There is increasing evidence that gardening provides substantial human health benefits. However no formal statistical assesment has been conducted to test this assertion. Here , we present the result of a meta analysis of research examining the effects of gardening, including horticultural therapy, on health. We peformed a literature search to collect studies that compared health outcomes in control(before participating in gardening or nor gardeners) and treatement groups( after participating in gardening or gardeners) in january 2016. The mean difference in health outcomes between the two groups was calculated for each studey and then the weighted effect size determined voth across all and sets of subgroup studies. Twenty two case studes were included in the meta analysis, which comprised 76 comparisions between control and treatment groups. Most studies came from the United States, followed by Eurpe, Aisa and the Middle East. Studies reported a wide range of health outomes, such as reductions in depression, anxiety, and body mass index, as well as increases in life satisfaction, quality of life and sense of community. Meta analytic estimates showed a significant positive effect of gardening on the health outcomes both for all and sets of subgroup studies, whilst effect sizes differed among eight subgroups. although egger's test indicated the presence of publications bias, significant positive effects of gardening remained after adjusting for this using trimand fill analysis. The positive effects of gardening on health. A regular dose of gardening can imporove public health .
Globally, the prevalence of the so-called “lifestyle diseases,” such as heart disease, stroke, depression, diabetes, and obesity is becoming a major public health issue (Caballero, 2007; Janssen et al., 2005; Moussavi et al., 2007). It is, for example, estimated that worldwide, ap- proximately 415 and 350 million people presently suffer from diabetes and depression, respectively, and hence both are costly to national health care budgets (IDF, 2015; WHO, 2016). Unfortunately, this trend is expected to continue for the foreseeable future as a high number and proportion of the world's population will be living in urban areas (Seto et al., 2012). Indeed, urban living is associated with various ad- verse health consequences, such as high-fat diets, sedentary lifestyles, and increased levels of social and psychological stress and environmen- tal pollutants (Clougherty et al., 2007; Lambert et al., 2015; Peer et al., 2003; Sodjinou et al., 2008). As a consequence, promoting health of urban populations has become one of the most challenging issues of the 21st century (Dye, 2008; Tzoulas et al., 2007).
(Groenewegen et al., 2006; Tzoulas et al., 2007). Indeed, there is mount- ing evidence that direct experience with natural environments offers a wide range of health benefits (Hartig et al., 2014; Keniger et al., 2013; Soga and Gaston, 2016). Louv (2005) argued that a decrease in contact with nature results in a number of health and behavioural problems, es- pecially for children, which in sum can constitute a “nature-deficit dis- order.” Recent studies suggest that daily contact with nature has a long-lasting and deep impact on health, including on depression and anxiety symptoms (Beyer et al., 2014), birth weight (Dadvand et al., 2012), diabetes, and obesity (Lachowycz and Jones, 2011), circulatory and heart disease (Maas et al., 2009), and longevity (Takano et al., 2002). It is therefore increasingly recognized that a regular contact with nature can promote human health and be used as a form of pre- ventive medicine (Groenewegen et al., 2006).
Gardening is arguably one of the most common ways of interacting with nature and indeed is enjoyed as a popular pastime in many coun- tries. In the UK, there are estimated to be 27 million people, approxi- mately 40% of the total population, who actively participate in gardening (Bisgrove and Hadley, 2002). Likewise, it is estimated that in the US, 117 million people, one in three, participate in gardening (Statista, 2015), and that in Japan, 32 million people, one in four, partic- ipate in daily gardening as a hobby (Statistics Bureau, Ministry of Internal Affairs and Communications, 2011). Gardening requires, at most, a relatively small piece of land, and in many parts of the world, such gardens are today common. In the UK, it is estimated that 22.7 mil- lion households (87%) have access to a domestic garden, which com- prise 432,924 ha of land in total (Davies et al., 2009). Mathieu et al. (2007) also showed that more than a third of the land in the city of Dunedin, New Zealand, was used for domestic gardens. Alongside domestic gardens, allotment and community gardens, pieces of land with plots rented by an individual or group to grow plants for non-com- mercial use also offer places in which people can participate in gardening. The city of Stockholm, Sweden, for example, contains ap- proximately 10,000 allotment plots, which occupy 210 ha of land and involve 24,000 people (c.f. Barthel et al., 2010). Given the scale of gar- dening activities, and the apparent feasibility of accommodating them in cities and towns, these have great potential for limiting the ongoing loss of human–nature interaction—the extinction of experience.
