Immigrant health assimilation is often framed as a linear, individualistic process. Yet new assimilation theory and structural theories of health behavior imply variation in health assimilation as immigrants and their families interact with different US social institutions throughout the day. We test this idea by analyzing how two indicators of dietary assimilation—food acculturation and healthy eating—vary throughout the day as Mexican children in immigrant households consume food in different institutional settings. Using individual fixed-effects models and data from the National Health and Nutrition Examination Survey, we find that Mexican children in immigrant households (N = 2,337) engage in “dietary code-switching,” eating more acculturated but not necessarily less healthy food in schools and more acculturated but less healthy food in restaurants compared to homes. Findings advance theory and knowledge about how social institutions condition dietary assimilation in particular and health assimilation more broadly.