Abstract Data of the 1992-1993 Mexican Survey of Chronic Diseases in the Urban Adult Population were analyzed to investigate the effects of age and sex on the association between overweight and hypertension. Blood pressure, body weight, and height were measured in a group of 13 945 Mexicans aged 20 to 69 years living in towns and cities larger than 15 000 people. Hypertension was defined following the recommendations of the Joint National Committee for Detection, Evaluation, and Treatment of High Blood Pressure-V. Overweight was defined following the recommendations of the National Institutes of Health Consensus on Health and Obesity. The prevalence of types of hypertension was higher in men than in women, particularly in the groups of 20 to 39 years of age. Cross-classification of subjects according to the presence of hypertension and overweight confirmed the association between both variables. The odds ratio and 95% confidence intervals indicated that overweight was associated with systolic and diastolic hypertension and with isolated diastolic hypertension in women and, to a lesser extent, in men. This association was not found in the 60- to 69-year group in both sexes. Overweight did not show significant association with isolated systolic hypertension in both sexes. Results of a multiple logistic regression analysis of overweight on hypertension, controlling for age and sex, were consistent with these findings. It is suggested that other factors, independent of overweight, explain the observed gender-specific differences in the prevalence of hypertension in younger age groups. The hormonal environment of young women is one of the mentioned factors modifying the prevalence of hypertension in this group of the Mexican urban adult population.
The association between overweight and hypertension has been known for several years. However, this association depends on the variability of the prevalence of overweight in a given population,1 2 and age, sex, and ethnicity are potential confounders.3 4 In 1992 to 1993, the National Survey of Chronic Diseases (NSCHD) was conducted in Mexico.5 The NSCHD showed high prevalence of hypertension in the urban adult population. However, women presented lower rates in the younger age groups. To understand some of the factors underlying these gender-specific differences, we examined the association between overweight in a group of the same population, controlling for the effects of sex and age. The presentation of results of the analysis is the purpose of this article.
The data analyzed are from the 1992-1993 NSCHD. A more detailed description of the sample, methods, and results is presented elsewhere (Arroyo P, Fernandez V, Kuri-Morales P, Orozco-Rivade-Neyra S, Avila-Rosas H, Loria A, Olaiz G, Tapia-Conyer R. Hypertension in Urban Mexico: the 1992-1993 National Survey of Chronic Diseases; submitted for publication). A multistage sampling process was employed, and Mexico was divided into four regions which will be referred to as Northern, Central, Southern, and Metropolitan. The Metropolitan region consisted of Mexico City and the 10 surrounding municipalities. The regions differed in urbanization, industrialization, and mortality indicators.6 7 Within each of the first three regions (Northern, Central, and Southern) there are 10 states, of which 5 were randomly chosen for inclusion in the study. The entire Metropolitan area was included. Within each state, basic geographical statistical units were randomly selected. From this sample, cities and towns with populations greater than 15 000 were selected, and a random sample of 51% of these towns was included in the study. A total of 416 towns and cities with populations greater than 15 000 were included in the survey. Within each of these 416 towns and cities, random samples of the neighborhood blocks were drawn, and the survey was administered to people between the ages 20 and 69 in each of the households within each selected neighborhood block. Individuals living in military, religious, or health institutions were excluded.
A quota of 4731 individuals per region was calculated using the household as the sampling unit (according to the 1990 Census, there was a ratio of 2.33 persons per household), which gave a target of 2030 households per region. The criterion used in the estimation of the sample size was established in a preliminary pilot survey of diabetes, ie, the ability to detect risk factors with a minimum prevalence of 4% and a 95% confidence interval (CI). A relative maximum permissible error in the estimation of 0.289 and a nonresponse rate of 30% (global for nonresponses either by household or by individuals) were also applied criteria.
The fieldwork was carried out from September 1992 to August 1993. It was done sequentially by regions starting in the Northern region and ending in the Metropolitan region. A team of 30 nurses and social workers performed the survey in the four regions. They had been previously trained and standardized by experts of the National Institutes of Health of Mexico. This training lasted 28 weeks and included six dry runs in which 200 households were visited in an area near Mexico City (Delegación Tlahuac).
A questionnaire was administered to every eligible adult living in the selected households to collect data on family history, clinical symptomatology, and therapy taken in regard to arterial hypertension and several chronic diseases. Blood pressure (BP) was measured manually by trained observers using a standard mercury sphygmomanometer. An appropriately sized cuff was placed on the participant’s right arm. After the subject had rested 5 minutes in the seated position, the cuff was inflated, and the pulse obliteration was measured. The I and V Korotkoff phases were the criteria for systolic and diastolic pressures (SBP and DBP). The individuals were classified according to the criteria of the Joint National Committee for Detection, Evaluation and Treatment of High Blood Pressure (JNC-V),8 ie, hypertensive if the SBP was ≥140 mm Hg and/or the DBP was ≥90 mm Hg, and/or self-reported treatment with antihypertensive medications. Since the purpose of this study was to analyze the association between the distribution of BP and body mass index (BMI), treated hypertensives were excluded from the statistical analysis. The weight and height of the individuals, barefooted and in light clothing, were measured using a flat spring scale and a metric tape adhered to a wall. BMI was calculated as weight/height2, and overweight was considered to exist if the BMI was equal to or higher than 27.3/m2 of height for women and 27.8 for men.9
The information was transferred to a database in a PC computer. Range and logic checks were used to detect erroneous information. Rates (%) of prevalence of types of hypertension and overweight by groups of age by sex were calculated. We evaluated the risk of hypertension in the presence of overweight calculating the odds ratio (OR), and the 95% CI for groups of sex and age.10 A multivariate logistic model was applied to evaluate the adjusted effects of overweight and age on the risk of types of hypertension for both men and women. The corresponding adjusted OR, CI, and standardized β coefficients are presented. To assess the weight of each independent variable, the standardization procedure consisted of the multiplication of the β coefficient by the standard deviation of each variable.11
Of a total of 18 036 potential subjects, 14 657 were interviewed at home, giving a response rate of 81.2%. The group analyzed consisted of 13 945 subjects (95.1% of total), divided as 41.6% men and 58.4% women. The study group resulted from the exclusion of 380 (2.6%) subjects with pharmacological treatment and 232 subjects (1.6%) missing information. Women were overrepresented in the sample, a phenomenon which could be attributed, in part, to the absence of men in the household during working hours and to migration. However, as can be observed in Table 1⇓, the age and sex distribution of the study group, compared to the 1990 general population,6 did not present major differences except in men of ages 30 to 39 years, with 1.1%, and women of 40 to 49 years, with 1.0%. The ratio of men to women in the census population was 0.93, while in the study group it was 0.71.
