Marques-Vidal et al. BMC Public Health 2011, 11:772 http://www.biomedcentral.com/1471-2458/11/772 Page 6 of 8 Table 3 Prevalence of overweight and obesity, by survey year, sex and educational group 1995-6 42.9 (42.0 - 43.7) 35.4 (33.9 - 36.9) 31.1 (28.5 - 33.7) 11.8 (11.2 - 12.3) 6.6 (5.8 - 7.3) 6.1 (4.8 - 7.5) 36.9 (36.2 - 37.7) 20.1 (18.7 - 21.4) 11.6 (9.9 - 13.2) 15.3 (14.7 - 15.9) 4.8 (4.1 - 5.5) 2.2 (1.4 - 2.9) 1998-9 45.7 (44.8 - 46.6) 36.4 (35.0 - 37.8) 34.4 (31.9 - 36.9) 13.6 (13.0 - 14.2) 7.6 (6.8 - 8.3) 5.0 (3.8 - 6.1) 37.8 (37.0 - 38.5) 20.3 (19.1 - 21.5) 14.8 (13.2 - 16.4) 17.8 (17.2 - 18.4) 5.6 (4.9 - 6.3) 3.1 (2.3 - 3.8) 2005-6 45.5 (44.4 - 46.7) 35.6 (34.1 - 37.1) 38.3 (35.6 - 41.1) 17.5 (16.6 - 18.3) 9.3 (8.4 - 10.2) 9.7 (8.0 - 11.3) 38.7 (37.7 - 39.8) 21.6 (20.3 - 22.9) 19.1 (17.2 - 21.1) 21.4 (20.5 - 22.3) 8.4 (7.4 - 9.3) 6.1 (4.9 - 7.2) Gender/age group Men Overweight (%) Primary Secondary University Obesity (%) Primary Secondary University Women Overweight (%) Primary Secondary University Obesity (%) Primary Secondary University Survey year 1995-6 1998-9 2005-6 Educational level (%) Primary Secondary University Age group (%) 18-24 25-34 35-44 45-54 55-64 65-74 75+ Overweight was defined as a body mass index (BMI) ≥25 and <30 kg/m2; obesity was defined as a BMI ≥30 kg/m2. Results are expressed as percentage and (95% confidence interval). the presence of an “educational obesity gap” in the Por- tuguese population, but not it’s widening. Several rea- sons for the similar increase in BMI levels among subjects with basic and university education can be pro- posed. For instance, Portuguese subjects with a univer- sity degree tend to exercise less [31] and to remain sit longer [32] than lower educated subjects; subjects with Table 4 Multivariate analysis of the determinants of obesity in the Portuguese population, by gender Men 1 (ref.) 1.15 [1.07 - 1.23] 1.51 [1.41 - 1.62] 1.79 [1.57 - 2.05] 1.30 [1.13 - 1.50] 1 (ref.) 1 (ref.) 2.43 [2.05 - 2.88] 4.11 [3.50 - 4.82] 5.30 [4.53 - 6.21] 5.67 [4.84 - 6.66] 4.75 [4.04 - 5.59] 2.97 [2.48 - 3.56] Women 1 (ref.) 1.18 [1.11 - 1.25] 1.49 [1.40 - 1.59] 3.80 [3.25 - 4.43] 1.76 [1.49 - 2.09] 1 (ref.) 1 (ref.) 2.40 [1.98 - 2.91] 4.19 [3.49 - 5.03] 6.41 [5.35 - 7.68] 6.37 [5.31 - 7.63] 5.56 [4.63 - 6.68] 3.78 [3.13 - 4.57] Statistical analysis by logistic regression. Results are expressed as Odds-ratio and [95% confidence interval]. The results for each variable are adjusted for the other two. Obesity was defined as a body mass index ≥30 kg/m 2 . higher education also tend to have a lower quality diet [33] with a higher fat content [34]. Conversely, lower educated subjects are less likely to engage in weight control, to consider obesity as a disease and to consider weight loss as a health priority [15,16]. Hence, and con- trary to what is observed in other countries [35,36], it seems that the highly educated Portuguese tend to adhere to unhealthy lifestyles leading to increased obe- sity rates. This strong increase in overweight prevalence rates among subjects with university calls for preventive measures, as it is possible that, in a not-so-far future, the increase in obesity levels will further accelerate in this group. The lower increase in BMI and prevalence rates among subjects with secondary education was unexpected and the reasons for such a difference await further investigation. The major advantage of this study is that it is based in nationally representative samples, and that the data has been collected using the same methodology throughout time, thus enabling assessing trends with confidence. Still, this study also has several limitations worth men- tioning. Namely, height and weight were self-reported, leading to an underestimation of obesity prevalence. Although we used a correction method, still the best issue would be to conduct a nationwide examination survey, which unfortunately is not programmed for the next years. Further, selective underreporting of weight and over reporting of height are influenced by actual body size as well as sociodemographic characteristics [37]. Therefore, it is possible that bias increases over