3.4.1 Study characteristics

The dominant study design of the 28 included studies was cross-sectional (n = 20), the others longitudinal (n = 8).18, 29, 53-57 One of the longitudinal studies conducted a quasi-experiment, which considered a pre-park and post-park time frame and dividing the children into those who live near the park (the exposure group) and those who live further from the park (the control group) to examine how exposure to a newly built park translates to changes in BMI z score over time.58 Almost half of the studies were conducted in the United States (n = 13), 10 studies were conducted in Europe, four of which in the United Kingdom,29, 56, 59, 60 two studies from Germany61, 62 and Spain,63, 64 and one from the Netherlands18 and Lithuania.65 The sample sizes ranged from 93 to 41,283. Seven studies used small cohorts with less than 1000 subjects,47, 48, 53, 64, 66-68 most studies used medium size cohorts (n = 15) not exceeding 7000 participants, four studies included larger samples over 10,000 participants,69-72 and two studies included very large samples of around 40,000 subjects.52, 73 Five studies considered a wide age range up to 18 years. Seven studies included children under the age of 9 years,48, 59, 60, 62, 65, 70, 74 and four studies included exclusively adolescents of at least 10 years.61, 64, 73, 75 Twelve studies included both children and adolescents with ages ranging from 4 to 18 years.

Most studies analyzed park accessibility and availability based on children's place of residence, and two studies focus on the school environment.62, 72 The definition of the sphere of influence was in 14 studies based on circular or network buffers ranging from 100 to 3000 m in radius from the pace of residence, one study that considered a 10-mile (16,000 m) radius.53 Eight studies based their analysis on official administrative or statistical boundaries and three studies analyzed distance from the nearest park, without defining a sphere of influence.52, 58, 64 The remaining studies used neighborhood area without further specifications on the delimitations. The most used exposure metric was the relative amount of park surface in the sphere of influence (n = 11). Other studies quantified exposure through the dichotomous variable presence/absence of parks, the number or density of parks, and the distance from the nearest park. Four studies used the satellite-derived Normalized Difference Vegetation Index (NDVI) to quantify the greenness of the surrounding environment. The definition of park/greenspace was inconsistent across studies. Most studies identified areas intended as urban free-usable greenspace. Some studies identified specific features (e.g., children playgrounds), and others used a broad approach (e.g., NDVI), considering the total amount of vegetation without distinct function.

The outcomes analyzed were BMI z score, BMI trajectories, BMI percentiles, and weight status. Anthropometric measures were rarely used: waist circumference (n = 3),55, 76, 77 waist-to-height ratio (n = 1),55 sum of skinfold (n = 1),48 and percentage body fat (n = 2).47, 77

The quality of the studies was either fair (n = 6) or good (n = 22). The main reasons for fair quality were small sample sizes, self-reported outcomes (height and weight), or study population scarcely representative of the population (see Table S5).

3.4.2 Summary of findings

Due to the great variability in exposure metrics, we synthesized findings across the following exposure categories: distance to the nearest park (n = 9), park area (n = 10), number of parks (n = 8), and presence/absence of parks (n = 5). Only three studies analyzed NDVI, which was insufficient for meta-analysis according to our criteria (Table 2). The z test and related p value suggest that there was insufficient evidence to support an association of distance to park and childhood obesity (p = 0.170). Out of the nine studies, only one found a significant association.67 Two studies concluded with mixed findings: One study found a significant association in boys of all ages and girls of high school age but not in younger girls,52 and one study found an significant association in children living in urban areas but not those in rural areas.18 The p value suggested weak evidence of an association with percentage of park area (p = 0.014). Three studies found significant associations, six studies found no statistically significant effects, and two studies had mixed results, with effects only found in boys and older children. The p value showed little evidence of an effect of number or density of parks on childhood obesity (p = 0.148). One study found a significant association, five studies did not find significant associations, and one study reported mixed results with effects only in girls.69 The intervention study did, however, find an effect in the intervention group, which could not be replicated in the control group.53 We identified strong evidence on the presence of a park within the sphere of influence and childhood obesity (p < 0.001). Out of the five studies, four studies found statistically significant effects. Results from the three studies that explored the effect of greenness via the NDVI suggest a potential association in the more proximal environment of less than 250 m.18, 63, 65 Three studies specifically focused on playgrounds, and none of them found statistically significative associations.