Obesity has become one of the most serious public health challenges of modern societies [1–4]. Excess body fat essentially results from a long-term positive energy balance, which in turn arises from an interaction between genetic predisposition and environmental factors [4, 5]. Among the latter, socioeconomic factors are particularly important because they can influence a variety of more proximal factors in the causal chain such as dietary intake, physical activity, and psychosocial characteristics [6]. Income is the indicator of socioeconomic status (SES) that most directly measures the material resources component [7]. Three main mechanisms through which income may affect body fat accumulation have been proposed: (1) Low-income groups preferably purchase low-cost food, which is typically characterised by high energy density, leading to chronic overconsumption of energy [8–12]. (2) Low income sorts people into deprived neighbourhoods with fewer opportunities for healthy diets and physical activity [8, 9, 11–13]. (3) Economic deprivation is related to psychosocial traits that favour increased energy intake in energy-rich environments, such as low levels of self-esteem, self-control, and social support, as well as social anxiety [9, 11, 12, 14, 15]. Against this background, obesity has been postulated to be “the toxic consequence of economic insecurity and a failing economic environment” [8].
In contrast to this hypothesis, income was frequently not related to obesity measures in adult populations of highly developed economies (in 45% and 61% of the reviewed studies in women and men, respectively) [16]. Observed associations were mainly inverse in women (in 88% of the studies), but more frequently positive in men (in 63% of the studies). In the European Union as a whole, obesity was found to be concentrated among the less affluent, although the inequality was low in magnitude [17]. It was again observed more consistently in women than in men. A small number of studies has investigated associations of income with obesity in the German general population [18–22]. Inverse associations were found in all of them. In line with the observations in other high-income countries, relations were seen more often in women (in all studies) than in men (in two of the five studies).
Despite numerous studies on the income-obesity association conducted so far, some methodological and content issues warrant a more thorough investigation:
Most studies relied only on body mass index (BMI), which is weight divided by height (kg/m2), as a measure of obesity. BMI is strongly correlated with whole-body fat mass [23], yet it is questioned that it adequately captures abdominal fat, which may be most relevant for the risk of cardiovascular and metabolic diseases [24]. Therefore, the need to consider other indicators in addition to BMI in the context of income-related inequalities in obesity has been emphasised [25, 26].
Usually household income is measured and adjusted for household size, resulting in the net equivalised income (NEI) [7]. For data analysis, quantiles based on the NEI distribution in the study sample are frequently computed. Alternatively, NEI can be categorised based on the distribution in the study region. Individuals in households with a NEI of less than 60% of the regional median NEI are commonly considered at risk of poverty, while those with more than 150% of the median NEI are classified as affluent [27–29]. Extreme income groups relative to the regional income distribution are not well characterised in terms of obesity by the available literature.
Other SES indicators, particularly education and occupation, are strongly related to both body size [16] and income [7]. The resulting confounding of the associations of income with obesity measures has not been thoroughly examined in the literature.
The relevance of income for obesity is likely to vary by population group. The direction of the relationship is known to differ by the economic development of a country [16], but also within countries of an economic region [17]. In East Germany, income inequality is a quite new phenomenon due to the egalitarian income policy of the former German Democratic Republic. Variation of the income-obesity association within Germany has rarely been studied, and an only slightly weaker relation in the East German than in the West German population was observed [18]. As outlined above, while an inverse association of income with body size is well established among women in western societies, the relation is less clear in men. Further, the meaning of current income may be most sensitive during the prime earning years, yet for older adults a less reliable indicator of their true SES [7]. In the few studies exploring this issue, income inequality in obesity varied inconsistently by age [17, 21]. Lastly, education may protect against the obesogenic effects of income disparities through knowledge acquisition and social capital [7]. However, epidemiological evidence for education being a modifier of the income-obesity association is scarce [30, 31].
Our investigation aimed at characterising income as a possible cause of obesity in an East German general population. We used cross-sectional data to examine: (1) Is income related to obesity measures in the adult population of the city of Leipzig? (2) What is the impact of other SES indicators on the observed associations? (3) Do sociodemographic factors modify the associations of income with body fat measures? We addressed existing research deficits by (a) focusing on an East German population, (b) considering not only BMI but additionally waist circumference (WC) as an indicator of the metabolically probably most relevant body fat distribution, (c) describing societally relevant income groups in terms of the distribution of obesity measures, (d) adjusting for confounding by other status indicators, namely education, occupation, and employment, and (e) evaluating the relevance of income for body fatness in as yet insufficiently characterised population groups: women versus (vs.) men, persons of working age vs. older persons, people with higher vs. lower educational level.