome (n=176). After these exclusions, 2737 men were available for analysis. 327 events for coronary heart disease were recorded (myocardial infarction, angina, or death due to coronary heart disease), 440 events of cardiovascular disease (myocardial infarction, angina, stroke, all deaths due to cardiovascular disease), and 105 events of type 2 diabetes during the mean follow-up of 7 years.
shows that individuals in BRHS who developed cardiovascular disease were older and more likely to have higher systolic blood pressure, a significantly lower HDL-cholesterol, and a higher triglyceride than were those who did not develop cardiovascular disease. However, waist circumference and fasting glucose did not differ between the groups that did or did not develop incident cardiovascular disease. Individuals who subsequently developed diabetes had significantly higher baseline fasting glucose, waist circumference, triglyceride and systolic blood pressure, and lower HDL-cholesterol than did those who did not develop diabetes.
Comparison of baseline characteristics by incidence of cardiovascular disease and diabetes in British Regional Heart Study (BRHS)
shows the prevalence of metabolic syndrome and its individual components in both PROSPER and BRHS. Overall, 1335 (28%) individuals satisfied the criterion for the metabolic syndrome in PROSPER of having at least three separate metabolic abnormalities at baseline. We recorded no evidence that people with or without existing vascular disease at baseline differed with respect to the prevalence of metabolic syndrome (χ2
=0·24, p=0·63; data not shown). Nor did we note any interaction of treatment allocation on any of the endpoints of interest (data not shown). In BRHS, 744 (27%) men had metabolic syndrome.
Participants in PROSPER (all and those with no cardiovascular disease at baseline) and in BRHS who fulfil each definition of metabolic factor
shows the Kaplan-Meier estimates of cumulative rates of cardiovascular disease and diabetes events in participants without existing vascular disease or diabetes at baseline in PROSPER, stratified by the metabolic syndrome and every component metabolic factor and adjusted for age, sex, country, and treatment allocation. This analysis shows that metabolic syndrome and most of its components are substantially associated with risk for new onset diabetes, but not with events of incident cardiovascular disease. The criterion of impaired fasting glucose (6·1—6·9 mmol/L) alone is linked to a pronounced increased risk of incident diabetes (figure
Cumulative event rates by presence or absence of metabolic syndrome and every metabolic factor at baseline, in participants without baseline vascular disease in PROSPER
(A) Events of cardiovascular disease. (B) Events of incident diabetes. Criteria were: BMI ≥30 kg/m2; triglyceride ≥1·69 mmol/L; HDL cholesterol <1·04 mmol/L (men) or <1·29 mmol/L (women); fasting glucose ≥6·1 mmol/L; and systolic blood pressure ≥130 mm Hg, diastolic blood pressure ≥85 mm Hg, or treated hypertensive. CVD=cardiovascular disease. BMI=body-mass index.
provides the estimated hazard ratios associated with each component of the metabolic syndrome in those without vascular disease at baseline in both studies. In PROSPER we noted no association between events of cardiovascular disease and metabolic syndrome or individual metabolic factors. By contrast, metabolic syndrome, and each individual metabolic factor, was significantly associated with incident diabetes. The criterion of impaired fasting glucose showed an especially strong association with incident diabetes; despite only 336 (7%) participants having impaired fasting glucose compared with 1335 (28%) having metabolic syndrome, the population attributable fraction was similar at 46% for impaired fasting glucose and 45% for metabolic syndrome. These findings were broadly similar in individuals with baseline vascular disease for whom metabolic syndrome was not associated with higher risk of cardiovascular disease events (HR 1·01 [95% CI 0·79—1·28]) but was associated with diabetes risk (2·93 [1·87—4·58]). We noted the same finding for individual components (data not shown).
Hazard ratios (95% CIs) for events of cardiovascular disease and incident diabetes associated with metabolic syndrome, and individual criteria for metabolic syndrome
We related metabolic syndrome to coronary heart disease alone (coronary death or non-fatal myocardial infarction) in individuals without baseline vascular disease (the primary endpoint included stroke). The HR for coronary heart disease for metabolic syndrome was thus 1·11 (0·86—1·45), which accords with the results for total events of cardiovascular disease. The same finding was also true for all individual components of the syndrome.
In BRHS, the metabolic syndrome was weakly associated with incident cardiovascular disease (relative risk [RR] 1·27 [1·04—1·56)]) despite being strongly related to diabetes (7·47 [4·90—11·46]; table 4
). Additionally, three of the five components of metabolic syndrome did not predict risk of cardiovascular disease events—namely, raised waist circumference, triglyceride, or glucose—whereas the other two cutoff points for HDL cholesterol and blood pressure were only modestly associated (table 4
). By contrast, all five components were more strongly associated with risk of incident diabetes, with raised fasting glucose particularly strong (table 4
Finally, when we used the 148 events of coronary heart disease specifically (myocardial infarction or fatal death due to coronary heart disease), the age-adjusted relative risk was 1·20 (0·84—1·30), and the results for the individual components were much the same to that seen for all events of cardiovascular disease (data not shown).
