In this study of nationally representative data on American adults from 2015 to 2016, we found that after controlling for possible confounding factors, there is an interaction between gender and age, BMI and age, race and age on hypertension. We found that the risk of hypertension increased for both men and women after the age of 49, and the tendency was more significant in women. With the increase of age, especially in the elderly, compared with overweight and BMI < 25 kg/m2, the effect of obesity on hypertension has not changed much. In addition, the interaction of age and race showed that Other Hispanic had the highest prevalence of hypertension and among Mexican Americans aged 60–80 is 8 times that of those aged 40–59.
This study found a significant interaction between age and gender on the prevalence of hypertension. Previous studies had also found differences in the incidence of hypertension between men and women[5, 15]and it was consistent with our findings; in particular, the effect of hypertension on women was stronger than men. A review by the U.S. Centers for Disease Control and Prevention (CDC) noted that blood pressure may rise especially in women after menopause. By the age of 60 and 70, 70% of women had high blood pressure, and after 75, the number rose to nearly 80%. This was in line with the findings of the increased prevalence of hypertension among women in our study after 49 years old. Although our research cannot provide a clear reason, this may be explained by following aspects. After entering menopause, estrogen level dropped and activated the renin-angiotensin system (RAS) and the sympathetic nervous system to maintain the body's electrolytes and fluid balance, which can lead to increased blood pressure. In addition, there was also evidence that estrogen lost can cause endothelial dysfunction, which may lead to the development of hypertension. Acute increase in estradiol-mediated intracellular calcium will activate endothelial nitric oxide synthase to produce nitric oxide, while estradiol can increase mRNA expression of endothelial nitric oxide synthase in the long run. Because nitric oxide can effectively promote vasodilation, which can reduce blood pressure. In short, decreased estrogen may explain increased risk of hypertension after the menopause. Unlike women, men's hormones would not drop apparently until they after 60 years old. The main reason for the greatly increased prevalence of hypertension in elderly men may be the decline of androgen level. Related studies have shown that androgen deficiency was associated with increased prevalence of hypertension. As we all know, men's overweight or obesity rate will increase due to their slow metabolism after middle age. Visceral obesity can reduce the level of sex hormone-binding globulin and luteinizing hormone (LH), thereby affecting the bioavailability of male testosterone and androgens, while insufficient androgen can decrease atrial natriuretic peptide level then led to increase vasoconstriction blood pressure. It can be seen that the reason why elderly women suffer from hypertension higher than men was that women enter menopause ten years earlier than men enter old age, so the level of estrogen declines more severely than androgen.
Some previous longitudinal cohort studies have found the effect of overweight/obesity on the incidence of hypertension[20–23]. This was consistent with our research results. Our results based on a large U.S. population provided a comprehensive interpretation of the association between overweight/obesity and hypertension of a wide range of ages, and after adjusting for confounding factors, interaction analysis showed obesity has a reduced effect on hypertension in the elderly compared to overweight. A study in Italy showed that with the increase of BMI in the elderly, the risk of hypertension is gradually reduced. This was consistent with our research results. It was suggested that the obesity of the elderly population may not be the decisive factor for the incidence of hypertension. It may be that obese elderly people receive more treatment and led to a reduction in cardiovascular risk factors. This study was also consistent with previous research results, that is, the effect of BMI on the risk of hypertension depends on age, that is, the effect in the young and middle-aged population is stronger than that of the elderly[25, 26]. In contrast, a 22-year follow-up study in China showed that the risk of hypertension was increasing with age and BMI. The possible reason is related to ethnic and physical differences.
The interaction between age and race showed that Other Hispanic had the highest prevalence of hypertension. However, Mexican Americans have the lowest risk of disease between 40–59 years of age and the second highest risk of disease risk of between 60–80 years. One reason for the high prevalence of other Hispanic may be the high birth rate. The large-scale population increase prevents them from getting better formal education and poor awareness of prevention. In addition, Hispanics have a high obesity rate and a relatively higher risk of developing hypertension. This may be related to their overall lower socioeconomic status. Studies in developed countries have shown that obesity was inversely proportional to socioeconomic status, and American Indian descent also played a part, especially among Mexicans. Another reason for the rapid increase in the risk of high blood pressure for elderly Mexicans is the same as that of other Hispanics. Because the proportion of the population is growing rapidly, and Mexican Americans are traditionally known for their low awareness, treatment and control rates of hypertension[30–32], Putting them at high risk of cardiovascular disease morbidity and mortality. In addition, an analysis of Mexican Americans had shown that hypertension was significantly correlated with genetic variation associated with angiotensin converting enzyme (ACE) activity[33, 34].
The main advantage of this study was the use of a large, continuous, nationally representative survey. Secondly, the blood pressure value was the average of three measurements and results were more reliable. Finally, it was the first study on the effect of the interaction of gender, age, BMI and race on hypertension in the US population, this would help implement corresponding antihypertensive measures among men and women of different ages, BMIs and races. However, the findings in this report were subject to some limitations. First, notes on cross-sectional studies also applied to our findings, so causal inference cannot be made. Secondly, some research data depended on self-reported information, it may be affected by recall bias. If the information they provide was fallacious, it may lead to misclassification. For instance, when individuals who were taking anti-hypertensive medications but did not acknowledge taking medications to lower their blood pressure and BP was normal at the time of examination and this condition would be classified as normotensive. However, in the process of collecting data, professional staff tried to maximize the authenticity of data such as age, height, weight, and whether they suffered from hypertension. Finally, although we have controlled some key confounding factors-especially smoking, drinking and physical exercises, but we cannot rule out the possibility of unobserved confounding factors.