To our knowledge, this is the first study to identify characteristics associated with the disparity between objective and self-reported PA on a continuous scale. In this cohort of recently diagnosed T2DM patients from Eastern England, individuals with greater PA disparity were more likely to be male, be in younger age- and lower socio-economic categories and have a lower BMI. Furthermore, when examining the characteristics of those more likely to overestimate their PA (by the PAEE equivalent of 30 minutes of moderate activity per day), overestimators (19%, n = 80) were more likely to be in lower socio-economic categories. Objective PA levels were relatively low in these recently diagnosed T2DM patients. Indeed, compared with a representative sample of 10,000 healthy adults from ten European countries which used the same objective method of assessing PA — Interact Study — mean objective PAEE levels were 7.9 kJ · kg-1 · day-1 lower in our population of recently diagnosed T2DM patients (34.4 and 42.3 kJ · kg-1 · day, respectively). Differences in population characteristics may explain some of the between-study PAEE differences; for example, here the average age was 60.3 ± 7.5 years, compared to 53.8 ± 9.4 years in Interact. Nevertheless, given the benefits of PA for people with diabetes, these data highlight health promotion opportunities in terms of increasing PA levels.
The association between greater PA disparity and low BMI could be partly explained by the observation that those who underestimate their PA level have substantially higher objective PA levels compared to those who are PA overestimators or PA aware. In addition, leaner individuals may spend less time reflecting on their PA levels than more overweight individuals. Individuals with greater PA disparity were more likely to be male, potentially driven by higher male objective PA levels, coupled with gender differences in the ability to self-assess activity levels in the face of gender norms regarding PA. For example, social desirability/approval may influence female self-reported PA more than males. Several factors may contribute to the observation that PA disparity is greater in younger participants. Social approval is associated with PA underestimation  and this factor may be more important in younger age groups. Indeed, post-hoc analyses show that underestimation is most prevalent in the youngest age group (data not shown). In addition, our self-report PA measure may not have adequately captured the activity of this age group, also leading to PA underestimation. Occupational PA is a known key determinant of total PA . Thus, inadequate perception of occupational PA in people with routine/manual relative to managerial/professional occupations may help explain the association between socio-economic category and increased PA disparity. An alternative explanation could be that individuals in lower socio-economic categories may experience more difficulty in completing the questionnaire, resulting in higher PA disparity.
Previous studies assessing ‘PA awareness’ – the difference between an individual’s belief and measured attainment of PA guidelines – have predominantly relied on self-reported and self-rated assessments. We have extended previous work by incorporating an objective measure of PA, which likely reflects true PA more accurately than self-reported or self-rated PA. The different approaches used to classify PA awareness make it difficult to directly compare proportions and characteristics of PA overestimators and underestimators. Previous studies report a lower proportion of underestimators (ranged from 6.1 to 22.5%) and a slightly higher proportion of overestimators (ranged from 15% and 35%) compared with our study (67% and 19%, respectively), with the remaining individuals falling into two other awareness categories [11–14]. In this study, the proportions of individuals in the three awareness groups (over- and under-estimator and aware) did not differ by trial arm suggesting that the intervention was not responsible for the large percentage of under estimators. Reasons for the lower proportion of underestimators in prior studies include the fact that the self-reported PA questionnaire captures specific activities over four domains. Thus, inevitably there will be some activities people engage in that are not included in questionnaires and we expect lower total activity levels from these questionnaires, compared to complete 24-hour recall obtained from continuous wear of objective monitors which capture all activity. Furthermore, the method used here to convert self-reported PA into PAEE removes resting metabolic rate (RMR) to produce estimates that better reflect PA in its own right and not total energy expenditure . Studies which include RMR may produce inflated PA estimates. Despite methodological differences, in terms of the characteristics of overestimators, one previous study found individuals with a family history of T2DM are more likely to be overestimators if they are in lower socio-economic categories . An association between PA overestimation and lower BMI has been reported in healthy adults [11–13] and those at high risk of developing T2DM , but similar associations were not shown using PA disparity as an outcome. Further work combining the various awareness measures in one study would be useful.
Strengths and limitations
The strengths of this study include the use of an objective measure of PA and our derivation of a novel continuous measure of PA disparity based on converting objective and self-report measures to the same units. Our method of objectively assessing PA has been validated against indirect calorimetry during simulated daily activities [41–43] and during free-living against doubly labelled water  and likely reflects true PA more accurately than self-report. Use of an objective PA measure to classify PA disparity avoids issues with correlated error arising from use of two self-report measures. Discounting the basal metabolic rate from our self-reported PA data ensures self-reported PA estimates more accurately reflect objective PA. In England, GP registers typically cover ~99% of residents  and, as nearly half of GPs approached participated in ADDITION-Plus, participants were drawn from a large population-based sample ensuring generalisability to similar locations. Follow-up at one year was also high (93% of living patients).
One limitation of this study is the cross-sectional design which precludes the establishment of causality. Generalisability to ethnically diverse UK populations and/or more deprived areas may be limited due to the recruitment of GPs from a single geographical area, East Anglia England. Observed discrepancies between objective and self-reported PA could partly reflect differences in reference period (4 days for objective and one year for self-reported PA). However, results from sensitivity analyses do not support a major role of between-instrument differences in the time frames captured. Self-reported PA may be underestimated (e.g. if individuals engage in activities that are not listed on the questionnaire) or overestimated (e.g. if the energy costs of activities are overestimated). Indeed, it is possible that assigning MET values to contemporary occupational activities using historical literature overestimated occupational PA in those with routine/manual occupations, which could bias PA disparity with respect to social class (occupationally defined). Sensitivity analyses assigning lower occupational MET values to occupational classes did not alter our findings, suggesting this source of bias is likely to be minimal but cannot be ruled out. Similarly, the scaling of self-reported activity estimates to daily quantities of activity energy expenditure has its limitations, which may be exacerbated by significant proportions of time not accounted for.
What implications do these results have for PA behaviour change interventions? Firstly, given the relatively low level of PA in this sample of individuals with recently diagnosed T2DM, interventions aimed at increasing PA levels are needed and may help improve overall health as well as slow or prevent T2DM progression, as observed in individuals with impaired glucose tolerance [46–48]. Secondly, as high PA disparity (and poor PA awareness) can reduce the effectiveness of interventions aimed at changing PA intentions and behaviour , decreasing PA disparity may play an important role in promoting PA. The proportion of people overestimating their PA observed in this study suggests that improving people’s awareness of PA levels may form an important step in interventions aimed at increasing PA. Decreasing PA disparity can be achieved through self-monitoring of, and feedback on PA levels [50, 51]. Thirdly, focusing on helping individuals from lower socio-economic categories to become more aware of their PA may be one way of maximising the efficacy of PA interventions.