Author(s): Yan Kestens 1 , 2 , * , Alexandre Lebel 2 , 3 , Basile Chaix 4 , Christelle Clary 2 , Mark Daniel 5 , Robert Pampalon 6 , Marius Theriault 7 , S. V. p Subramanian 3
Towards better understanding the current obesity epidemic, there has been a concerted effort to examining the association between food environments that an individual is exposed to and their body mass index . Studies have been looking at relations between food environments and food purchasing, diet, or more distal health outcomes like BMI, cardio-vascular outcomes or mortality , , . But findings are mixed. For instance, association between fast food access and diet or BMI have been positive, negative or null , , .
The majority of studies on food environments and health have relied on measures of foodstore accessibility . Some studies have considered more specific elements, such as food availability and costs , portion sizes , visual food cues, or availability of specific food types , , . Geographic analyses of food environments generally use spatial proximity or density estimates to measure accessibility or exposure to foodstores . Measures are established for point data such as postal codes or addresses, or for areal units, most often administratively defined and sometimes purposely designed, for example using ego-centered circular  or road-network buffers . Proximity generally accounts for travel times or distance between the reference units and the closest foodstores . Alternative accessibility measures based on gravity theory or space-time geography principles have more rarely been used . Density measures are usually computed within a chosen areal unit by dividing the count of observation by the area or the population, or using kernel density estimation methods , .
In spatial epidemiology, the relationship between environmental exposures and individuals - and their corresponding health behaviors or disease outcome - is traditionally grounded to one reference location - most often, place of residence. Some have looked at exposure in non-residential locations such as schools , . However, even then, the relation between access and health outcomes is assessed for one reference location only. A study integrating exposure to both residential and five non-residential regular activity places showed that ignoring non-residential exposures underestimated the association between residential exposure and self-reported health . Another multi-location exposure study assessed the relation between BMI and accessibility to restaurants including fast food outlets around both home and the workplace . No association was found for women, and for men, a significant inverse relation between BMI and restaurant proximity was found around workplaces only, and not around home.
Limiting measures of exposure to the local residential area may constitute a 'local'  or 'residential'  trap, and thus ignores actual 'spatial polygamy' , or the fact that we live and spatially relate to more than one 'anchor point', through a network of usual places . Already in the 1950's, researchers in sociology and geography documented how daily activities included destinations outside of the residential neighbourhood . The resulting...