Step 1: socioeconomic disadvantage causes psychological and emotional distress
There is a wealth of evidence linking socioeconomic disadvantage with disease, including weight gain and obesity, as well as mental health problems, poor self-rated health and mortality [16, 17]. The effects of socioeconomic factors on mental health problems are particularly noteworthy, with a more or less linear association between income inequality and mood disorders such as depression and anxiety [18-20]. As an illustration, a UK study found that a poor self-rated health was 5.5 times more common at age 50 in individuals born in the lowest socioeconomic quintile compared to the top quintile . The adverse health effects of socioeconomic disadvantage are also found in children [22, 23].
There is clear evidence linking downward social mobility, e.g. by immigrating from a poor to a rich country, to depression and other mental health problems [24, 25]. Another marker of socioeconomic disadvantage is unemployment, which is also strongly associated with numerous adverse health outcomes, including increased mortality . Unemployed adults report 2.5–3 times higher rates of self-reported poor health in cross-country studies . The effect was also more pronounced in countries with less welfare .
Longitudinal data from the Whitehall II study found that job insecurity was strongly associated with poor self-rated health and depression . Data from the United States showed that adults with insecure employment were more likely to report minor depression (odds ratio [OR]: 6.8) and anxiety attacks (OR: 3.7) than secure workers, after adjustment for multiple confounding variables . Other studies show similar associations between insecure employment and mental health problems in Japan .
Families suffering from outright poverty have been found to eat more processed junk food with high sugar and fat content, but minimal nutritional value [31, 32]. Interestingly, there is a significant association between the price of a Big Mac, as a global indicator of junk food prices, and prevalence of obesity .
Having a low education is associated with an increased risk of low self-esteem , a major protective factor against poor mental health and strongly related to obesity . Likewise, living in a slum or otherwise deprived neighbourhood predicted poor self-rated health, inflammation, obesity and the metabolic syndrome [36-38], as was living in a neighbourhood with high racial segregation .
Another effect of socioeconomic disadvantage is an increased risk of persistent negative emotions (emotions comprise affective, cognitive and behavioural traits that tend to cluster) . Negative emotions are further characterized by their inappropriateness, frequency, intensity and duration, e.g. anger, depression, anxiety, anger, hostility and hopelessness, and are closely linked with a negative self-belief and self-doubt . Negative emotions have been identified as an important link between socioeconomic adversity and a coronary heart disease [40, 41], and socioeconomic disadvantage has been found to greatly increase risk of negative emotions such as anger, life dissatisfaction and hopelessness [40-42].
Step 2: a disharmonious family environment
The distress originally caused by socioeconomic disadvantage is thereafter transferred to the family (Fig. 1) and increases the risk of a disharmonious family environment, characterized by externalization of psychological distress and frustrations, marital discord, negative belief systems and pervading pessimism, inter-parental violence, lack of family cohesion and support, neglect, abuse, parental addiction, excessively harsh or disinterested upbringing methods, and food insecurity [43, 44].
Figure 1. Proposed step-by-step model of obesity causation. Although the figure only shows reverse causality in step 6, all steps in the model are likely to be more or less bidirectional, especially once severe obesity has been established. Skipping of intermediary steps can also occur, e.g. in adult-onset obesity.
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Starting a family and becoming a parent is a major life-changing event that drastically elevates demands and responsibilities, not least financially. Parenting is associated with stress and depression in both mother and fathers, especially when there is pre-existing psychopathology [45-47]. Interpersonal risk factors for increased family distress include parents with pre-existing low self-esteem, low education, addiction and relationship strain, i.e. common results of socioeconomic disadvantage [48-50].
Belonging to a certain ethnic group also appears to play an important part in determining stress levels for parents, and the creation of the disharmonious family environment, particularly in the United States through its marked structural disadvantages, segregation and inequality, with African-American, Hispanics and Asians experiencing significantly higher stress levels when starting a family than Caucasian . A lower community cohesion is likewise linked to increased stress levels for new parents .
Step 3: psychological and emotional distress in the offspring
A disharmonious family environment and parental mental health problems have been associated with a range of negative psychological and emotional outcomes in the offspring, including eating disorders, addiction, violence, heightened sensitivity to stress, poor school performance, negative emotions, behavioural problems, low self-esteem, low self-worth, lower education, a lack of confidence, sleeping problems, eating disorders, anxiety and depression [52-62].
Children also tend to adopt parental values and beliefs. If parental values are mainly negative, e.g. excessive hopelessness and pessimism as a result of socioeconomic disadvantage, the child is likely to adopt the same negative values, thereby creating a negative mindset and the perception that change for the better is not possible .
