research has focused mostly on cross-sectional data, comparing different people living in different neighborhoods. A great need is longitudinal evidence relating changes in healthy food availability to changes in diet over time.

A few longitudinal studies have taken place, even though causality is not yet clear. For example, using MESA data, people who live in neighborhoods with higher healthy food availability scores had a 45 percent reduced incidence of diabetes over a five-year period (Auchincloss et al., unpublished). Another study (Sturm and Datar, 2005, 2008) confirmed that higher prices of fruits and vegetables were linked to greater increases in children’s weight over time. In the Moving to Opportunity study of families who had moved from poor to non-poor neighborhoods (Kling et al., 2007), BMI was significantly reduced, although the reasons for this are not understood. Also, as discussed further in this chapter and in Chapter 5, “natural experiments” occurred in Glasgow and Leeds in the United Kingdom, where supermarkets opened in public housing areas and the areas could be studied before and after.

Challenges to understanding the causal links remain, including determining which aspects of the local food environment (e.g., availability, price, convenience) are most relevant to health, how to measure and what to use as a proxy, the scale at which changes to the local food environment are most effective, and reasonable time lags and critical periods in which to expect any effects to occur. In addition, dynamic processes are