This study showed a higher frequency of cardiovascular risk factors such as truncal obesity and elevated systolic blood pressure among individuals living with type 2 diabetes compared to the controls who did not have type 2 diabetes. In another study involving Nigerians with type 2 diabetes mellitus, the researchers posited that cardiovascular risk factors were more common among the individuals with type 2 diabetes compared with those without type 2 diabetes.16 Several authors in the past have noted this cluster of risk factors and that was the genesis of the studies on metabolic syndrome.27
Previous studies have also documented a higher frequency of elevated systolic blood pressure among individuals with diabetes compared with the general population, just as it was found in the present study.28,29 Type 2 diabetes causes arterial stiffening especially in the presence of sub-optimal metabolic control and this is believed to be one of the links between type 2 diabetes and elevated systolic blood pressure.30 Other mechanisms that have been documented as the plausible explanations for a higher frequency of elevated systolic blood pressure among individuals with diabetes are endothelial dysfunction, sodium retention, sympathetic over-activity, renin-angiotensin-aldosterone activation and nephropathy.31 However, as hypothesized by some authors, the relationship between type 2 diabetes and hypertension is more reciprocal than causal.32
As noted in the present study, truncal obesity, represented by weight circumference, was higher among individuals with type 2 diabetes and similar observation has been reported from previous studies.33 Abdominal distribution of fat is a better determinant of the risk of developing type 2 diabetes than the total body fat mass.34 Truncal obesity is characterized with increased inflammatory cytokines and non-esterified fatty acids as well as hormonal dysregulation which are thought to be the underlying processes leading to insulin resistance and eventually type 2 diabetes mellitus.35
Quite striking was the observation, in this study, that the HDL-C and fasting triglycerides of those with or without type 2 diabetes mellitus were not significantly different. In the general population, an epidemiological study quoted by Laakso has demonstrated that the lipid profiles of individuals with type 2 diabetes were not remarkably different from that of the general population.36 However, our study was a hospital based study and previous hospital-based studies have documented a significantly lower HDL-C and higher triglycerides (a cluster sometimes called diabetic dyslipidaemia) among individuals with type 2 diabetes.37,38 Nevertheless, Zheng et al posited that the association between hypertriglyceridemia and type 2 diabetes is better appreciated in the presence of poor glycaemic control. Since the participants in this present study had averagely good glycaemic control, evidenced by the Hba1c (6.99 ± 0.73%), it would not be out of place to get a triglyceride and HDL-C profile that are not remarkably different from those of the controls without type 2 diabetes.38
As expected, HOMA-IR, a marker of insulin resistance was higher among the cases with type 2 diabetes when compared with that of the controls. Insulin resistance is a core pathophysiological pathway in the development of type 2 diabetes mellitus.39 Also, this study showed a significantly higher 10-year cardiovascular risk score among Nigerians with type 2 diabetes when compared with those without type 2 diabetes. This is not surprising because type 2 diabetes is associated with a concurrent cluster of other cardiovascular risk factors such as obesity and hypertension, as demonstrated in this study and other previous studies.40,41 Again, insulin resistance has been suggested as a potential culprit behind this observation.40
Going by the results of this study, the prevalence of metabolic syndrome in people living with type 2 diabetes depends on the diagnostic criteria used. The frequency of metabolic syndrome, using the modified NCEP ATP III criteria, was 71.6%. However, the frequency was slightly lower (65.7%) when IDF criteria were applied. A previous study done among Nigerians with type 2 diabetes mellitus patients was 66.7%.42 However, while the previous study used the conventional NCEP ATP III criteria, this present study used the modified NCEP ATP III criteria and this may account for the lower frequency in the previous study (66.7% vs 71.6%) as it has been suggested that the modified criteria have a better performance.11 In support of this assertion, a study done in Ethiopia that used the Modified NCEP ATP III criteria, reported a prevalence of 70.1% which is quite similar to 71.6% found in this present study. The frequency of metabolic syndrome among type 2 diabetes mellitus patients diagnosed with the IDF criteria in this present study (65.7%) is quite similar to what was found in a previous study in Nigeria that also used the IDF criteria (63.6%).43
The prevalence of metabolic syndrome is significantly higher among patients with type 2 diabetes when compared with the general population whether the modified NCEP ATP III criteria (71.6% vs 17.9%; p < 0.0001) or the IDF criteria (65.7% vs 10.4%; p < 0.0001) were used in making the diagnosis. A study done in Nigeria, using the IDF criteria for metabolic syndrome in the general population without type 2 diabetes mellitus, found a prevalence of 8.8% which is comparable to 10.4% documented in the present study.44 Another study done in Nigeria that used the NCEP ATP III criteria to diagnose metabolic syndrome in apparently healthy individuals not previously diagnosed with type 2 diabetes reported a prevalence rate of 12.1% which is lower than 17.9% found in this present study. This may be because while the previous study used the old NCEP ATP II criteria, the present study used the modified NCEP ATP III criteria which has been shown to have a better performance.
Using point biserial correlation, there was no statistically significant association between HOMA-IR (a marker of insulin resistance) and the presence of metabolic syndrome in type 2 diabetes mellitus whether the IDF criteria (p = 0.810) or the modified NCEP ATP III criteria (p = 0.909) were used. There is now a paradigm shift in what is believed to be the core component of metabolic syndrome. It is now thought that waist circumference, or truncal obesity, is more important than insulin resistance in the diagnosis of metabolic syndrome and this informed the IDF criteria which insist on the presence of increased waist circumference as a prerequisite for the diagnosis of metabolic syndrome.4 Interestingly, this present study found a significant association between metabolic syndrome, whether the modified NCEP ATP III or IDF criteria were used, and waist circumference (p < 0.0001). Previous studies have also demonstrated an association between metabolic syndrome and waist circumference.45,46
This study showed a significant association between metabolic syndrome and 10-year cardiovascular risk score only when the modified NCEP ATP III criteria were used in diagnosing metabolic syndrome (p = 0.04) although the strength of the association was weak. This association was not demonstrable using the IDF criteria. This is in keeping with the hypothesis by previous researchers that the modified NCEP ATP III criteria have a better performance than the IDF criteria.11
This study was able to demonstrate an almost perfect agreement between using NCEP ATP II and IDF criteria in the diagnosis of metabolic syndrome among Nigerians with type 2 diabetes mellitus. (κ = 0.862; p < 0.0001). It is still worthy of note that the IDF criteria missed 8.3% of the participants with type 2 diabetes which met the NCEP ATP criteria. However, neither NCEP ATP criteria (κ = 0.143; p = 0.116) nor the IDF criteria (κ = 0.144; p = 0.273) had a significant agreement with insulin resistance (using HOMA-IR). Again, this is in agreement with the hypothesis that insulin resistance is not a prerequisite in the diagnosis of metabolic syndrome. This study also found a fair but significant agreement between metabolic syndrome diagnosed with the modified NCEP ATP criteria (κ = 0.213; p = 0.029) and intermediate-to-high cardiovascular risk (using ASCVD risk categories) but this was not found with metabolic syndrome diagnosed with the IDF criteria. This suggests that the modified NCEP ATP III criteria predict cardiovascular risk much better than the IDF criteria.