4.1 Summary of findings
This study aimed to assess the potential impact of exposure to greenspace and blue space on the dietary intake and food choices of adults. Several studies have investigated the association between greenspace and different health outcomes, including improved mental health (Annerstedt et al. 2012; Triguero-Mas, Donaire-Gonzalez, Seto, Valentín, Smith, et al. 2017; Britton et al. 2018: Korn et al. 2018; Lee and Lee 2019), reduced prevalence of type 2 diabetes (Teufel-Shone et al., 2014; Burkart et al., 2016), reduced mortality (Burkart et al., 2016; Orioli et al., 2019) and improved pregnancy outcomes (Abelt and McLafferty 2017). However, this is the first review to examine changes in dietary patterns following exposure to green and blue spaces, and no conclusive data were found on the association between them.
The first significant finding of this review was the lack of evidence and studies available concerning blue space. A small number of articles were retrieved; however, none met the inclusion criteria (Goeminne et al., 2015; Burkart et al., 2016; Britton et al., 2018). Further research should be conducted on the potential health benefits of blue space on dietary patterns, changes in dietary habits, and the associated health benefits.
In contrast, although greenspace is well researched, limited evidence is available on exposure to greenspace and impact or changes to dietary patterns, and only four studies were included in this review. Interpreting the results is further complicated because of the heterogeneity of the documented interventions and outcomes.
However, despite the limited findings in this review, emerging evidence suggests the use of urban horticulture as a tool to improve population health and urban biodiversity, particularly in socially disadvantaged neighbourhoods. Urban horticulture is related to an increase in healthy food intake, exercise, exposure to nature, and social cohesion among communities with several recent studies reporting on these health benefits (Algert et al. 2014, Clendenning et al. 2016, Martin et al. 2017). However, more studies are needed to strengthen the evidence available on its benefits to human health (Piedrahita et al. 2020).
The four observational studies included in this review identified an association between greenspace exposure and obesity. Yuen et al. (2019) indicated that exposure to greenspace was not correlated directly with healthy eating patterns but with physical activity levels. This finding is consistent with that of von Hippel and Benson (2014), who found that diet had no effect on the association between obesity and the natural environment, and that the relationship was mediated by physical activity. A similar finding was also reported by Michimi and Wimberly (2012), who discovered that as recreational opportunities and natural amenities increased, physical activity levels increased, and the prevalence of obesity decreased. An association between obesity and physical activity was also identified in the final study conducted by Choi and Yoon 2020, in which a negative association between BMI and walkability in urban neighbourhoods was identified. While the main research question addressed in this review explicitly concerns dietary pattern changes, secondary outcomes concerning exposure to greenspace and obesity were addressed.
4.2 Greenspace and obesity
The health benefits of exposure to greenspace and obesity have been well researched; however, the mechanisms and pathways underlying these associations are complex. Obesity is a complex disease and several factors play a role in gaining weight. However, one of the significant causes of obesity (BMI > 30) and overweight (BMI > 25) is the consumption of excessive energy dense foods combined with a lack of exercise (World Health Organisation, 2021). One of the potential mechanisms explored to explain the association between greenspace and health outcomes is the accessibility and availability of greenspace, which may influence individuals’ physical activity levels and subsequently, weight status. Yuen et al. (2019) found that physical activity levels were positively related to the percentage of greenspace and accessibility of open space facilities. In a study by von Hippel and Benson (2014), obesity was reported to be more prevalent in hot, dark, cold, and rainy counties in the US. These findings raise the possibility that individuals within these counties, owing to adverse weather conditions, are not engaging in outdoor physical activity, resulting in an increased prevalence of obesity. Attempts to promote physical activity should consider temperature extremes. Although little is known about how exercise affects dietary habits, evidence suggests that exercise motivates people to adopt healthier eating habits (Joo et al. 2019). Moderate physical activity can also serve as a gateway behaviour, leading to the adoption of other healthy habits (Blakely et al. 2004; Grant et al. 2020; Lounassalo et al. 2021).
Previous studies in this area have noted positive associations between greenspace and obesity-related indicators, including physical activity, BMI, and obesity-related health outcomes. According to a systematic review (Luo et al. 2020), there may be a link between increased access to greenspace and a lower risk of obesity. The results of the meta-analysis showed that individuals exposed to increased levels of NDVI were less likely to be overweight/obese [OR]: (0.88; 95% CI:0.84, 0.91); however, the review also identified that further high-quality studies are required to assess the evidence for this causal relationship. These findings are consistent with an earlier systematic review, which found a positive link between greenspace and obesity but mixed evidence on the association between greenspace access and physical activity (Lachowycz and Jones 2011). A study in the US reported lower BMI levels in individuals exposed to increased forest cover, and this association was stronger for those who participated in outdoor recreation (Ghimire et al. 2017). These studies reinforce the connection between opportunities for physical activity and a lower prevalence of obesity.
However, the results concerning greenspace exposure and obesity have been mixed across studies. One interesting finding in this review was that more urban leisure amenities in a neighbourhood were associated with higher BMI levels (Choi and Yoon 2020). This finding suggests that promoting walkability in neighbourhoods may be more successful in targeting obesity than leisure amenities.
