The current research shows the presence of socioeconomic inequalities in obesity and sedentary lifestyles during the 2006-2017 period in Spain. Obesity and sedentary lifestyle were mainly concentrated in disadvantaged social classes and lower educational levels.
Gender differences
Regarding the differences between male and female participants observed in our study, previous research has found that gender is an important factor that may help us to explain this variability [34]. For example, it has been suggested that differences in obesity between men and women may emerge not only through sexually dimorphic physiology (e.g., during and after menopause, the drop in estrogen leads to an overall increase in total body fat) but also through different forms of discrimination against women [35].
We believe that our results are consistent with other studies conducted in Spain, which show that women report worse health than men [36], especially after the economic crisis of 2008 [37, 38] . We find in our research that health inequalities for men are greater for social class than for educational level. Meanwhile, for women these differences are greater for education than for social class. Studies conducted in other European countries, like Sweden, present results in line with our research [39].
Practical implications
Obesity is a complex and multifactorial problem [40], and although sedentary lifestyle plays an important role in the development of this clinical condition [41], other social determinants have also been described to influence the apparition and maintenance of this disease [42]. We believe it is important, from a Public Health perspective [43], to try to help social disadvantaged groups with greater risk to obesity [44]. For this reason, we think that encouraging social policies oriented to income redistribution may be an effective strategy in the fight against the obesity epidemic [45]. In addition, economic interventions like taxes (e.g. surcharge placed upon fattening food and beverages) may shift the relative prices of unhealthy foods to nudge consumers toward healthier options [46].
We believe that an outcome-wide epidemiology approach is necessary to show an accurate picture of health inequalities, as previous investigations have suggested [47], because some exposures may influence different outcomes heterogeneously in beneficial or harmful ways [48]. Considering that few studies to date have used this outcome-wide perspective, we think that our research is especially relevant in the current situation of rising inequalities after the COVID pandemic crisis [49].
Future research
It is important to remark that our results were obtained from cross-sectional data in three different time periods (before, during, and after the 2008 recession). Longer longitudinal studies would be necessary to truly understand the evolution of these health inequalities, especially considering that it usually takes more than 10 years to observe the consequences of social inequalities in health after an economic crisis [50].
Strengths and limits
This research is the first documented work, to the extent of our knowledge, that uses an outcome-wide epidemiology approach to analyze socioeconomic inequalities in obesity in Spain. However, it is not without limitations. Firstly, it has been carried out using cross-sectional data only. Nonetheless, NHSs from 2006 to 2017 were used to investigate trends between men and women in the socioeconomic patterns of obesity. Secondly, results were based on self-reported data. However, previous studies comparing self-reported data with administrative data show similar results [51]. Finally, it is important to remark that minority groups (e.g., immigrants) do not usually participate in this type of surveys [52]. Despite all these limitations, we believe that the NHS have proven their worth for epidemiological studies as they provide large sample sizes with a low non-response rate.