Younger age and diabetes detected through screening emerged as the most consistent predictors of dietary change over the year of study, and objective physical activity at one year. Our finding that younger patients engaged in higher levels of physical activity supports previous literature . Patients with screen-detected diabetes were diagnosed more recently than clinically diagnosed patients. The diagnosis may have acted as a cue to adopt healthier behaviours over the year of our study, whereas patients with clinically diagnosed diabetes may have faced challenges to maintain any changes in behaviour made soon after diagnosis. Our findings contribute to previous literature by suggesting the existence of a window of opportunity to facilitate behaviour change early in the disease trajectory.
There were significant improvements in self-reported PA in women, and in self-reported and objectively measured dietary variables in men and women over one year in this cohort of recently diagnosed diabetes patients. Intervention studies focusing on lifestyle change among people with recently diagnosed [7, 11] and established diabetes have also reported positive changes in self-reported PA and diet over 12 months .
There were significant associations between younger age, male sex, a smaller waist circumference, and lower systolic blood pressure values at baseline and higher PAEE levels at one year. These results are partially in agreement with other studies. A study among offspring of people with diabetes (mean age 40 years) identified male sex and a higher level of baseline fitness as predictors of change in objectively measured PA . A review of correlates of adults’ participation in self-reported PA showed that PA participation was consistently higher among men than women and was inversely associated with age . A cohort study revealed that adults (≥55 years) with a large waist circumference were more likely to be physically inactive than those with smaller waists . An association was also observed between baseline diastolic blood pressure and PAEE at one year, but this was of borderline statistical significance.
We did not find any significant baseline predictors of change in self-reported PA. Previous studies identified the following predictors of self-reported PA in patients with recently diagnosed diabetes: age , sex , BMI [11, 13], future-oriented thinking , proactive competence , and baseline exercise behaviour . These findings are similar to our results regarding objectively measured PA levels at one year, but not with change in self-reported PA. Differences may be due to length of follow-up (short-term predictors of change may differ from long-term predictors), the larger age ranges targeted by other interventions, and the difference in measurement method [12, 13].
Greater increases in plasma vitamin C levels were observed in women, which is consistent with other studies that have shown higher plasma vitamin C concentrations in women [32, 33]. We observed a greater change in plasma vitamin C levels and in fruit intake among patients with screen-detected diabetes than in clinically diagnosed patients. This might reflect a greater capacity for change in dietary behaviour in the first year after diagnosis among screen-detected patients, while clinically diagnosed patients have lived with the condition for longer and may have already made significant changes to their diet. While the difference in the increase in plasma vitamin C levels between screen- and clinically-diagnosed patients was small (6 μmol/l), the difference between the 25th and 75th percentiles was 24 μmol/l, suggesting that patients in the top quartile of plasma vitamin C consumed one more portion of fruit and vegetables per day compared with those in the bottom quartile. A similar difference was seen in self-reported fruit intake, where screen-detected diabetes patients increased on average 74 g/d (which equals a fresh apricot) compared with clinically diagnosed diabetes patients.
ADDITION-Plus participants who reported better health at baseline were also more likely to report increases in fruit intake. A cross-sectional study among US adults indicated that the odds of consuming ≥5 servings of fruits and vegetables per day were higher among those who rated their health as excellent/very good compared to those who rated their health as poor . We did not observe any significant associations between baseline predictors and change in self-reported vegetable intake in multivariable analysis.
Younger patients were more likely to report a greater reduction in fat and energy intake. Other studies among patients with recently diagnosed diabetes showed that only baseline dietary behaviour [11, 12] and proactive competence  predicted fat consumption at follow-up. Having a clinical diagnosis of diabetes was a predictor of greater decrease in fat intake (approx. 5 g/d which equals, for example, one teaspoon of mayonnaise) indicating a better management of fat intake among patients who lived with the condition for longer.
Intention, perceived behavioural control and beliefs about becoming more physically active and eating a low fat diet did not predict change in objectively measured or self-reported behaviours in the ADDITION-Plus cohort. These findings replicate results in a study by Thoolen et al.  in which exercise and diet intentions did not predict these behaviours at follow-up in patients with recently diagnosed diabetes. Lack of associations is unlikely to be due to lack of power as our sample size was large compared to most studies in this area. Instead it may be due to inaccurate or unrealistic beliefs at baseline when patients had limited experience with behaviour change, the one-year interval between measurement of beliefs and behaviour, or behaviour change through automatic routes (e.g. activation of previous goals) rather than reflective routes . Although previous research suggested that the TPB is useful for the prediction of health behaviours in the general population , studies of prediction of behaviour change using the TPB are few, and findings for people at risk for diabetes are inconsistent [37, 38]. Our results suggest that the TPB might not be a useful framework for predicting changes in health-related behaviours in individuals with recently diagnosed diabetes. Other psychological predictors also did not predict change in health-related behaviours in our cohort. This might be partially attributed to the low reliability of some measures (Cronbach’s alpha at baseline was 0.56 for treatment control) or common method variance . When a behaviour and its determinants are measured using the same method e.g. by self-report questionnaire, associations may be due, at least in part, to commonality in response patterns to these measures. When the behaviour is measured by a different method, e.g. objectively, part of the correlation explained by common method variance disappears, leading to lower or non-significant associations. Some studies have shown, for example, that psychological determinants of dietary behaviour predict self-rated fruit and vegetable intake (asking people to rate their own fruit and vegetable consumption) better than intake assessed by FFQ (general food intake questionnaire) [40, 41]. Finally, since it has been shown that environmental factors are significantly related to health behaviours , and that social support might be important in health behaviour change [28, 42], it would be desirable to incorporate these variables as correlates of behaviour change in future studies.
The present study has several strengths. ADDITION-Plus included objective and self-report measurement of two key health behaviours over 12 months in a well-defined group of patients at high cardiovascular risk who could benefit from positive changes in diet and PA. The use of an objective measure of PA, which has been extensively validated in the laboratory and during free-living conditions [43, 44], reduces the error and bias commonly associated with self-report measures. There was a high follow-up rate (95%) and a wide range of potential predictors from a variety of domains were examined. The study also has several limitations. While the sample was population-based, it was largely Caucasian and middle-aged, which restricts generalizability to other populations. Other limitations include the fact that PAEE was only measured at one year, and that self-reported health behaviours may be subject to recall and social desirability bias. Furthermore, we also explored a number of associations and conducted multiple significance tests, which mean that our results should be interpreted with caution as some significant associations may have occurred by chance (alpha inflation).