Socio-economic position and risk of T2DM
The risk of type 2 diabetes (T2DM) increases with decreasing socioeconomic position. Hence, people with low educational level, low income level, low occupational status or people with adverse lifecourse socioeconomic conditions have higher risk of developing T2DM than people with higher socioeconomic position. The relationship is poorly understood; however, health behaviour, contextual factors and psychosocial stress partly explain the association between socioeconomic position and T2DM.
Socioeconomic position (SEP) refers to the social position of an individual in relation to the society in which she or he lives. The relationship between SEP and health is highly complex and involves personal factors such as health behaviour and structural factors including access to health care and quality of care, access to healthy food, opportunities for exercise, infrastructure and community characteristics.
Socioeconomic position has no direct biological connection to disease, although its relation to health is mediated by other factors that have biological consequences, such smoking, overweight and physical inactivity. SEP cannot be measured directly and individual-based measures of educational level, income level and occupational status are therefore often used as indicators of SEP. These different indicators are correlated but they do not express the same features and cannot therefore be used interchangeably.
Education is a frequently used indicator as it is easy to assess and the measures are regarded as valid. Education is established early in life and reflects therefor also the person’s SEP during childhood as well as the parent’s SEP4. Since education is a stable factor, it is not affected by poor health in adulthood. Furthermore, people with a high educational level may be expected to understand and interpret preventive messages, and thus more able to change behaviour and to benefit from the health care system.
Income indicates social standing and the potential to access such things as healthy food and good living conditions. Income can therefore influence health, just as health can influence income, which is why there is a risk of reverse causality. Occupation reflects social standing, income and the individual’s connection to the labour force. Consequently, there is also the possibility of reversal causality, since occupational status may reflect a person’s health. Over and above these individual indicators, broader contextual factors such as social infrastructure are important for health outcomes. Such influences include neighbourhood, access to health care, educational opportunities, social network, and the wider social environment.
Most SEP indicators are measured only at just one point in life, although life-course factors including childhood socio-economic position are considered to be important health indicators , as these also take into account early life exposures relevant to the development of non-communicable diseases.
Socioeconomic position and risk of T2DM
The risk of T2DM is higher among people with low SEP in high-, middle-, and low-income countries. This relationship is consistently found in high-income countries, although more studies are needed in low- and middle-income countries. In a systematic review which included 23 studies from high-, middle-, and low-income countries, Agardh found a 41% higher risk for T2DM among people with low SEP compared to people with higher SEP. Similar results were found by Espelt in the Survey of Health Aging and Retirement in Europe among people aged 50 years and over. Likewise, the InterAct project found a 67% higher risk of T2DM among men with low educational level and 88% higher risk of T2DM among women with low educational level in a multicentre study including 8 European countries.
Factors explaining the relationship between socioeconomic position and T2DM
The excess risk of T2DM among people with lower SEP is poorly understood, although SEP may contribute to the development of diabetes through such factors as health behaviour, access to health care, process of care and psychosocial stress.
Lifestyle factors, socioeconomic position and T2DM
Well-known risk factors as obesity, physical inactivity, smoking are all found to be more commonly present among people with low socioeconomic position, as compared to people with higher SEP. Various potential risk factors for developing T2DM have been examined, and BMI was found to mediate the relationship between SEP and T2DM and to explain part of the relationship – especially among women. Thus, Espelt found obesity to explain 23% and 16% of the inequality in the prevalence and the incidence of T2DM in women, respectively. The InterAct project found likewise BMI to explain part of the relationship between educational level and T2DM while risk factors as smoking, physical activity and diet could not explain the excess risk of T2DM among people with low educational level.
Psychosocial stress, socioeconomic position and T2DM
Lifestyle factors cannot however fully account for the increased risk of T2DM among people with low SEP. There appears to be higher vulnerability to adverse health outcomes among people with low SEP, in particular higher vulnerability to T2DM among the deprived in the younger age groups.
People living in poor and unsafe conditions are also more exposed to psychosocial stress. Several pathways are suggested to link psychosocial stress to T2DM. Thus (1) psychosocial stress might affect inflammation through the hypothalamic-pituitary adrenal (HPA) axis which is associated with dysregulation of cortisol. Dysfunction of this axis is associated with obesity and T2DM. Likewise, (2) living with psychosocial stress may lead to adverse health behaviours e.g. sedentary lifestyle, smoking, increased alcohol consumption and intake of unhealthy food which in return will increase the risk of T2DM.
Few studies have examined psychosocial stress among people with low SEP. In an 18-year follow-up of the Whitehall II study chronic inflammation was found to explain a quarter of the excessed risk for T2DM among people with cumulative socioeconomic adversity over the lifecourse and a third of the excess risk for T2DM among people with low SEP. This study was performed in a selected population which is why the authors call for more studies to confirm the findings.
Gender difference in the relationship between SEP and T2DM
In general, the relationship between SEP and T2DM was stronger among women than among men and some studies found social inequality in incidence of T2DM only among women. This gender difference might partly be because women with low SEP are more likely to be overweight, to have a sedentary lifestyle, and to experience psychosocial stress.
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