The present study is the first study to prospectively assess the effect of objectively measured urban form on change in weight and incidence of obesity in African-American women, and is the largest to date in terms of numbers of participants and geographic scope. In this population, 6-year weight gain and the risk of incident obesity were lower among women who lived in dense urban neighborhoods compared to those who lived in suburban or rural neighborhoods. The effect of urbanicity on weight change and risk of obesity was more pronounced in women aged <40 years.
Our prospective findings agree with findings on various measures of urbanicity and BMI from cross-sectional studies.5-7, 9-13
The most consistent associations have been with indices of housing or population density and mixed land use. The two largest cross-sectional studies (over 10,000 participants each) were conducted in Atlanta 10
and New York City 24
. Each study included over 2000 black subjects and presented race-specific analyses. In the Atlanta study, a continuous index of land use mix ranging from single use to highly mixed use was most strongly associated with risk of obesity (OR for each quartile increase = 0.88, 95% CI 0.84, 0.92).10
In race-specific analyses, the relationship between the land use mix and BMI was significant among only white participants. In a similar analysis of the Atlanta data, a different measure of walkability was significantly associated the BMI among white men only.24
In the New York City study, associations between BMI and most measures of urbanicity (i.e., population density, land use mix, public transit availability) were in the expected inverse direction 6
but were stronger in white than in black subjects.25
A prospective study that used ordinal regression to predict change in BMI over 6 years found no association between objective measures of neighborhood walkability and BMI change, but the study (n
=500) had limited statistical power.26
In an attempt to isolate the effects on weight change of moving from one type of neighborhood to another in the present study, an analysis was conducted among women who moved, and change in urbanicity was defined as an increase or decrease of at least one quintile of the urbanicity score. There was no appreciable difference in weight change or risk of obesity among the three groups of movers. However, few women moved among areas of drastically different urbanicity: only 1% of movers moved from the lowest to the highest quintile or vice versa, and among the groups who changed their score by at least one quintile, the mean change was little more than one quintile. Furthermore, each of the three compared groups included a range of neighborhood types at baseline. Thus for example, the referent group included women who moved among low-density suburbs or among inner-city neighborhoods. Thus the overlap in neighborhood types may have masked any differences in weight change among movers. In addition, 6 years may be too short a time to discern an effect of change in the environment on weight change.
Three previous studies have assessed weight change by moving status among participants in the NLSY. 4, 15, 16
Since 1979, the NLSY has periodically enrolled adolescents aged 14–22 years and followed them over time. In the first analysis of NLSY participants enrolled in 1979, a continuous sprawl index was assigned at the county level.4
Among 262 participants who moved during the follow-up years 1996–2000, there was an association between BMI change and change in the sprawl index such that people who moved to a more-sprawling county gained more weight than those who moved to a less-sprawling county.4
In another analysis of the 1979 NLSY cohort, sprawl was calculated for a 2-mile radius around participant residences during follow-up years 1988–1994.15
Among the approximately 5400 people who moved between 1988 and 1994, sprawl had no effect on change in BMI during the time period. In the third analysis of a new NLSY cohort enrolled in 1997, there was no association between county sprawl and 6-year change in BMI among 2427 people who moved among counties.16
In all three analyses, sprawl was a continuous variable and it is unclear how many people moved from low- to high-sprawl areas and vice versa.
Strengths of the current study include assessment of African-American women who have been understudied in regard to the built environment, the prospective analyses, the large size of the cohort, the inclusion of three major metropolitan areas, the detailed assessment of urbanicity at the level of the residential address, and control for a range of individual and neighborhood-level confounders.
Study limitations include the use of self-reported weight, although a validation study showed acceptable accuracy. Accuracy of self-report has been related to age,27
but in the present study change in reporting accuracy over time is unlikely to have been associated with urban form. Data on land use or distance to specific destinations, which other studies have found to be important,6,9-13
were not available. Census data from the year 2000 were assigned to locations in earlier years, although it is not likely that values changed greatly between 1995 and 2000. The BWHS includes few women with less than a high school education so results may not be generalizable to this group. Finally, although the analyses were prospective and were adjusted for a range of variables known to influence weight gain, residual confounding due to selection bias may have played some role in the results.
In summary, in prospective analyses urbanicity was inversely associated with weight gain and risk of obesity. This strengthens the evidence that living in denser urban areas is associated with less weight gain and obesity than living in suburban or rural areas. Policies that encourage more dense and urban residential development may have a positive role to play in addressing the obesity epidemic. Residents of rural and suburban areas should be aware that such low-density environments may contribute to weight gain, and efforts to maintain a healthy weight may be needed.