Results from this study show that metabolic syndrome has negligible clinical association with incident vascular events in elderly people despite strong associations with risk for incident type 2 diabetes in two prospective studies. Moreover, we have shown that the individual cutoff points in the criteria for metabolic syndrome predict risk of new onset diabetes with similar strength to data in younger populations,2
but results from PROSPER show that these cutoff points do not predict risk of vascular events in this population, irrespective of whether or not individuals had vascular disease at baseline. The findings were broadly similar when we extended analyses to the BRHS population-based study in men. Moreover, the same finding was also true in both studies if we used coronary heart disease alone as the vascular endpoint. We noted that cutoff points for fasting glucose, triglyceride, and waist circumference had no association with risk of incident coronary heart disease in either study despite strong associations with incident diabetes. These findings suggest that the pattern of risk factors for new-onset diabetes differs in many respects to that which predicts vascular events in elderly people.
Of wider relevance, our findings concur with data in middle-aged populations for whom criteria for metabolic syndrome are inferior to, and do not enhance conventional methods for, risk prediction of coronary heart disease.3
For example, in the 20 year follow-up of the original BRHS3
(of men aged 40—59 years at baseline), the area under the ROC curve for prediction of events of coronary heart disease was 0·59 for metabolic syndrome versus 0·68 for Framingham risk score. After adjustment for the Framingham risk score, metabolic syndrome was not associated with risk (RR 1·14 [95% CI 0·96—1·35]). Possible explanations for this weaker prediction are that the criteria for metabolic syndrome are more strongly aligned to new onset diabetes,2
use a weaker dichotomous design instead of continuous distributions of individual components, and omit major risk factors for cardiovascular disease (eg, smoking). Thus, inclusion of strong diabetes, but, at best, weak predictors of cardiovascular disease—namely, waist circumference or BMI, triglyceride, and glucose—in criteria for metabolic syndrome significantly attenuate associations to events of coronary heart disease, and even more so when these variables are dichotomised.
Although our results accord with a significant association between criteria for metabolic syndrome and incident diabetes in this elderly cohort, impaired fasting glucose alone was strongly linked to risk for incident diabetes in PROSPER, with nearly 50% of participants with a fasting glucose greater than 6·1 mmol/L progressing to develop overt diabetes. In BRHS, which included a younger cohort, fasting glucose alone was similar to metabolic syndrome in prediction of diabetes. The same finding has been recorded in other studies such as an 8-year follow-up of the Framingham offspring population, in which the glucose cutoff was by far the best predictor of diabetes (RR 12·5 [95% CI 9·1—17·3]).25
Thus, metabolic syndrome per se, as presently defined, is also not necessary or clinically needed to identify those at increased risk for new onset type 2 diabetes. Rather, if systematic screening for prevalent diabetes or high subsequent risk is ever widely adopted—and there is no guarantee it will—it will probably be a two stage process and targeted to those who are initially identified at substantially greater risk with use of simple questions (eg, age, family history of diabetes, ethnic origin, simple activity, and diet questions) combined with simple measurements (eg, BMI, waist, blood pressure). Thus, simple non-laboratory measures can be used to identify high-risk individuals before any blood sample—primarily for fasting or postprandial glucose measurements—is taken. In support of this approach, simple questionnaires show good prediction of diabetes risk in several cohorts.26
There are other implications of our findings. First, although metabolic syndrome as an entity might be of conceptual value, it has no benefit in risk stratification for cardiovascular disease in elderly people. This finding is clinically relevant since most vascular events occur in individuals older than 60 years. Of course, the association of established cardiovascular risk factors with risk of vascular events is generally weaker in elderly populations (particularly LDL cholesterol, but also HDL cholesterol and blood pressure) than in middle-aged people, and our findings of even weaker associations of metabolic syndrome with incident vascular events is in keeping with this observation. Nevertheless, even though metabolic syndrome is modestly associated with risk of vascular events in middle-aged populations, the strength of independent association for events of coronary heart disease in cohorts without baseline disease—pooled RR of 1·49 (1·37—1·61) in a recent meta-analysis4
—is simply too weak to enhance risk prediction beyond established Framingham-based algorithms.3
This point is well made by Pepe and colleagues,28
who argue that a single measure of association such as an odds ratio does not meaningfully describe a marker's (or in this case criteria's) ability to classify patients.28
Second, previous studies have clearly shown that the sum of the components is not more than the individual parts of the syndrome with respect to risk prediction of cardiovascular disease:1
a finding that we now extend to include two cohorts in the present report.
