Mindfulness-based programmes for mental health promotion in adults in nonclinical settings: A systematic review and meta-analysis of randomised controlled trials

Background There is an urgent need for mental health promotion in nonclinical settings. Mindfulness–based programmes (MBPs) are being widely implemented to reduce stress, but a comprehensive evidence synthesis is lacking. We reviewed trials to assess whether MBPs promote mental health relative to no intervention or comparator interventions. Methods and findings Following a detailed preregistered protocol (PROSPERO CRD42018105213) developed with public and professional stakeholders, 13 databases were searched to August 2020 for randomised controlled trials (RCTs) examining in–person, expert–defined MBPs in nonclinical settings. Two researchers independently selected, extracted, and appraised trials using the Cochrane Risk–of–Bias Tool 2.0. Primary outcomes were psychometrically validated anxiety, depression, psychological distress, and mental well–being questionnaires at 1 to 6 months after programme completion. Multiple testing was performed using p < 0.0125 (Bonferroni) for statistical significance. Secondary outcomes, meta–regression and sensitivity analyses were prespecified. Pairwise random–effects multivariate meta–analyses and prediction intervals (PIs) were calculated. A total of 11,605 participants in 136 trials were included (29 countries, 77% women, age range 18 to 73 years). Compared with no intervention, in most but not all scenarios MBPs improved average anxiety (8 trials; standardised mean difference (SMD) = −0.56; 95% confidence interval (CI) −0.80 to −0.33; p–value < 0.001; 95% PI −1.19 to 0.06), depression (14 trials; SMD = −0.53; 95% CI −0.72 to −0.34; p–value < 0.001; 95% PI −1.14 to 0.07), distress (27 trials; SMD = −0.45; 95% CI −0.58 to −0.31; p–value < 0.001; 95% PI −1.04 to 0.14), and well–being (9 trials; SMD = 0.33; 95% CI 0.11 to 0.54; p–value = 0.003; 95% PI −0.29 to 0.94). Compared with nonspecific active control conditions, in most but not all scenarios MBPs improved average depression (6 trials; SMD = −0.46; 95% CI −0.81 to −0.10; p–value = 0.012, 95% PI −1.57 to 0.66), with no statistically significant evidence for improving anxiety or distress and no reliable data on well–being. Compared with specific active control conditions, there is no statistically significant evidence of MBPs’ superiority. Only effects on distress remained when higher–risk trials were excluded. USA–based trials reported smaller effects. MBPs targeted at higher–risk populations had larger effects than universal MBPs. The main limitation of this review is that confidence according to the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach is moderate to very low, mainly due to inconsistency and high risk of bias in many trials. Conclusions Compared with taking no action, MBPs of the included studies promote mental health in nonclinical settings, but given the heterogeneity between studies, the findings do not support generalisation of MBP effects across every setting. MBPs may have specific effects on some common mental health symptoms. Other preventative interventions may be equally effective. Implementation of MBPs in nonclinical settings should be partnered with thorough research to confirm findings and learn which settings are most likely to benefit.

Comparison of MBPs with active non-specific control groups: multivariate meta-analysis of anxiety, depression, distress, and wellbeing at post-intervention, (post-int), 1-6 months follow-  Table 30. Sensitivity analysis of methodological quality removing high-risk-of-bias trials.

Form Tab Information collected
Study Identification Sponsorship sources, conflicts of interest, country, study setting, corresponding authors, institution, emails, date recruitment started, and year first published.

Methods
Study design, conceptualisation of mindfulness, incentives for participants, number of participants (total randomised and per arm), and power calculation.

Population
Inclusion and exclusion criteria, type of participant, group differences, baseline characteristics

Interventions
Intervention name, mention of intervention manual, whether it was an adaptation of another intervention (rationale), intensity, mindfulness components (type, frequency and duration), non-mindfulness components (type, frequency and duration), home practice length and type, group size, any individual tailoring, any booster sessions or support after the end of the programme, adherence to intervention manual, intervention setting, teacher competence, teacher characteristics, response to intervention (attendance, satisfaction, reasons for missing sessions), and whether participants paid to do the course.