Evidence suggests that a disharmonious family environment is particularly damaging for children . Firstly, children have not yet built up sufficient protective mechanisms, such as resilience, coping skills or self-esteem. Secondly, children also tend to internalize (negative emotions, depression, anxiety) parent's frustrations, thereby promoting further psychological and emotional distress . Externalization (aggressiveness, hyperactivity, behavioural problems) of parent psychological and emotional problems is also common [57, 60]. Thirdly, childhood is a very intensive developmental period, meaning that children are extra sensitive to disharmonious social environments [57, 60, 64]. Furthermore, earlier parts of the proposed causal chain, such as family socioeconomic disadvantage, have also been found to increase risk of poor self-rated health, and emotional and behavioural problems in the children [65-69].
Step 4: psychological and emotional overload
Step 4 symbolizes the point where the individual, often a child, is not able to cope with the high amount of experienced psychological and emotional distress, since buffering mechanisms such as coping, resilience and external support are materially insufficient.
The psychological and emotional overload leads to maladaptive coping strategies (e.g. eating to suppress negative emotions and stress, eating disorders, passivity, addiction, increased stress sensitivity), over-reactive emotional responses to outside stressors, feeling insecure and a need for increased security [61, 65, 70-79].
There are also direct physiological perturbations in the energy homeostasis systems, possibly as a response to the increased need for security and survival . Stress levels are now much increased causing hypothalamic–pituitary–adrenal axis dysregulation and increased levels of cortisol, ghrelin, insulin and pro-inflammatory cytokines [80-84]. Stress, negative emotions and inflammation therefore appear to be important mediators between the psycho-emotional overload and disruption of energy homeostasis and metabolism [79, 81-83, 85-87].
Step 5: disruption of energy homeostasis and start of weight gain
The increased circulation of appetite stimulating hormones and peptides, such as ghrelin, cortisol and insulin, causes an increase in appetite and attraction to calorie-dense foods, with concomitant changes in eating behaviour and energy intake [82, 83, 86, 87].
Stress could also have adverse effects on energy expenditure, arguably the main physiological regulator of energy homeostasis and therefore body weight [88-90]. Interestingly, the resting energy expenditure per kilogram has been found to be much lower in the obese than normal weight , suggesting that the obese are indeed in calorie-saving mode, possibly as a result of stress (Habash, Fagundes, Kiecolt-Glaser et al., unpubl. data) and thereby increased ability to handle future insults. Stress also appears to induce other adverse metabolic effects such as reduced triglyceride clearance, which promotes fat storage .
Behavioural disturbances in energy homeostasis include emotional eating, i.e. eating to find relief from negative emotions and stress [79, 81, 83, 86, 93]. Food, and especially so-called comfort foods rich in fat and sugar, is a readily available form of self-medication from many types of negative emotions and stress [86, 94]. Sleep patterns and physical activity levels are also affected adversely by the more or less chronic exposure to stress, which will further increase metabolic disturbances and inflammation .
Possibly the largest adverse impact of psychological and emotional distress, particularly in children, is that the altered profile of hormones and metabolites (e.g. glucocorticoids) affecting the amygdala and hippocampus as a result of negative emotions, facilitate memory and formation of habits . Children in disharmonious families are therefore very likely to rapidly learn that eating comfort foods significantly reduce distress, which then starts to form a maladaptive habit. Emotions have a profound impact on habituation of behaviour through basal ganglia, with little interference from the conscious parts of the brain [83, 86].
Moreover, individuals with this type of maladaptive emotional conditioning can easily progress to using the same strategy after smaller stressors such as school-related problems, tiredness or body dissatisfaction, promoting a reflex-like automatic chain of events that the cognitive parts of the brain struggles to contain [83, 86]. Development of other maladaptive coping behaviours such as eating disorders can also occur .
A deliberate psychological reason behind disrupting energy homeostasis is the desire to induce obesity as protection from childhood abuse, particularly sexual abuse .
Step 6: manifested obesity and reverse causality
Once obesity has been established, there exist a large number of reverse causality situations. This includes the direct impact of obesity on mental health, social disadvantage, psychological and emotional distress, stress, inflammation and hyperinsulinemia, thereby creating a very potent negative circle. Although Fig. 1 only shows a bidirectional association for step 6, all steps in the model are likely to be more or less bidirectional, especially for severe obesity. There can also be causal processes that bypass intermediary steps in the model, e.g. where obesity (step 6) connects all the way back to socioeconomic disadvantage, and so on.
There are several other examples of reverse causality. Obese children have, e.g. been found to be twice as likely as normal weight children to develop mental health problems . Childhood obesity is also a major risk factor for developing low self-esteem [98, 99], social discrimination and bullying [74, 100, 101], and body dissatisfaction [73, 102]. Moreover, obesity makes it harder to move up the social ladder, and increases the risk of moving down the social ladder . Similarly, obese young men are less likely than normal weight to achieve higher education . In adults, obesity is associated with low self-esteem, body dissatisfaction and a pre-occupation with dieting, mainly due to childhood weight-related teasing and failure to live up to unrealistic body shape ideals .
In terms of physiology, adipose tissue is a major source of pro-inflammatory cytokines, which fuels pre-existing inflammation even further , leading to multiple knock-on effects such as cognitive impairment and depression . Other examples include obesity as a barrier to be physically active [107-109].