Background The objective of this study was to assess the trend in socioeconomic inequalities in obesity from 1988-2012 for adult Mexican women. We hypothesised that education would be inversely associated with obesity in richer urban areas of Mexico but not in rural areas, and that the gradient in urban areas would become steeper as the nutrition transition progressed.

Methods Data came from four nationally representative surveys carried out in 1988, 1999, 2006 and 2012. Response rate ranged from 80% to 97% and sample size of non-pregnant women age 20 to 49 from 10,318 to 14,531. Weight and height were measured by a nurse and education level (higher education, high school, secondary, primary or less) was self-reported. Analyses were adjusted for survey design. The relative index of inequality (RII) was computed and its trend from 1988-2012 tested.

Results Nationally, age-standardised obesity prevalence increased from 9.3% to 33.7% over the 25 year period. An inverse association between education and obesity was observed in urban areas. In rural areas, obesity prevalence increased markedly but there was no association with education level. Obesity increased 5.92 fold (95% CI 4.03, 8.70) among urban women with higher education in the period 1988-2012 compared to 3.23 fold (2.88, 3.63) for urban women with primary or no education. The RII in urban areas declined over the period from 2.87 (1.94, 4.25) in 1988 to 1.55 (1.33, 1.80) in 2012, trend p < 0.001.

Discussion As expected, an inverse association between education and obesity was observed in urban areas of Mexico. However, the stepwise decline in obesity inequalities was because obesity prevalence increased faster among higher educated groups. These findings simulate trends observed in the USA and not in other middle income countries undergoing nutrition transition like Brazil and China. In rural areas there was no association between education level and obesity across the four surveys.