As defined by the WHO (1948), health is “a state of complete phys- ical, mental and social wellbeing and not merely the absence of disease or infirmity,” we interpret “health” in a broad sense to include physical and psychological wellbeing. Positive effects on health are thus not sim- ply the amelioration of symptoms associated with chronic illness (e.g. depression, anxiety, obesity) but include the presence of positive emo- tions (e.g. quality of life, life satisfaction, sense of community, happi- ness) and the absence of negative emotions (e.g. anger, loneliness, confusion), and the state of being able to perform the normal actions of daily life without the hindrance of both physical and psychological dysfunction. Increased physical activity level was also included as a pos- itive health outcome, as it has proven to be a good indicator of risk for obesity-related diseases (Janssen et al., 2005). We use the term “garden- ing” for “an activity in which people grow, cultivate, and take care of plants (flowers and vegetables) for non-commercial use,” which is not simply limited to an activity in domestic gardens but includes that in al- lotment and community gardens. In this study, horticultural therapy, a practice of engaging patients in gardening activities to improve their physical, psychological, and social health, was also considered as a form of gardening.
We focused on studies that collected data on people's health out- comes in the context of gardening, were published in peer-reviewed scientific journals after 2001, and were written in English. This study followed the PRISMA statement (Moher et al., 2009). We performed the search, assessed eligibility, and extracted data. Literature search was conducted using the PubMed database in January 2016. We used the following terms in the keyword search: Physical activity OR Health OR Restoration OR Recovery OR Therapy OR Well-being OR Wellbeing OR Well being OR Psychology OR Quality of life OR Life satisfaction OR Hap- piness OR Anxiety OR Depression OR Stress OR mood OR Pain OR Obesity OR Social AND Gardening OR Allotment OR Allotment gardening OR Horti- culture OR Horticultural therapy OR Community Garden. The PubMed search resulted in 2456 records. We also ran similar queries on Google Scholar in January 2016 to identify studies that had previously been missed. We searched using all possible combinations of the above 19 health terms and 6 gardening terms (114 combinations), and examined the first 50 hits from each (5700 records in total). Studies identified through PubMed and Google Scholar were screened on title, abstract, or both, and 79 full-articles were assessed for eligibility. The eligible ar- ticles were obtained from the Internet, via the University of Tokyo elec- tronic library, or by personal contact with the authors. To be included in our meta-analysis, a study had to (1) conduct a quantitative survey rather than a qualitative one, (2) focus on outdoor gardening, (3) have control (before participating in gardening or non-gardeners) and treat- ment groups (after participating in gardening or gardeners), (4) report sample size and mean and standard deviation (SD) or error (SE) of health outcomes both for the control and treatment groups, and (5) have N 11 participants (sample size). Reviews of previous work and study protocols were ignored.
We finally included 21 articles [22 case studies; one paper (Gonzalez et al., 2011a) reported two independent studies] in the meta-analysis (see Table 1). The study selection process (PRISMA diagram) is shown in Fig. 1. Basic information was collected for these studies, including the first author's name, year and name of publication, country of origin, and details of settings (duration and types of gardening), participants (mean age, female ratio, and health condition), and types of health out- comes measured. We also extracted mean values of health outcomes, sample size (n), and SD for both the control and treatment groups. If a single study reported data on more than one health outcome, then we considered each comparison between the control and treatment groups (hereafter comparison) independently. Duplicate results that were de- rived from repeated analyses (e.g. subgroup analysis) were ignored. For studies that measured health outcomes during gardening on multi- ple occasions, we used only data points at the start (control) and end (treatment). One study (Park et al., 2009) compared health outcomes of people with multiple levels of gardening activity (non-gardeners, gardeners, and active gardeners; here, active gardeners were defined as those who met or exceeded recommended physical activity levels by gardening, and gardeners as those who did not meet this recommen- dation by gardening but did garden as moderate intensity). In this case, we compared only non-gardeners and gardeners. We finally obtained 76 comparisons. The full dataset is listed in Table S1.
