This study did not show an effect of a worksite mindfulness-based multi-component intervention on vigorous physical activity, fruit intake and behavioral determinants after 6 and 12 months. A significant interaction effect was found for gender and sedentary behavior at work, which had decreased significantly in women in the control group after 6 months. After 12 months there were no differences in sedentary behavior at work between women in intervention and control group.
A possible explanation for not finding the expected effects could be that we aimed at stimulating healthy behavior in a population without specific risks. Previous research on the effectiveness of mindfulness training on lifestyle behavior had study populations that were obese or overweight [18, 19, 21]. It might be that the mindfulness training as under study is less effective in populations not at risk, i.e. with a healthy body weight. In other words, it might be that ceiling effects occurred. This does however not imply that mindfulness is not effective among this group, but that another type, intensity and duration of a mindfulness intervention might be effective. Due to lack of studies to the effect of mindfulness among relatively healthy persons, future research is relevant from a population health approach. Serious health consequences (such as all-cause mortality) associated with body weight, usually start with a BMI higher than 30 . Our study population consisted of very few participants with a BMI higher than 30, which made itimpossible to perform a subgroup analysis to explore the effectiveness in a high risk subgroup.
Another possibility of ceiling effects might have occurred in the behavioral determinants.Although the targeted lifestyle behaviors were selected on their room for improvement (for example, 60.1% of the study population did not engage in vigorous physical activity at baseline at all; and the recommended target of 2 pieces of fruit per day was not the average consumption at baseline ), it appeared that the scores on the key determinants were already close to optimal. This implies that there was no room for improvement (so-called ceiling-effect), even though the corresponding behaviors seem susceptible to change. It might be that the measurement of the determinants was not sensitive enough to distinguish small changes, which was especially important since the population under study was relatively healthy.
Next to the aforementioned explanations, the timing of the measurements could also be relevant to explain the lack of effect. Other studies evaluating the effectiveness of mindfulness based interventions on lifestyle behaviors and weight-related outcomes found effects at the immediate follow-up measurement, that is immediately after the training [18, 19, 21]. Both Tapper and colleagues and Van Dalen and colleagues found reduced effects at the follow-up measurement at 6 months and 3 months respectively [18, 19]. Kearny and colleagues , however, did not find any effects at their follow-up measurement of 4 months. It might be that the effects of the mindfulness training component in our intervention wore of before our first follow-up measurement took place, after the total intervention duration of 6 months.
To enhance fruit intake, participants in our intervention were offered free fruit at the workplace. This fruit was offered during the whole intervention period of six months. In contrast to our findings, other interventions showed significant effects on fruit intake following offering free fruit during 6 months . Despite this, the fruit was well appreciated by the participants and the reach among participants was reasonably good (69%) . Possibly, the participants who made use of the fruit at the workplace, were the ones who already ate fruit before the intervention and now ate the provided fruit instead of bringing their own.
The difference in effect modification between the primary analyses and the sensitivity analyses for sedentary behavior at work after 6 months is probably caused by the difference in handling missing data. The primary analyses concerned intention-to-treat analyses, whereas the sensitivity analysis concerned complete-case-analyses. For the sedentary behavior at work questions, there were relatively more missing values than for the other questions (T1: 18% missing and T2: 19% missing). Therefore, we believe that the effect modification for gender we found in the sensitivity analyses was a Type 1 error, especially given the number of analyses performed. In addition, the use of a questionnaire that had not been validated, though more often used in worksite health promotion intervention studies, may have contributed to less reliable findings. Therefore, the results should be interpreted with caution.
Strengths and limitations
The major strength of this study is that it is the first study to examine the effectiveness of a worksite mindfulness-based intervention targeting lifestyle behaviors in a randomized controlled trial design, which is the most reliable design for intervention studies. Second, the number of participants is quite large compared to other studies on the efficacy of mindfulness interventions on lifestyle behaviors, overweight and obesity (range n = 12–84) [18, 19, 21, 36, 39]. In addition, the duration of follow-up was 12 months. Long term effectiveness is especially important for interventions aimed at behavioral change, since sustaining a changed behavioral pattern is difficult, especially for weight maintenance . Another strength, is that the intervention was tailored for the target population. In addition, loss to follow-up was very limited (less than 10%). A last strength, is that we measured physical activity objectively using accelerometers in a subgroup in addition to subjectively using a questionnaire.
A first limitation of this study, is the lack of an immediate post-intervention measurement of the program component mindfulness training. Long term effects are relevant, but when an immediate effect is present, this implies that the maintenance of the direct effects deserve attention, rather than focussing on changing the intervention component itself.
Another limitation of this study - given the complexity of overweight and obesity- is that environmental factors at the participating research institutes have not been taken into account. A meta-analytic review showed that worksite physical activity and dietary interventions containing an environmental component were more effective than individual interventions . Examples of an environmental component are a construction design where the stairs are more easily to find then elevators, sit-stand work stations, or standing meeting facilities.
Implications for research and practice
For the future development of worksite health promotion interventions, it is recommended to assess the selected key determinants on the potential for improvement among the study population, next to the potential effects of selected behaviors. Furthermore, it is recommended for multi-component intervention studies to perform intermediate measurement, that is, after a single component, to gain insight in the possible attrition of effects, if present.
This study aiming at health promotion for all workers at two research institutes did not show any effects. For future mindfulness research, intervention aims besides the cognitive dimension of lifestyle behavior could be explored. Given the complexity of overweight and obesity, it is recommended for worksite health promotion to also address other dimensions and combine environmental and individual components in an intervention. In addition, it is recommended to develop and validate a reliable questionnaire to measure sedentary behavior at work, which to date does not yet exist.