Adversities of acculturation? Prevalence of obesity among immigrants

I study factors associated with prevalence of obesity among immigrants as the length of their stay in the US becomes longer. Using the National Health Interview Surveys for 1990-2004, I create 'synthetic cohorts' of immigrants that allow controlling for two important determinants of obesity: age at arrival and period of arrival in the US. I find that immigrants with a BA degree did not experience any change in obesity as their stay in the US advanced. Obesity among those without a BA degree, however, increased with duration of stay. Similarly, while some ethnic groups experienced increase in obesity with duration, others did not. For those who experienced increase in obesity with duration, the rise was higher during the first five years of residence than later and greater for those who arrived at a relatively young age.