The major findings of our analysis were that poor hypertension control may negatively influence the independence in ADL/IADL in older adults, but this adverse association could be changed if taking BMI status into account. After adjusted for demographics and chronic conditions, among underweight subset, poor hypertension control turned into a protective factor for ADL/IADL disability not only versus without hypertension, but also versus hypertension control; however, at the same time, among obese subset, poor hypertension control was a certain risk factor for ADL/IADL disability versus without hypertension. Our results thus contribute to the current knowledge by providing evidence that BMI status may completely change the association between hypertension and ADL/IADL disability.
Although hypertension is one of the most important preventable contributors to disease and death[30], several studies also show different result when focusing on the relationship between hypertension and disability[11, 12, 31, 32]. For example, Uddin et al. found hypertension was significantly associated with lower ADL scores in Bangladesh[31]; our previous study indicated that simple hypertension wasn’t significantly associated with ADL/IADL disability[24]. In present study, we found only poor hypertension control was associated with ADL/IADL disability, while hypertension control was not. These results remind that the statues of hypertension control might modify the relationship between hypertension and disability among older adults. Based on the finding in our study, we suggest that more attention should be paid to those with poor hypertension control, although the proportion of poor hypertension control subset was low (only 6.54% in our sample).
In further comparison, when BMI status was taken into consideration, the relationship between hypertension and disability became much more complicated. For obese subset, poor hypertension control indicated a high prevalence rate of both ADL and IADL disability; even hypertension control was associated with IADL disability. As previous studies proved, obesity not only independently predicted the risk of development ADL/IADL disability[21, 33], but also was one of the most common risk factors for hypertension[17]. Our findings prompt that poor hypertension control and obesity show synergistic effects on disability (for ADL disability, OR of the whole participants = 1.47, OR of the obese participants = 1.73; for IADL disability, OR of the whole participants = 1.55, OR of the obese participants = 1.80). Accordingly, taking measures to control blood pressure for obese older adults may deserve special attention.
Generally, underweight is associated with a low prevalence rate of hypertension[34], but also regarded as a red flag of disability[35–37]. In this study, for underweight subset, poor hypertension control indicated a low prevalence rate of disability. This result was completely different with the whole sample. There are several plausible explanations. Firstly, the age of the underweight subset was relatively older. More specifically, the mean age of the underweight subset was 76.09 ± 10.19, while for the normal weight, overweight, and obese subset was respectively 70.12 ± 8.53, 69.06 ± 7.39, and 70.21 ± 7.94. Studies showed, for older individuals, high blood pressure might be a compensatory mechanism to maintain organ perfusion, which can have a survival benefit while ensuring perfusion in critical organs, and ultimately prevention of physical decline[15, 38]. In addition, the share of underweight subset was quite low (only 4.89%). Therefore, the influence of underweight status could be hided among the whole group. The finding reminds that more studies are needed to explore whether or not to take antihypertensive therapy for underweight hypertensive older individuals.
Furthermore, the association of hypertension statuses and ADL disability or IADL disability appeared different changes between normal weight and overweight subsets. For ADL disability, poor hypertension control was a definitely risk factor compared with without hypertension only among normal weight subset; for IADL disability, poor hypertension control was a definitely risk factor compared with without hypertension and hypertension control only among overweight subset. The prevalence rate of IADL disability was over triple that of ADL disability (for ADL disability = 9.27%; for IADL disability = 32.47%). The huge difference between the prevalence rates of ADL and IADL disability may induce the statistical differences. At the same time, it is noteworthy that the different directions of the association showed the distributions of ADL disability and IADL disability among older people might be different. Therefore, we suggest that ADL and IADL should be used together to enforce the integrity and reliability of independence and disability evaluation.
Our study has several limitations that warrant consideration. First, the cross-sectional design of the study limits the causality of the findings and only associations can be drawn, providing hypotheses that can be verified in future studies. A second limitation is more than 700 hypertensive respondents were excluded from this study because of missing blood pressure information, which might cause potential effect to the results. In addition, some characteristics, such as education, nutrition status, and muscle strength, that were associated with ADL/IADL disability, were not included in this survey[39–41].
To our best knowledge, this is the first study aimed at investigating the possible interactive effect of hypertension and BMI status on dependency in activities of daily living among older people. As previously discussed, hypertension control and poor hypertension control could play different roles among different BMI statuses. Thus, these findings provide some evidence that hypertension and BMI status have interactive effect on ADL and IADL disability.