Another finding of this review (von Hippel and Benson 2014) was that exposure to natural elements, such as wind, trees, waterfronts, and mountains, had little or no association with obesity. Recent Irish research (Dempsey et al. 2018) discovered evidence of a U-shaped relationship between obesity and greenspace. The study identified that people living in the highest and lowest areas assessed for greenspace had higher probabilities of obesity. These preliminary findings suggest that even though individuals may have access to greenspaces and other amenities in their neighbourhood, it does not mean they will avail of them to promote health. These relationships may partly be explained by other confounding and mediating factors.
Healthy eating habits also moderate obesity. However, this study discovered that healthy eating habits were not related to greenspace but rather to other demographic and quality of life (QoL) variables, such as education and age (Yuen et al. 2019). Again, this finding is consistent with the literature, and several factors are associated with food choices, including socioeconomic and demographic factors, and social mobility (Arruda et al. 2014).
Further research should be undertaken to investigate the implications of dietary patterns and physical activities on greenspace exposure to reveal a more solid relationship and determine causal pathways. Moreover, health interventions should target both health-promoting behavioural patterns (dietary habits and physical activity), as evidence suggests that individuals implementing one behaviour change are more likely to successfully implement another (Lippke et al. 2011). The features of the built environment should also be investigated, as walkability appears to be more strongly associated with obesity than with other urban or environmental amenities. Health interventions should consider temperature extremes as they are not conducive to outdoor physical activity.
4.3 Greenspace and Socio-economic Status (SES)
Evidence suggests that socioeconomic status (SES) is a significant indicator of obesity and food choice (Wham et al. 2015; Thorpe et al. 2019). There is also mounting evidence that greenspace exposure maybe “equigenic” (Mitchell et al., 2015), implying that those in the lowest socio-economic groups may experience the greatest benefits from greenspace exposure. This finding was reported by (Yuen et al., 2019), who identified that lower socioeconomic groups, such as the elderly, youth, and the less educated, appeared to benefit more from greenspace in their living urban environment. A similar finding was identified in a study by Maas et al., 2006), who reported that percentage greenspace within one and three kilometres radius was significantly related to improved self-perceived general health by individuals. This positive association was stronger for lower socioeconomic groups, including the elderly, youth, and second-level education. These results are consistent with those of Choi and Yoon (2020), who reported that SES has a negative effect on BMI, and identified that the impact of SES is much more significant on obesity than physical activity and access to environmental amenities. Similarly, Rigolon et al. 2021, discovered that greenspace has greater health benefits in low-income countries, and public greenspace has a greater protective effect for lower SES groups. There is a strong link between obesity and SES. According to a cross-sectional study conducted in the United States (Wen et al. 2017), obesity rates were found to be higher in rural areas, which was attributed to education level and median household income. Thorpe et al. (2019) found that a lower SES in education was a predictor of more deficient dietary patterns.
Other characteristics of greenspaces, including accessibility, quality, size, and facilities for certain activities, offer different opportunities for physical activity and other health benefits for individuals. Further research is necessary to determine the characteristics required for specific health outcomes. Individual user determinants are also crucial, including age, gender, ethnicity, and safety when using greenspace, and these should also be taken into account.
4.4 Limitations and Strengths of the study
The strengths of this study include the broad inclusion criteria used for greenspace and blue space and their effects on dietary outcomes to ensure that all relevant data were considered. Studies were not excluded based on the study design, type of greenspace, or measurement of exposure to greenspace. Consequently, a variety of greenspace exposures and outcomes were identified. However, this can also result in high heterogeneity across studies and difficulties in comparing and interpreting study results. Despite the inclusivity of this review, only a few articles which met the inclusion criteria were retrieved. Therefore, these findings should be cautiously interpreted. This review demonstrates the lack of research in this area, and further studies on dietary patterns, including food choices and dietary intake, should be undertaken.
Another limitation of this study was its methodological design. All included studies were observational, with a cross-sectional design being the most common. This review emphasises the need for longitudinal and experimental research in this area. In observational studies, there is a potential for bias, including selection bias, information bias, and confounding. In two of the studies used in this review (Michimi and Wimberly 2012; von Hippel and Benson 2014), BMI measurements for participants were based on self-reported heights and weights collected by the Behavioural Risk Factor Surveillance System (BRFSS), a telephone survey of more than 350,000 US adults per year. As this information is self-reported, there is a risk of over- and under-reporting, and compromised objectivity. To correct for self-reported bias in one study (von Hippel and Benson 2014) newly released bias-corrected estimates of county obesity prevalence were used.
Further bias can be introduced through confounding, and in two studies, quality appraisals (Michimi and Wimberly 2012; Yuen et al. 2019), it is unclear whether confounding factors were identified and strategies were implemented to deal with these. This can result in a biased estimate of the outcomes. A third study within the review (von Hippel and Benson 2014) identified that the associations reported in the study might be attributable to confounding factors that were not controlled for, and reverse causality is also possible. Due to complex associations and limited evidence on causal pathways for specific health outcomes in greenspaces, many confounding factors are possible. Strategies to avoid potential sources of bias should be adequately considered by careful planning, and future research should consider this.