To comprehensively determine factors affecting the progression of Type 2 diabetes and the recovery of NGT from IFG and/or IGT, we conducted a study in which we prospectively investigated Japanese workers with IFG and/or IGT, who are at high risk of developing diabetes [14
]. We found that, in addition to FPG levels, social and psychological factors such as status at work (night duty), stress in daily life and social position (administrative position) are also independent risk factors for the progression to DM from IFG and/or IGT, while being a white-collar worker and a non-smoker are factors associated with the recovery of NGT.
In our study, the rate of development of overt diabetes from IFG and/or IGT was equal to or relatively low (28.1%) and the rate (30.5%) of reversal of NGT was relatively high, compared with other reports [14
]. There are several possible reasons for these differences: (i) participants were educated about dietary and exercise therapy once or twice a year by dieticians and medical doctors; (ii) participants were non-obese (mean BMI less than 25.0 kg/m2
); and (iii) the DM group gained only a small amount of weight (less than 1 kg over 3.2 years), whereas the recovered group lost a small amount of weight, approximately 0.1 kg (data not shown). These factors might reduce the development of diabetes.
Furthermore, known risk factors such as obesity and elevated liver enzyme levels [1
] were not found to be risk factors for the development of diabetes from IFG and/or IGT in our study, probably because the majority of participants analysed were not obese and they were already at the highest risk; i.e. they had IFG and/or IGT. Among participants with IFG and/or IGT at baseline, those with higher FPG levels (6.1–6.9 mmol/l) might have greater impairment of insulin secretion than those with lower FPG levels, which were associated with the recovery of NGT from IFG and/or IGT.
The social and psychological factors, such as night duty, stress in daily life and administrative position, resulted in the progression from IFG and/or IGT to overt diabetes, possibly as a result of increasing insulin resistance. In our study, night duty was the highest risk factor for progression to diabetes. Subjects who are on night duty experience sleep disorders, which are known to affect the sympathetic nervous system and to be associated with impaired glucose tolerance [24
]. In addition, short sleep duration is a risk factor for developing diabetes, independent of confounding factors [25
]. Furthermore, stress in daily life and status at work (administrative position) were risk factors for the development of diabetes. Persons in administrative positions often experience both physical and mental stress [26
]. In our case, because they were middle managers, they may have experienced strong job strain and stress as a result of high job demands combined with low job decision latitude and effort–reward imbalance, factors which are associated with Type 2 diabetes [7
] and cardiovascular diseases [27
It has been proposed that stress activates the hypothalamo–pituitary–adrenal axis and the central sympathetic system and leads to the development of endocrine perturbation and obesity, which increases insulin resistance, causing Type 2 diabetes [30
]. In addition, increased levels of stress hormones such as catecholamines and glucocorticoids may impair insulin secretion [32
]. Furthermore, stress may induce pro-inflammatory cytokines [33
] and DNA damage [34
], factors which are related to Type 2 diabetes.
Interestingly, approximately one-third (30.5%) of participants with IFG and/or IGT at baseline returned to NGT. A multivariate analysis indicated that baseline factors related to improvement of glucose intolerance also included social factors (white-collar worker) and lifestyle (non-smoking), as well as lower levels of FPG (6.1–6.9 mmol/ll) and serum ALT levels. Smoking is a risk factor for IGT and Type 2 diabetes [35
], while serum ALT level is a risk factor for Type 2 diabetes [3
]. Therefore, non-smoking and low serum ALT levels may be associated with a return to NGT from IFG and/or IGT.
Although this is one of the only studies to prospectively examine relationships between psychosocial factors and risk of diabetes, further study is necessary. One limitation of this study is that many variables were examined with only a small sample size.
In summary, the results of the present study indicate that, in addition to glucose levels, social and psychological factors also affect progression to Type 2 diabetes or recovery of NGT from IFG and/or IGT in Japanese workers and social and psychological interventions may need to be considered to prevent the development of Type 2 diabetes in those at high risk.