The present study aimed to estimate the incidence rate and risk factors associated with hypertension (HTN) among people aged 40–70 years in Kharameh, southern Iran. The main findings of our study demonstrated that the cumulative incidence and incidence density rate of HTN were 10.98% and 21.54 per 1000 person-years, respectively. Among the demographic and behavioral variables, old age, female gender, high BMI (>30 kg/m2), and sleeping pill use were risk factors for HTN. However, green iris color was a protective factor for HTN. Among the clinical factors, pre-HTN, a history of HTN in first-degree relatives, rheumatoid disease, kidney stone, recurrent headache attacks, a history of head injury with anesthesia, and high TG levels were risk factors for HTN. Finally, among the behavioral variables, opium use was found to be a risk factor, while smoking and alcohol consumption had a protective role. However, only drug use was significant in multivariate analysis.
Incidence of hypertension
In this study, we calculated the incidence rates of hypertension based on person-years and cumulative incidence, taking into account a four-year follow-up period. It is important to note that previous studies have used varying follow-up periods, which has made it difficult to compare cumulative incidence rates across studies. Therefore, our goal was to compare the person-year incidence rate with similar studies in the field to enable more meaningful comparisons. Our study found that the incidence of hypertension in both sexes was estimated to be 21.54 per 1000 person-years, with rates of 16.06 and 27.37 per 1000 person-years in men and women, respectively. Notably, the incidence of hypertension was higher in women than in men. Our findings differed from other studies, with lower incidence rates reported in Iran (Ahvaz- 65 per 1000 person-years in general and 67.7 and 57.8 per 1000 person-years in men and women), Portugal (52.7 in men and 43.4 in women per 1000 person‑years), and India (93.1 in men and 70.9 in women per 1000 person-years). However, our results were consistent with a study conducted in Canada (22.7 in men and 21.6 in women per 1000 person-years)(2, 31-35).In a cohort study conducted in Thailand with a 4-year follow-up, researchers reported a lower cumulative incidence rate of 3.5% compared to our study(35). Most studies have found that the incidence of HTN in men is higher than in women, in contrast to our study. There are several possible reasons for the observed difference between our results and those of other studies. Firstly, in the Thai study, as mentioned previously, most of the participants were young, which may explain why the incidence rate was lower. Secondly, the lower incidence of HTN in our study may be due to the lifestyle of our participants, who consumed herbal teas such as thyme, which have been reported in other studies to lower blood pressure(36).Thirdly, the higher incidence of HTN in women compared to men in our study may be attributed to the fact that most of the women were postmenopausal and had a higher prevalence of tubectomy, hysterectomy, and ovary removal. Previous studies have shown that menopausal women and those who have undergone hysterectomy or ovary removal are at a higher risk of developing cardiovascular, blood pressure, and metabolic diseases(37).
The association between the Demographic and lifestyle variables with incidence of hypertension
The findings of the present study suggest that older adults are at a higher risk of developing hypertension (HTN) compared to younger age groups. This is in line with previous research which has also shown an increase in the risk of HTN with increasing age. As people age, their arteries become stiffer and less elastic, which can lead to higher blood pressure.(38-41). Additionally, previous studies have found a positive relationship between body mass index (BMI) and HTN, which is consistent with the results of this study. Obese individuals have a higher likelihood of developing HTN, and the mechanisms behind this relationship are complex and involve various physiological processes such as sympathetic nervous system activation, renin-angiotensin-aldosterone system stimulation, changes in adipose-derived cytokines, insulin resistance, and renal structural and functional alterations.(23, 42-44). While the incidence of HTN is typically higher in men than in women, the findings of this study suggest that the opposite is true. However, the relationship between drug use and HTN is still unclear. In our study, in line with the studies conducted by Najafipour et al., Rahimi et al, Nakhaee et al., the opium use increased the odds of developing HTN(45-49). Contrary to our study, studies by Rezaianzadeh et al, a Deris et al., Masoomi et al. have reported an inverse relationship between drug use and HTN(19, 50, 51). The available data regarding the association between drug use and alterations in blood pressure are generally insufficient and inconsistent. Various reasons may have caused these differences. Firstly, the illegal nature of drug use may restrict the availability of accurate information. Secondly, potential confounding variables and interactions between drug use and other factors may not have been taken into account in some studies. Thirdly, the drugs used by individuals may not be pure and could contain additives that may either pose risk factors or have beneficial effects on HTN. Lastly, further research is necessary to address this matter conclusively. Interestingly, the present study found an inverse relationship between green eye color and blood pressure, which is consistent with previous research conducted by Friedman et al. While no confirmed genetic connection has been found between eye color and blood pressure, it's possible that certain genes may impact both eye color and blood pressure in some way.(52). Finally, while a study by Sasaki et al. found a significant percentage of individuals using sleeping pills in the HTN group, the present study only found weak evidence for an association between sleeping pill use and HTN. Further studies are needed to assess the impact of individual sleeping pills on blood pressure, as the ingredients in these medications may contribute to HTN.(53, 54).
