Obesity and body mass index in Spain: the 'single population' theory revisited.

OBJECTIVE

The 'single population' theory suggests that the distribution of body mass index (BMI) moves up and down as a whole. We test directly whether this theory is valid among the adult population within one country over time, by examining the association between median BMI and the prevalence of obesity in the adult population of Spain's 17 regions between 1987 and 1993.

DESIGN AND SETTING

Ecological study using data from two national health surveys.

SUBJECTS AND INTERVENTIONS

Self-reported weight and height for persons aged 20-64 y were drawn from representative Spanish-population samples from two similar National Health Surveys carried out in 1987 (n = 20 705) and 1993 (n = 15 490).

MAIN OUTCOME MEASURES

Correlation and linear regression between the difference in median BMI and the difference in the prevalence of obesity (BMI > or = 30 kg/m(2)) between 1987 and 1993.

RESULTS

Between 1987 and 1993 the change in median BMI was in general strongly correlated with the change in the prevalence of obesity (r = 0.85; P < 0.001). The correlation is greater in women, persons over 45 y, and those with lower educational levels. Between 1987 and 1993 each unit of increase in median BMI is associated with an absolute increase of 5.1% (95% CI 3.3-6.8%) in the prevalence of obesity.

CONCLUSIONS

The single population hypothesis holds true among the adult population within a whole country over time. This could be useful for monitoring and understanding the prevalence of obesity within a population.