The results of this study demonstrated that the association between CVH and cognitive function was not significant, and that the level of hs-CRP did not affect the risk of cognitive dysfunction. CVH assessment results obtained using the CANHEART health index did not show an association with cognition in either men or women. A stratified analysis across hs-CRP tertiles revealed no association between hs-CRP level and cognitive function. Although the presence of hypertension in men was associated with cognitive dysfunction, no other component had a significant association with cognition in either men or women.
Several explanations for these results may be suggested. First, the relatively small number of participants in the low cognitive function group should be considered. In this study, only 87 participants of the 2,622 total study population had an MMSE score below 24. The number of participants with low cognitive function in this study was relatively small compared to that in previous studies. A previous research study conducted in France included 745 dementia patients in a total study population of 6,626 [6]. Another study conducted in the UK included 347 dementia patients in a total population of 7,899 [5]. Consequently, the smaller sizes of the low cognitive function group and total population in our study could have lowered the accuracy of the analysis.
Second, some factors may have contributed to higher cognitive function in our study participants compared to that in previous studies, notably, the differences between the study populations. For example, the fruit and vegetable consumption in Korea is 452 g per day [18], whereas that in France is 342 g/day and in the UK is 258 g/day [19]. Comparatively high consumption of fruits and vegetables could contribute to a delay in cognitive decline [20]. It is possible that unknown differences in Eastern/Western population health-related distribution might have an effect. In addition, application of different criteria for measuring cognitive decline in the studies could lead to differences in results. In our study, the criteria for cognitive decline included mild cognitive impairment (MCI), whereas the criterion in previous studies was dementia [5, 6]. Since MCI is a pre-dementia stage and its severity is less than that of dementia [21, 22], the correlation between CVH and MCI and that between CVH and dementia could be different. Differences in the methods for measuring cognitive dysfunction could be another reason for inconsistent study outcomes. In our study, the Korean version of the MMSE-DS was used, whereas previous studies used a combination of cognitive tests [5].
Additionally, the differences between the AHA’s Life’s Simple 7 tools and the CANHEART model might also cause dissimilar results [23, 24]. The AHA’s Life’s Simple 7 tools includes smoking, physical activity, healthy diet, BMI, total cholesterol, blood pressure, and fasting plasma glucose [14, 23, 24]. In comparison, the CANHEART index includes smoking, leisure physical activity, fruit and vegetable consumption, BMI, hypertension, and diabetes; several of these components are subjective and may be easily affected by recall bias [25]. Consequently, the participant CVH level could be underestimated when using the CANHEART components. In this case, participants with normal CVH levels could be classified as having low CVH level, making the relationship between low CVH and low cognitive function appear weaker. Compared to the CANHEART model, the AHA’s Life’s Simple 7 tools includes objective components, such as total cholesterol and fasting plasma glucose levels; therefore, the influence of recall bias in previous studies could be less than that observed in our study.
The current study has implications for the design of future studies. Primarily, our study and the previous studies obtained very different results. We found that CVH levels did not show an association with cognitive function in our study population. It is still unknown whether the differences in results were related to statistics or differences between Korean and European populations. The current study also confirmed that inflammation levels had no significant association with cognitive dysfunction. This will potentially simplify future study designs.
Our study had several limitations. First, the CANHEART model used for measuring CVH has a low applicability. This model was developed in Canada and has been applied extremely infrequently elsewhere. Second, our study had a cross-sectional design; therefore, the temporal association between CVH and cognitive function could not be verified. Third, the elements that may have caused conflicting results in studies investigating associations between CVH and cognitive function in different populations are unknown. Several factors such as genetic differences between races [26], degree of fruit and vegetable consumption [27, 28], or environmental and cultural influences could be relevant. Further investigations to develop an appropriate index for the Korean population are needed.
In conclusion, our study revealed that the association between low CVH levels and cognitive decline was not significant and that inflammation levels were not relevant to cognitive decline. Several recommendations can be made for designing future studies to ascertain why our results conflicted with those of previous studies. A larger study population is important, since in our study, the low number of participants was potentially the cause of low significance. In addition, the AHA’s Life’s Simple 7 model is cautiously recommended to reduce potential subjectivity. The methods used to evaluate cognitive function in future studies should be similar to those used in previous studies, allowing accurate comparisons. Consequently, efforts to resolve the issue of conflicting outcomes between this study and previous studies could be focused on for distinguishing the characteristics of the study populations.