Third, although impaired fasting glucose (>6·1 mmol/L and <7·0 mmol/L) is a strong risk factor for development of type 2 diabetes, it seems not to be associated with risk for vascular events in elderly people, either in those with or without existing vascular disease. This finding was noted in both studies, and re-emphasises that fasting glucose within the non-diabetes range is unlikely to be helpful in risk stratification for vascular events. Rather, the use of traditional risk factors is necessary for estimation of vascular risk in individuals without diabetes.
Fourth, the other major limitation of metabolic syndrome is of course the use of dichotomous cutoff points. This limitation is notable in the example of HDL cholesterol in PROSPER or triglyceride in BRHS. In continuous analyses, HDL cholesterol is significantly lower in PROSPER participants who have a subsequent event, but not so with the dichotomous cutoff points that were chosen for the criteria for metabolic syndrome (table 3
). Quintiles of HDL cholesterol are associated with risk in PROSPER,30
but clearly a simple dichotomous cutoff has reduced power. In BRHS, although triglyceride was higher in continuous analysis for events of cardiovascular disease, its significance was attenuated with the dichotomous approach.
Finally, our work emphasises that patterns of risk factors related to development of new diabetes differ in many ways to those for vascular events. Thus, metabolic syndrome does not offer a common risk factor pathway or unified criteria to define risk for both diabetes and vascular disease.
Our study has several limitations and strengths. We acknowledge that the absence of an oral glucose tolerance test means that some individuals will have developed type 2 diabetes which was not detectable by changes in fasting glucose alone or by clinical history. However, oral glucose tolerance tests for all participants over the course of PROSPER or BRHS were not feasible for pragmatic reasons and logistics. Our studies are not uncommon in this respect, and in line with expectations, predictors of incident diabetes in both populations included the expected triad of fasting glucose, BMI or waist circumference, and triglyceride. We also acknowledge that, as with BRHS, most studies reporting incident diabetes use doctor diagnosis plus participant self-report, and thus PROSPER is more thorough than are most other studies with additional biochemical confirmation.
We did not have waist circumference in PROSPER but did so in BRHS, and results were nearly identical. This finding concurs with a recent meta-analysis reporting no difference in outcomes irrespective of whether waist circumference or BMI were used in criteria for metabolic syndrome to predict events of cardiovascular disease.4
Crucially, when treated in a dichotomous way, any anthropometric measure is either very weakly or not at all associated with risk of incident cardiovascular disease events. We also acknowledge that the data for blood pressure should be interpreted with caution, since the recruitment criteria for PROSPER required that individuals without hypertension had other risk factors. Nevertheless, somewhat consistent with our data for diastolic blood pressure, some other studies have reported higher mortality in the very elderly participants with lower diastolic blood pressure.31
We also recognise that we used criteria from the NCEP and not the International Diabetes Federation to test associations; however, a meta-analysis has shown that the NCEP-based definition is better than IDF in its associations with vascular risk, albeit, and as discussed above, modestly so.4
We recognise that attrition bias could have led us to record slightly weaker associations of metabolic syndrome with events of cardiovascular disease in elderly people. However, we3
have shown that even in younger populations, metabolic syndrome is at best a modest predictor of events of coronary heart disease or cardiovascular disease, which is clearly worse than traditional algorithms that use continuous or categorical measures and which contain the key risk factors for cardiovascular disease: age, LDL cholesterol, and smoking. Thus our results are likely to be generalisable to adults of all relevant age groups.
With regards to strengths, the very similar findings in two independent prospective studies with differing designs but both with ascertainment of incident cardiovascular disease and diabetes, provide substantial reassurance about the general validity of the data. PROSPER is a large study in terms of number of events that simultaneously tests the association of metabolic syndrome with events of cardiovascular disease and incident diabetes, and moreover, we were able to test associations in individuals with and without cardiovascular disease at baseline, showing clearly a closer association with diabetes. Although many risk variables lessen in their association with vascular events in elderly populations, a large number of new onset diabetes and vascular events (increasingly) occur in this population, and so the results have both clinical and scientific value.
Finally, our clear finding in both studies of substantial differing diabetes versus cardiovascular disease associations of metabolic syndrome and its components, should aid better general understanding of differing risk patterns for these two diseases, which therefore should not be considered together. Our findings should help other investigators to think about their data in a similar critical way.
NS and SGW conceived the idea for the relevant analyses of PROSPER and BRHS, respectively, and drafted the paper. NS, AM, and SGW were responsible for the analyses. All other authors (except NGF and DJF) were involved in the design and conduct of either PROSPER or BRHS. NGF and DJF, together with all other authors, contributed to the interpretation of the data. All authors critically reviewed the report and approved the final version.
We declare that we have no conflict of interest.