Prevalence of Hypertension and Overweight
The prevalence of hypertension by type and groups of age and sex is described in Table 2⇓. Men presented an overall rate of 35.9% and women 24.9%. Younger women showed lower rates than men, but the difference disappeared after 50 years of age. Isolated systolic hypertension had the lowest frequency, whereas the combination of both forms of hypertension was the most frequent type. In all forms of hypertension, men were affected more frequently than women, mainly when both types coexisted.
Table 3⇓ presents the prevalence of overweight in the study group by age and sex. In all groups of age, the frequency of overweight was higher in women than men; the rates increased with increasing age in both sexes and was highest in the 40- to 49-year group of men and in the 50- to 59-year group of women.
Tables 4⇓ and 5⇓ show the prevalence of different types of hypertension by age group in normal and overweight male and female subjects, respectively. In the age group from 20 to 59 years, overweight subjects of both sexes show a higher prevalence of hypertension, independent of the type, than normal subjects, and this difference disappears in the 60- to 69-year group.
Effect of Overweight Adjusted by Sex and Age on BP
Tables 6⇓ and 7⇓ show the OR and 95% CI of the risk of types of hypertension in relation to overweight, by groups of age, in men and women, respectively. The risk of hypertension associated with overweight was related mainly to the isolated diastolic and systolic and diastolic types; the ORs associated with isolated systolic hypertension were not significant in most age groups. This pattern of association disappears in the age group of 60 to 69 years in both sexes.
Table 8⇓ presents the OR, CI, and β standardized coefficients of the multiple logistic regression of overweight, age, and sex on the types of hypertension. In women, overweight had a stronger effect on isolated diastolic hypertension and on systolic and diastolic hypertension, while age had a stronger influence on isolated systolic hypertension. In men, the effect of overweight on isolated diastolic hypertension was lower than in women. In men, the effect of age on the isolated systolic hypertension was still high, although to a lesser extent than in women. Within the same age bracket, the effect of overweight on hypertension was stronger in women than in men, particularly for isolated diastolic hypertension.
The results of the present study confirm the association of BMI with BP in the urban Mexican adult population. However, several distinctive features are worth commenting on: (1) with increasing age, a higher prevalence of hypertension in both sexes was observed; (2) lower rates of hypertension in women than in men were found in the age bracket of 20 to 39 years; (3) women attained a similar prevalence of hypertension than men in the fourth and fifth decades of life; and (4) women showed higher prevalence of overweight than men did in most age categories.
The OR and the β regression coefficients were consistent with these observations; their magnitudes and changes confirm the different pattern of association between BMI and BP in both sexes in the urban Mexican population. However, this association does not explain the lower prevalence of hypertension found in women from 20 to 39 years of age, since overweight showed the opposite effect on them. Thus, other factors acting in the opposite direction should be considered in the explanation of this phenomenon. One hypothesis to investigate is the effect of the hormonal environment of women on the vascular reactivity, as has been shown in experimental conditions and clinical conditions, which suggests the presence of gender-specific differences.12 13 Another hypothetical factor to be considered is the effect of the reproductive history of women on BP during this period of life. Although declining, relatively high fertility rates in Mexican women are still present. It is difficult to assign any influence to ethnic differences across regions in the present survey, since most of the urban Mexican population is of mestizo background, and relatively pure Indian populations live in rather small rural communities and were not surveyed.
A recent study in a white Canadian population14 presented data on the effect of overweight and age over a large lifespan, ie, from 6 to 74 years. The multiple regression coefficients between BP and BMI calculated by these authors are also lower for both SBP and DBP in women in the age bracket of 15 to 39 years than in men. The difference between male and female subjects was not observed in older age groups, as in the Mexican sample.
As an additional comment, the prevalence of overweight and hypertension may be related to the influence of strong urbanizing forces operating in Mexico in the last generation.15 Associated with these forces are all the changes in the lifestyle observed in urban populations, and it is within this context that the results of the present study have to be evaluated.
As was commented on earlier, Chen et al14 studied the association of BP with BMI in a group of Canadians of Caucasian origin. The multiple regression coefficients between BP and BMI presented by these authors are also lower for both SBP and DBP in women in the age bracket of 15 to 34 years than in men. These differences between sexes are not observed in older age groups, suggesting the same pattern observed in the Mexican sample.
- Received March 16, 1997.
- Revision received April 22, 1997.
- Accepted May 9, 1997.
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