The association between the clinical variables with incidence of hypertension
Our study provides strong evidence supporting the relationship between pre-hypertension and hypertension, which is consistent with findings from other studies (55-58). Many cross-sectional studies have reported hypertension as a risk factor for rheumatoid disease(59, 60). Whether it is a risk factor for rheumatoid disease or not requires more longitudinal studies. However, our study indicates that the likelihood of developing HTN is greater in individuals with rheumatoid disease compared to those who are healthy. This may be due to several reasons. Firstly, chronic inflammation (a common characteristic of rheumatoid patients) can increase the risk of hypertension. Secondly, some medications used to treat rheumatoid disease, such as non-steroidal anti-inflammatory drugs, may also increase the risk of hypertension. Finally, most rheumatoid patients possess other risk factors that contribute to the development of hypertension (61-64). Our study also confirms the association between nephrolithiasis and an increased risk of hypertension, which is consistent with previous research findings(65). Furthermore, we found that individuals with a family history of hypertension were more likely to develop the condition. This finding is consistent with results from other studies, further confirming the relationship between family history and hypertension(66-68). In our study, we observed that individuals with a history of frequent headache attacks were more likely to develop hypertension than those without such a history. Several studies have found an association between migraine and hypertension, with some suggesting that people with migraine may have a higher risk of developing hypertension later in life. Other studies have found that people with hypertension may be more likely to experience migraine or other types of headaches. More research is needed to fully understand the relationship between these two conditions.(69-73).
Our study found that individuals with a history of head trauma are at a higher risk of developing hypertension. Although some studies have explored the immediate effects of head trauma on blood pressure, we found no research examining the long-term effects. Izzy et al.'s study further supports the notion that head trauma may be associated with a higher risk of chronic cardiovascular, endocrine, and neurological diseases (74). It is possible that the increased risk of comorbidities after traumatic brain injury (TBI) can be attributed to various behavioral and lifestyle changes, such as physical inactivity, unhealthy diet, social isolation, and an increased likelihood of other risk diseases like sleep disorders and depression. Additionally, recent clinical and experimental studies indicate that TBI could potentially impact systemic metabolomics, gut flora, and immune pathways. Hence, the greater risk of comorbidities after TBI may stem from a combination of direct factors (such as hormonal and inflammatory changes caused by injury) and indirect factors (such as psychosocial risk factors). This has also been mentioned in other studies(74-80). Finally, our study found that high triglyceride levels are a risk factor for hypertension, which is consistent with research conducted in Japan(81), Norway(39), Iran(12, 32), and Lebanon(82). However, some cross-sectional studies have not found an association between triglyceride levels and HTN(83). This may be because people change their lifestyle after developing diseases such as diabetes, cardiovascular disease, and hypertension. Additionally, they may take triglyceride-lowering medication. Although high cholesterol was found to be a risk factor for hypertension in the univariate analysis, it was not statistically significant in the multiple models, possibly due to the inclusion of other diseases in the model and the fact that some participants were taking cholesterol-lowering medication. Nonetheless, previous studies on cardiovascular diseases and blood pressure have reported cholesterol as a risk factor, and our study confirmed that high triglyceride levels are also a risk factor (84, 85).
Strengths and limitations of the study
Our study has several strengths. First, it is a prospective cohort study that overcomes the limitations of cross-sectional studies, such as the issue of temporality. Second, we used Firth's regression model to analyze the data. This model, which is a Penalized Maximum Likelihood (PML) logistic regression, can provide unbiased estimates even in the presence of rare and unbalanced events. Third, we attempted to study multiple variables to control for confounding and examine their effects simultaneously. Finally, the data were collected by experienced experts, and two internal specialists confirmed cases of HTN.
However, our study also has some limitations. Firstly, due to the stigma associated with drug and alcohol use in Iran, participants may have concealed their behaviors, potentially causing bias. Secondly, we were unable to examine the association between participants' diets and HTN, as we did not have access to this information. Thirdly, we could not determine the roles of genetic, racial, and ethnic backgrounds, as this information was not available.