Discussion
The main finding in this study was that a 12-month lifestyle intervention, including high levels of exercise, significantly improved physical HRQoL in persons with T2D (diagnosis <10 years) compared with standard care. Specifically, 30% of the participants in the U-TURN group compared with 18% in standard care group achieved a change in score considered to represent a clinically significant improvement in regard to physical HRQoL. Despite the improvement in physical HRQoL, mental HRQoL, SWB, and mood were not significantly improved by the intervention.
We hypothesized that the U-TURN intervention would improve HRQoL and SWB. We based this hypothesis on the fact that the U-TURN participants achieved significant reductions in the need for glucose-lowering medications, with more than half of participants being discontinued from medications during the intervention.18 In addition, participants also managed to maintain glycemic control, improve fitness, and reduce several cardiovascular risk factors.18 43 These improvements are potentially important in relation to mental health for several reasons. First, glucose-lowering mediation has been shown to interfere with normal life44 because it is associated with discomfort45 and decreased QoL.46 Second, depression is prevalent in persons with T2D,47 and lifestyle interventions have been shown to improve signs of depression in persons with T2D10 and the healthy population.48 49 Third, epidemiological evidence suggests that an increased level of physical activity and a healthier diet are associated with better mental health; however, the (causal) effect is likely being bidirectional.50 However, the failure of the U-TURN intervention to improve mental outcomes is in line with several previous studies that have demonstrated that lifestyle interventions positively influence physical HRQoL in persons with T2D but do not provide improvements in the mental component of HRQoL. In the Diabetes Aerobic and Resistance Exercise trial, a combined exercise group did not show improvements in physical HRQoL compared with controls. The mental HRQoL was not significantly altered; however, due to deterioration in the controls the between-group difference was significant.51 Also, in the Health Benefits of Aerobic and Resistance Training in Individuals with Type 2 Diabetes study, a randomized 9-month exercise intervention with a control and three different exercise groups (aerobic, resistance and aerobic plus resistance training), it was reported that every intervention group demonstrated greater improvements in physical HRQoL compared with controls, but that the changes in mental HRQoL did not differ significantly between any of the intervention groups and controls at 9-month follow-up.52
The longest running study to date, the Look AHEAD study, evaluated the effect of a lifestyle intervention including healthy eating and physical activity aimed at achieving weight loss. During the 8-year follow-up period, physical HRQoL declined in the intervention group as well as in the control group. However, the decline was significantly greater in controls who received standard care.10 These results align with our findings. In our study, the between-group difference in the PCS was partly due to the decline in the PCS in the standard care group, contrasting with the positive change observed in the intervention group. In support of our results, the Look AHEAD study did not find significant differences between the groups in mental HRQoL at any time during the 8-year follow-up period.10 The IDES study demonstrated that higher levels of exercise volume increased physical HRQoL; however, the levels of physical activity had to exceed current recommended levels to reach significance.14 In contrast to our results as well as previous studies, the IDES study also showed an improvement in mental HRQoL at every level of physical activity in the intervention group after 12 months of intervention.14 This discrepancy could be explained by the fact that the IDES study included participants treated with insulin, and persons with a longer average duration of disease, both of which are indicative of more severe disease states. Both factors have been associated with worse HRQoL.53 In addition, the baseline levels of mental HRQoL in the IDES study are the lowest of all the studies discussed thus far, and these low baseline scores could explain why the IDES study found improvements in the mental HRQoL, while other studies did not. In other words, the low baseline scores could offer greater room for improvement. In contrast, our study participants had relatively high baseline mental HRQoL scores, the highest among studies discussed here. This difference may reflect a healthier volunteer bias, which would result in the inclusion of a selected subgroup of persons with good mental health at baseline. Although this possible explanation is speculative, the U-TURN intervention was intensive and required participants to allocate significant amounts of time and effort to the program. In addition, participants in the present study had relatively well-regulated glycemic control at baseline and had no severe diabetes-related complications known to be associated with a strong negative impact on HRQoL.54 55 Interestingly, while T2D has been associated with lower levels of HRQoL compared with the background population,56 57 persons without macrovascular complications appear to have an HRQoL that is relatively unaffected by the disease, and even persons with macrovascular complications appear to experience only small decreases in HRQoL.54 Another explanation for why mental health did not improve in this study could be that the intensive nature of the intervention blunted potential positive effects on mental health, as the efforts required to comply with the intervention may have resulted in a reduction in the mental HRQoL. The real-world acceptance and adherence to the intensive exercise program may differ across countries, as, for example, in Denmark, 29% are not adherent to the recommendations for physical activity for health compared with 37% in other high-income Western countries.58 However, since our results align with similar studies discussed here that prescribed lower levels of exercise and less intensive interventions, it is unlikely that the intensive training level obscured an otherwise positive effect on mental HRQoL.
Although speculative, it is possible that lifestyle interventions, in particular increased levels of physical activity and exercise, are effective in improving low affective states, but as our study and the other high-quality studies discussed here show, mental HRQoL and SWB may be relatively resistant to improvement in persons with T2D without major diabetes-related complications or depression.
In healthy populations, lifestyle interventions have demonstrated no or only modest effects on HRQoL.48 59 It can be hypothesized that unless an individual is starting with suboptimal mental health, mental health improvements are difficult to achieve due to ceiling effects. This hypothesis is supported by the theory that SWB is under the control of a homeostatic-like mechanism, fitness,60 that is resistant to long-term changes both in a positive and negative direction.61 SWB is characterized by two unique features: it has a natural positive offset and is highly stable over time.60 It has been proposed that individuals have a set-point range for SWB that is maintained through a psychological homeostasis system that draws on internal and external factors to maintain a stable level of SWB when challenges arise and overtime.60 This theory implies that in the absence of pathologies such as depression, long-term positive effects of lifestyle interventions on mental health are likely going to be absent.
There are some limitations to our study. First, the measurement instruments used to assess mental health are generic, and these scales may not be sensitive enough to detect and quantify small changes that are important to persons with T2D or subjective changes specifically related to exercise behavior. Second, exploring mental health was not the primary outcome of the U-TURN study and all findings have explorative character. Furthermore, participants in the present study had high baseline HRQoL which may have limited our ability to measure improvements in mental health. Third, the sample size might be underpowered to detect lower effect sizes. Fourth, the diversity in our trial population is limited, so further research should confirm our findings in populations of other ethnicities and/or socioeconomic backgrounds. Finally, due to the medical titration before baseline, participants were well controlled, making generalizability to less well-regulated persons questionable.