Body mass index and overweight in relation to residence distance and population density: experience from the Northern Finland birth cohort 1966

Abstract

Background: The effect of urban sprawl on body weight in Finland is not well known. To provide more information, we examined whether body mass index (BMI) and the prevalence of overweight are associated with an individual's distance to the local community centre and population density in his/her resident area.

Methods: The sample consisted of 5363 men and women, members of the Northern Finland Birth Cohort 1966 (NFBC), who filled in a postal questionnaire and attended a medical checkup in 1997, at the age of 31 years. Body mass index (BMI; kg/m(2)) and the prevalence of overweight (BMI ≥ 25.0 kg/m(2)) were regressed on each subject's road distance to the resident commune's centre and on population density in the 1 km(2) geographical grid in which he/she resided, using a generalized additive model. Adjustments were made for sex, marital status, occupational class, education, leisure-time and occupational physical activity, alcohol consumption and smoking.

Results: The mean BMI among the subjects was 24.7 kg/m(2), but it increased by increasing road distance (by 1.3 kg/m(2) from 5-10 to 20-184 km) and by decreasing population density (by 1.7 kg/m(2) from 1000-19,192 to 1-5 inhabitants/km(2)). The respective increases in overweight (overall prevalence 41%) were 13 per cent units for distance and 14 per cent units for population density. Adjusted regressions based on continuous explanatory variables showed an inverse L-shaped pattern with a mean BMI of 24.6 kg/m(2) at distances shorter than 5 km and a rise of 2.6 kg/m(2) at longer distances, and an increase of 2.5 kg/m(2) from highest to lowest population density. The associations with road distance were stronger for women than men, while the sex difference in association with population density remained indeterminate.

Conclusions: We conclude that young adults in Northern Finland who live far away from local centres or in the most sparsely populated areas are fatter than those who live close to local centres or in densely populated areas. The likely explanations include variations in everyday physical activity in different residential environments, although causality of the associations remains to be confirmed.

Figure 1

Northern Finland birth cohort study (1966): areas surveyed in 1997 shown as shaded.

Figure 2

Distribution of subjects according to residence distance and population density in the resident grid. Distribution of subjects according to distance to the midpoint of resident commune’s densest grid and according to population density of the resident grid. Smoothed by Gaussian kernel density function with smoothing windows of 0.5 km (distance) and 150 inhabitants per km² (population density). The modal values of each distribution are also indicated.

Figure 3

BMI (kg/m²) and overweight (BMI ≥ 25 kg/m²) according to residence distance and population density. Body mass index (BMI; kg/m²) and percentage of overweight (BMI ≥ 25 kg/m²) in relation to individual’s road distance (km) to the resident commune’s densest grid, and on population density of the resident grid (inhabitants/km²). Continuous line indicates the regression-based estimate for BMI and the prevalence of overweight, smoothed by a cubic spline with 4 degrees of freedom (95% confidence bands shown by dashed lines). Residential area types are marked by Arabic numerals: 1 scattered settlements; 2 rural areas proper; 3 transition zones; 4 built-up areas & suburbs; 5 high-rise centres. Upper row: crude BMI and prevalence of overweight. Lower row: regression-based gradients compared with the baseline, adjusted for sex, marital status, occupational class, education, leisure-time and occupational physical activity, alcohol consumption and smoking.