Marques-Vidal et al. BMC Public Health 2011, 11:772 http://www.biomedcentral.com/1471-2458/11/772 physical activity changed completely between 98-99 and 2005-6 [17], it was not possible to adjust for physical activity. Statistical analyses Statistical analyses were conducted using SAS Enterprise Guide v.4.2 (SAS Inc, Cary, NC, USA). Results were expressed as mean ± standard deviation, as number of subjects and (percentage) or as percentage and (95% confidence interval) for prevalence. Bivariate compari- sons were performed using Chi-square or analysis of variance (ANOVA). Yearly trends in BMI levels were assessed by bivariate or multivariate linear regression analysis and the results were expressed as slope (95% confidence interval). Multivariate analyses were per- formed using logistic regression for qualitative variables and a general linear model for quantitative variables; the corresponding results were expressed as Odds-ratio (OR) and [95% confidence interval] or as adjusted mean ± standard error. Statistical significance was considered for p < 0.05. Results Samples’ characteristics The main characteristics of the samples according to survey year are summarized in Table 1. Educational level as well as the percentage of subjects aged over 75 increased, while no differences were found regarding gender distribution. Trends in overweight and obesity Mean and median BMI as well as prevalence of obesity increased in both genders, while the prevalence of over- weight remained relatively constant (Table 1). After adjusting for age group and educational level, BMI increased from 24.5 ± 0.1 kg/m2 (adjusted mean ± stan- dard error) in 1995-6 to 25.1 ± 0.1 kg/m2 in 2005-6 (25.1 ± 0.1 to 25.7 ± 0.1 kg/m2 in men and 23.9 ± 0.1 to 24.6 ± 0.1 kg/m2 in women). Prevalence of overweight and obesity increased with age in both genders, but while obesity levels increased with time in almost all age groups, overweight levels remained relatively stable (Table 2). The prevalence of overweight and obesity was also higher among subjects with primary education, while subjects with secondary or university levels had rather comparable rates (Table 3). In men, the age-adjusted increases in mean BMI (expressed in kg/m2/year and 95% confidence interval) were 0.073 (0.062, 0.084), 0.016 (0.000, 0.031) and 0.073 (0.049, 0.098) in subjects with primary, secondary and university levels, respectively; the corresponding values in women were 0.085 (0.073, 0.097), 0.052 (0.035, 0.069) and 0.062 (0.038, 0.084). Obesity rates increased with time in all educational groups, while overweight levels Page 3 of 8 increased mostly if not exclusively in the better educated subjects (Table 3). Relative to 1995-6, in 2005-6 obesity rates increased by 48%, 41% and 59% in men and by 40%, 75% and 177% in women with primary, secondary and university levels, respectively. The corresponding values for overweight were 6%, 1% and 23% in men and 5%, 7% and 65% in women. Multivariate logistic regression showed that the likeli- hood of presenting with obesity was significantly higher in 2005-6 compared to 1998-9 and 1995-6 in both gen- ders. After adjusting for survey year, and age group, men with primary or secondary education had a signifi- cantly higher likelihood of presenting with obesity (1.79 and 1.30 fold higher likelihood, respectively) than sub- jects with university level. Women with primary or sec- ondary education also had a higher likelihood of presenting with obesity (3.80 and 1.76 fold higher likeli- hood, respectively) than women with university level (Table 4). Similar findings were obtained when the ana- lysis was applied to overweight, i.e. a higher likelihood of presenting with overweight among lesser educated subjects (Table 5). Discussion Overall, our results indicate that between 1995 and 2005, obesity increased while overweight remained stable in the adult Portuguese population. These find- ings are partly in agreement with previous studies [1,2]. This selective increase in obesity levels is of concern, as the impact of obesity on disease is stronger than for overweight. Our results thus suggest that strong preven- tive measures are necessary to curb this worrying trend, and it will be of interest to know the final results of the National Plan to Fight Obesity (Programa Nacional de Luta contra a Obesidade), which ended in 2010 [5,6]. In the 2005-6 survey, more than half of the Portu- guese population aged over 18 was overweight or obese, the rates being higher in men than in women. These values are in agreement with previously published stu- dies for the Portuguese population which used objec- tively measured data. For instance, the uncorrected prevalence of obesity among subjects aged 18-64 in the 2005-6 survey was 14.0%, close to the 14.2% reported by a national study conducted in 2003-5 within the same age group [2]. Correcting for underestimation due to self-report led to even higher overweight and obesity levels, with almost two thirds (63%) of the Portuguese population being overweight or obese, and over one fifth (21.3%) being obese (Table 5), a figure identical to the prevalence of obesity reported in another study which also used measured data [3]. Hence, the overall (un)corrected prevalence of obesity reported in this study appear to be in agreement with the results from studies using objectively measured data. Although self-