Outcomes
Outcome measure used, time points, group sizes, effect measures available and extracted effect sizes. Table 3. Definitions to refine primary outcome selection.

Primary outcome Requirement
Distress has to measure more than one negative emotion Wellbeing has to be more than one positive emotion Anxiety has to include physical symptoms and functioning impairment.
Depression has to include general anhedonia, worthlessness, physical symptoms and functioning Table 4 contains a summary of secondary outcome results, which are summarised below. The systematic review search date for the secondary outcomes was January 2020.

Comparison with passive control groups
At post-intervention (measured within one month of completing the intervention), in comparison with passive control groups, on average MBPs improved anxiety, depression, psychological distress, and mental wellbeing, ( Table 6). The prediction intervals indicated that post-intervention anxiety will be reduced following MBPs in more than 95% of the scenarios, but improvement is not homogeneous for the rest of the outcome domains. There was no evidence that improvements following MBPs in depression, distress or wellbeing remained six or more months postintervention (no studies for anxiety). However, only one study measured depression, four distress and three wellbeing at six or more months post-intervention, so results need to be interpreted with caution.
There is evidence for a modest and heterogeneous improvement in cognitive functioning following MBPs compared to passive controls shortly after intervention completion, with no significant differences at one-to-six-months follow-up (Table 9). MBPs improved real-life functioning at post-intervention in comparison with passive controls (small effect, Table 12). Effects may last for up to six months, with no reliable evidence on longer effects. MBPs improved the relationship with the self for up to at least six months (Table 15, no data on longer-term outcomes), and dispositional mindfulness for longer (Table 22). MBPs reduced psychosomatic symptoms shortly after course completion (Table 18), but no evidence supported effects persisting for any longer (Table 19). None of these effects was generalisable across settings.
Comparison with active non-specific control groups In comparison with active non-specific control groups at post-intervention, results supported improvements following MBPs in anxiety, depression, distress and wellbeing. However, reliability is low due to a mix of few studies contributing data, borderline p values, and prediction intervals that included adverse scenarios (Table 7). No studies measured these outcomes six or more months after the interventions. We found no evidence for improvements following MBPs in cognitive function (only three studies measuring this,  (Table 23). MBPs improved the relationship with the self for up to at least six months (Table 16, no data on longer-term outcomes). However, this effect is not generalisable to all implementation settings.
Comparison with active specific control groups Compared with active specific control groups, findings supported a modest superiority of MBPs in improving depression and wellbeing, but not distress and anxiety, at post-intervention (Table   8). Prediction intervals included null or unfavourable effects. Very few studies measured these outcomes six or more months after the intervention, with no significant differences between groups. We found no evidence for improvement in cognitive functioning (Table 11), real-life functioning (Table 14), relationship with the self (Table 17), psychosomatic symptoms (Table 21), or dispositional mindfulness (Table 24).

Risk-of-bias Source-specific Sensitivity Analyses
Source-specific sensitivity analyses could be conducted for risk-of-bias sources of randomisation, deviations from intended interventions, and missing outcome data; there was not enough risk variance for the other sources to meaningfully remove higher-risk trials. Source-specific analyses gave similar results to the overall-risk sensitivity analyses, except that in the comparison with passive controls the effects of MBPs on depression, distress and wellbeing remained significant and with narrower prediction intervals after removing trials at high risk of bias due to deviations from the intended interventions (a bias that tended to dilute intervention effects due to contamination between arms, Table 31). * Number of trials with non-reported data for the corresponding outcome. Abbreviations: 1-6m=1 to 6 months postintervention follow up, 6+ months= more than 6 months post intervention follow up, CI= confidence interval for overall mean, Cog. Func = Cognitive functioning, n=number, P-int=post-intervention, PI= prediction interval for new study, Psychosom = Psychosomatic outcomes, Real func = Real life functioning, Rel. Self = Relationship with self, SMD=standardised mean difference.