Discussion
fits, cancer, anxiety
CANCER, INFLAMMATION, CHRONIC DISEASES, DISORDERS, SECONDARY
To our knowledge, this is the first randomised controlled trial of community gardening in a population that was diverse in terms of age, ethnicity, and socioeconomic status. In this study, we found that participation in community gardening in Denver significantly changed the pattern of response over time for the primary outcomes of fibre intake and moderate-to-vigorous physical activity. Additionally, randomisation to community gardening reduced the secondary outcomes of perceived stress and anxiety, with greater reductions in those who started the trial with higher stress or anxiety. These key behaviours and experiences of stress and anxiety are pertinent to the prevention of cancer and other chronic diseases.Dietary fibre and whole-plant food intake are increasingly recognised as vital health indicators because of clear associations with improved metabolic health, healthy gut microflora, and decreased inflammation.Fruits and vegetables are individual components of fibre intake. A systematic review of 34 behaviour-based interventions designed to increase fruit and vegetable intake found that mean increases in total fruit and vegetable intake in studies of adults ranged from a low of 0·29 servings per day to a high of 2·74 servings per day, and that mean increases attributed to interventions were 1·13 servings.Within the physical activity domain, we found that although there was no significant difference between the two groups at T2, participants in the intervention group did approximately 5·8 min more of moderate-to-vigorous physical activity per day than participants in the control group at this timepoint. In a recent systematic review of physical activity intervention trials that assessed physical activity changes using device-based measures (eg, accelerometry), the authors found that for the 11 trials that included the measure of moderate-to-vigorous physical activity per day, the median baseline measure was 24·7 min per day (range: 4·3–46·3), with participants in the intervention groups completing 10·3 min per day more moderate-to-vigorous physical activity than comparison groups at last follow-up.In CAPS, participants randomly assigned to the gardening intervention had a larger decrease in the secondary outcomes of perceived stress and anxiety difference scores from T2 to T1, controlling for the baseline level, than those in the control group. Qualitative and observational studies of gardens across different cultural, geographical, and economic contexts have shown that participating in garden-based activity can reduce feelings of stress and anxietyOur results suggest that community gardening, as an example of a multicomponent intervention, could be beneficial in changing some key risk factors for cancer and other chronic diseases, thus warranting further investigation. Data from our qualitative interviews and our process assessment will be examined to contextualise the quantitative results and understand how the trial affected study participants.We included a broad range of outcomes in our study, which was necessary to measure the pleiotropic effects of community gardening. Multiple outcomes require multiple statistical tests, and multiple statistical tests increase the chance of an inflated type 1 error rate, and reduced replicability. In scientific research, researchers usually correct for multiple comparisons within each manuscript separately, and not over many papers based on data from a single trial. The rationale is that each report represents a separate experiment. A trial with multiple outcomes raises similar questions. The choice of whether or not to use a Bonferroni correction so that the entire trial has a total type 1 error rate of 0·05, or to use a total type 1 error rate of 0·05 per outcome is unclear. We chose the second approach, which controlled the type 1 error rate within each outcome, but did not correct for the multiple outcomes, because each outcome was a separate hypothesis.Strengths of our study include the randomised controlled design that was adequately powered to test hypotheses, the use of robust measurements for diet, and device-based measurements of physical activity duration. Both diet and physical activity were measured over multiple days at T1, T2 and T3, including three 24-h random diet recalls and 7-day thigh-mounted accelerometry. The inclusion of a diverse population and a high rate of completion of measurements (81%) provide a strong basis for generalisation of our findings. Another strength was the use of longitudinal repeated measures to test the time-byintervention hypothesis, which assessed the difference in the patterns of means across time for the intervention and control groups. This hypothesis test is often more powerful than the test of the main effect of an intervention alone. The analytical plan for the trial accounted for possible correlation within waiting lists and gardens, and repeated measures within individuals. Our analysis accounted for social desirability at baseline and assessed possible social desirability bias in the estimation of intervention effect. The groups were not different at baseline in social desirability, and sensitivity analysis showed no difference in intervention effect by social desirability score, which is known to be a source of bias in dietary self-reporting.Our study had several limitations. One limitation was the exclusion of data from wave 3 at T3 to account for the COVID-19 pandemic. The data were excluded because pandemic-related closures probably would have affected the primary and secondary outcomes and were excluded before unblinding and before any analysis had occurred. The effect of excluding the last timepoint of data resulted in a decreased sample size and thus might have diminished statistical power or attenuated results towards the null. By design, the trial did not have the power to assess intervention effects within subgroups stratified by race, ethnicity, sex, sexual orientation, and socioeconomic status. Diet, stress, and anxiety outcomes were self-reported, which might lead to information bias, although there was no evidence of social desirability bias for dietary reporting in a sensitivity analysis. Finally, a small number of participants refused their assignment or discontinued participation, and there might be other participants who did not report their status to the study staff. Crossover of participants between groups would only bias the results towards the null. Finally, this trial captured the community gardening experience only over 1 year. Whether the effects of intervention will be maintained beyond 1 year is unknown. Because of the time required to establish new activities such as gardening and maintain changes in health behaviours and health status, long-term follow-up would be useful to understand if and how gardeners maintain the changes they adopted in the first year of gardening.This randomised controlled trial strengthens evidence for community gardening as a comprehensive multicomponent nature-based social intervention that can improve some health behaviours and reduce perceived stress and anxiety in a diverse urban population. Both are important for the prevention of chronic diseases and mental health disorders. Gardening is a nature-based solution that fits within the broader context of urban agriculture systems. A community garden is a setting that could be within reach for citizens across the world and can be tailored to meet the needs of people across different social and economic groups, cultures, geographies, and local environments. Land planners, health officials, and policy makers together can integrate gardens into the fabric of communities, recognise gardens as a primary and permanent natural space, similar to playgrounds, farmers’ markets, bicycle lanes, and public plazas, and invest in programming that supports gardeners across the lifespan.