Blood pressure and socioeconomic status in low-income women in Mexico: a reverse gradient?

OBJECTIVES: In the developed world, there is a well-established inverse association between socioeconomic status (SES) and blood pressure. In the developing world, however, these relationships are not as clear, particularly in middle-income countries undergoing epidemiological and nutritional transition.
METHODS: A house-to-house cross-sectional survey was conducted in low-income regions of rural Mexico in 2003. A sample of women (n = 9362) aged 18-65 years (mean 35.2, SD 10.4) was assessed. Measurements of systolic blood pressure (SBP) and body mass index (BMI) were obtained using standardised techniques and equipment. Interviews were conducted to collect information about SES, both objective (education, income, housing and assets, occupation) and subjective (perceived social status).
RESULTS: Household income, housing and assets were positively and strongly associated with age-adjusted SBP; the associations were attenuated somewhat with the inclusion of BMI. SBP was also positively associated with perceived social status within one's community. In contrast, age and BMI-adjusted SBP was negatively associated with educational achievement. There was a significant education by BMI interaction; at equivalent values for BMI, women who had received at least some secondary education had lower SBP than those who had received less education.
CONCLUSIONS: In contrast to traditional assumptions about the associations between SES and health, women in low-income rural populations who are at the upper end of the income spectrum within their community were found to be more likely to have higher SBP, as were those who perceived that they had higher status in the community. These results challenge standard assumptions about the association of SES and health.