We performed the meta-analysis using the “metafor” package (Viechtbauer, 2010) in R (ver. 3.2.2) (R Core Team, 2015). The standard- ized mean difference Hedges' d (Hedges and Olkin, 1985) was used as the effect size metric for comparing mean differences in health out- comes between the treatment and control groups:
d ¼ ðMt−McÞ J=S
where Mt. and Mc are the mean values of the response variable (health outcomes) in the treatment and control groups, respectively, and S and J are the pooled SD of both groups and a term that corrects for bias due to small sample size, respectively. Here, positive effect sizes indicate that health condition is better in the treatment groups than in the control groups; we reversed the sign of health outcomes where higher values meant a less healthy condition (e.g. depression, anxiety, stress).
Based on the effect size of each comparison, we calculated the over- all pooled effect size and its 95% confidence interval (CI) as a weighted average of all 22 case studies (76 comparisons). Significance of the over- all effect size was assessed by determining whether the CI overlapped zero. Since preliminary analysis showed significant between-study het- erogeneity (see the Results section), we used a weighted random-effects model to estimate the overall effect size and CI. The random-ef- fects model assumes that different studies are not exactly identical in the survey methodologies and the characteristics of respondents. Het- erogeneity between studies was checked by the Q test and I2 statistic. In order to account for the possibility of pseudoreplication derived from using multiple comparisons from within studies, we recalculated the overall effect sizes after sampling one comparison from each sepa- rate study. The estimated mean and 95% CI of effect size were computed by bootstrap resampling 10,000 times in R.
The possibility of publication bias (a lower likelihood of studies being published that reported non-significant results than reported sig- nificant ones) was assessed using a funnel plot and Egger's test (Egger et al., 1997; Nakagawa and Santos, 2012). If publication bias was indicated by Egger's test, we performed a trim and fill analysis (with the R0 esti- mator) (Duval and Tweedie, 2000). This estimates the number of miss- ing studies (comparisons) in the original dataset and provides a true effect size: that is, an effect size when publication bias is not present. The trim and fill analysis was performed using the “trimfill” functions of the “metafor” package in R.
An overview of the 22 case studies is presented in Table 1. The sam- ple sizes ranged from 14 to 514 people (M = 117.2, SD = 144.5). Many of the studies came from the United States (9 studies), followed by Europe (7 studies), Asia (5 studies), and the Middle East (1 study). The participants ranged in average age from 8.5 to 84.7 years (M = 52.3, SD = 19.6), with the percentage of females ranging from 29.2 to 100.0% (M = 67.8, SD = 21.0). Eleven studies focused on patients (e.g. dementia, depression) and 11 on non-patients. Gardening types in- cluded horticultural therapy (12 studies), daily gardening (7 studies), and experimental short-term gardening (3 studies). Studies used a wide range of health outcomes (Table 1).
The results of the 76 comparisons and the meta-analytic estimates are shown in Fig. 2. Most studies reported positive effects of gardening, and none reported significant negative effects (Fig. 2). The 95% CI of the overall pooled effect size did not overlap zero (mean = 0.42, 95% CI: 0.36–0.48), suggesting a significant effect of gardening on the health outcomes (Fig. 2). We found significant between-study heterogeneity in the overall analysis (I2 = 40.47%, Q75 = 137.38, P b 0.001). After re- peated resampling using bootstrap simulation, the overall effect sizes remained significantly positive (mean = 0.47, 95% CI: 0.36–0.57) and its 95% CIs largely overlapped with those in the primary analysis (Fig. S1). Thus, the reported results overall would not be biased by pseudoreplication.
The 95% CI of effect size did not overlap zero for all eight subgroups (Table 2). The effect size of gardening on the health outcomes differed significantly between two subgroups for all four categories (outcome types: Q1 = 6.48, P = 0.01; gardening types: Q1 = 24.71, P b 0.001;
comparison types: Q1 = 18.31, P b 0.001; respondent types: Q1 = 17.96, P b 0.001) (Table 2). Studies focused on wellbeing variables, horticultur- al therapy, before/after comparison method, and patients showed higher effect sizes of gardening, compared to the other subgroups (Table 2). Of the eight subgroups, between-study heterogeneity was found for five (Table 2).
Egger's test indicated the presence of publication bias (t = 4.18, d.f.
= 64, P b 0.001). The trim and fill analysis suggested that 16 studies (comparisons) were missing from our dataset (white circles in Fig. 3). However, after adding those missing data to the original dataset, reported significant effects of gardening on the health outcomes were intact (mean = 0.35; 95% CI: 0.27–0.43, Fig. 3), suggesting that the ef- fects of publication bias on the overall results were negligible.
To our knowledge, this meta-analysis is the first to provide a quanti- tative synthesis of the evidence that gardening is beneficial for human health. Overall, the results suggest that participating in gardening activ- ities has a significant positive impact on health. Indeed, the positive as- sociation with gardening was observed for a wide range of health outcomes, such as reductions in depression and anxiety symptoms, stress, mood disturbance, and BMI, as well as increases in quality of life, sense of community, physical activity levels, and cognitive function. The 22 case studies were geographically dispersed, although more than one-third came from the United States. Publication bias is a common limitation of meta-analysis (Nakagawa and Santos, 2012). Nevertheless, our results remained quantitatively almost unchanged after using the trim and fill analysis, suggesting that the reported health benefits of gar- dening are robust. Given the recent rise in awareness of the health ben- efits derived from nature (Hartig et al., 2014; Keniger et al., 2013), these findings are particularly timely and support the argument that a regular dose of gardening can improve health.
Studies included in our analysis varied substantially with respect to the demographic characteristics of the participants and settings, which is likely to be the main reason for significant between-study heteroge- neity. To account for this issue, we performed subgroup analysis and de- termined that significant positive effects of gardening on health existed for all subgroups. Positive influences of gardening were particularly ev- ident on patients and horticultural therapy users. This is unsurprising because these groups would explicitly use, and be exposed to, gardens in a more health-supportive way than would non-patients. Likewise, studies using a “before and after” comparison method, which were commonly seen in horticultural therapy studies (Ghanbari et al., 2015; Gonzalez et al., 2010, 2011a, 2011b; Kotozaki, 2014; Min et al., 2014; Wichrowski et al., 2005), reported a larger effect size of gardening than those simply comparing a treatment (gardeners) to a control group (non-gardeners). Our subgroup analysis also indicated that wellbeing variables are more likely enhanced sharply by gardening than health variables. Although it is difficult to provide a precise expla- nation due to the limited sample size, one possible reason for this result is that the improvement of health variables would need a relatively lon- ger time, compared to wellbeing. As wellbeing variables were in many cases measured on a subjective scale (e.g. depression, anxiety, quality of life, life satisfaction) (Ghanbari et al., 2015; Kam and Siu, 2010; Kotozaki, 2014; van den Berg et al., 2010; Waliczek et al., 2005; Wood et al., 2016), they were more likely to respond immediately than objec- tive health outcomes (e.g. BMI) (Park et al., 2009; van den Berg et al., 2010; Wood et al., 2016; Zick et al., 2013).
Our meta-analysis has provided robust evidence for the positive ef- fects of gardening on health. With an increasing demand for reduction of health care costs worldwide, our findings have important policy impli- cations. The results presented here suggest that gardening can improve physical, psychological, and social health, which can, from a long-term perspective, alleviate and prevent various health issues facing today's so- ciety. We therefore suggest that government and health organizations should consider gardening as a beneficial health intervention and encour- age people to participate in regular exercise in gardens. To do so, policy makers need to increase people's opportunity and motivation to engage with gardening activities. The former requires enough spaces where peo- ple can enjoy gardening, and the latter needs the various advantages of gardening to be made apparent to a broad audience. Because gardens are accessible spaces for all kinds of people, including children, elderly people, and those with a disability, and relatively easily and quickly im- plemented in urban areas as a “land-sharing” strategy (Soga et al., 2015; Stott et al., 2015), we believe that such actions and policies would at the same time contribute greatly to redressing health inequalities.
Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.pmedr.2016.11.007.
MS was supported by the Japan Society of Promotion of Science (grant number 16K00631). KJG was supported by the Natural Environ- ment Research Council (grant number NE/J015237/1). YY was support- ed by JSPS KAKENHI (grant number 26292074).
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