Summary of Strength of Evidence, Stratified by Health Outcomes and Behaviors (n = 343)
| Health Outcomes . | Study Designs (%) . | Findings . | Strength of Evidencea,b . | |||
|---|---|---|---|---|---|---|
| RCTs . | Exp . | XS . | Long . | |||
| Physical activity (n = 108) | 2 | 18 | 82 | 6 | n = 71 studies found positive (health promoting) associations, n = 33 found mixed or null results, and n = 3 negative associations were reported. Most studies (n = 45) evaluated residential green space, n = 25 examined general greenspace activities. n = 59 (55%) used objective accelerometry based measurement to assess physical activity. | High (−) |
| Cognitive, behavioral, and mental health (n = 85) | 5 | 31 | 40 | 9 | n = 71 studies found positive associations, n = 12 found mixed and null results, and n = 2 negative associations were reported. Most studies (n = 34) evaluated residential green space, and most of the wilderness exposure studies (n = 8) reported on cognitive, behavioral, and mental health outcomes. This category had the largest proportion of experimental studies (37%) and RCTs (6%). | High (−) |
| Overweight/obesity (BMI) (n = 45) | 2 | 2 | 38 | 3 | All studies included BMI or BMI-defined categories for adiposity status (overweight/obesity); most used objectively measured height and wt to calculate BMI percentiles. n = 16 studies found positive associations, the majority (n = 27) found mixed or null results (n = 19 null), and n = 2 negative associations were reported. Most studies (n = 34) evaluated residential green space. | Moderate (−) |
| Academic and learning (n = 27) | 2 | 6 | 19 | 0 | n = 22 studies found positive associations, n = 4 found mixed or null results, and n = 1 negative association was reported. Most studies (n = 12) evaluated school green space proximity, and n = 6 examined residential green space proximity. | Low (+) |
| Asthma and allergy (n = 26) | 0 | 0 | 25 | 1 | n = 13 studies found positive associations, n = 5 found mixed or null results, and n = 8 reported negative associations. With the exception of a single study evaluating school green space proximity, all evaluated residential green space proximity (n = 25). Health outcomes (eg, conjunctivitis, wheezing, prevalent asthma, etc) and allergen measures (eg, trees, grasses, other flora specific to geographical regions such as the Euro-Siberia, etc) were vastly heterogenous. | Low (+) |
| Cardiovascular and metabolic (n = 16) | 3 | 4 | 8 | 1 | n = 7 studies found positive associations, n = 9 found mixed/null results, and no negative associations were reported. All studies (n = 16) evaluated physical outcomes, the most common being blood pressure (n = 8). Other outcomes included heart rate or heart rate variability (n = 3), insulin or glucose measures (n = 3), and blood lipids (n = 1). | Low (+) |
| Other (n = 33) | 5 | 7 | 19 | 2 | n = 17 studies found positive associations, n = 15 found mixed/null results, and n = 1 negative association was reported. Nearly all studies (n = 13) evaluated residential proximity to green space; many of the gardening studies (n = 6) were found in this category as well. Some of the more novel and compelling recent experimental studies were reported here (eg, gut microbiome, immune function, etc). | N/A |
| Health Outcomes . | Study Designs (%) . | Findings . | Strength of Evidencea,b . | |||
|---|---|---|---|---|---|---|
| RCTs . | Exp . | XS . | Long . | |||
| Physical activity (n = 108) | 2 | 18 | 82 | 6 | n = 71 studies found positive (health promoting) associations, n = 33 found mixed or null results, and n = 3 negative associations were reported. Most studies (n = 45) evaluated residential green space, n = 25 examined general greenspace activities. n = 59 (55%) used objective accelerometry based measurement to assess physical activity. | High (−) |
| Cognitive, behavioral, and mental health (n = 85) | 5 | 31 | 40 | 9 | n = 71 studies found positive associations, n = 12 found mixed and null results, and n = 2 negative associations were reported. Most studies (n = 34) evaluated residential green space, and most of the wilderness exposure studies (n = 8) reported on cognitive, behavioral, and mental health outcomes. This category had the largest proportion of experimental studies (37%) and RCTs (6%). | High (−) |
| Overweight/obesity (BMI) (n = 45) | 2 | 2 | 38 | 3 | All studies included BMI or BMI-defined categories for adiposity status (overweight/obesity); most used objectively measured height and wt to calculate BMI percentiles. n = 16 studies found positive associations, the majority (n = 27) found mixed or null results (n = 19 null), and n = 2 negative associations were reported. Most studies (n = 34) evaluated residential green space. | Moderate (−) |
| Academic and learning (n = 27) | 2 | 6 | 19 | 0 | n = 22 studies found positive associations, n = 4 found mixed or null results, and n = 1 negative association was reported. Most studies (n = 12) evaluated school green space proximity, and n = 6 examined residential green space proximity. | Low (+) |
| Asthma and allergy (n = 26) | 0 | 0 | 25 | 1 | n = 13 studies found positive associations, n = 5 found mixed or null results, and n = 8 reported negative associations. With the exception of a single study evaluating school green space proximity, all evaluated residential green space proximity (n = 25). Health outcomes (eg, conjunctivitis, wheezing, prevalent asthma, etc) and allergen measures (eg, trees, grasses, other flora specific to geographical regions such as the Euro-Siberia, etc) were vastly heterogenous. | Low (+) |
| Cardiovascular and metabolic (n = 16) | 3 | 4 | 8 | 1 | n = 7 studies found positive associations, n = 9 found mixed/null results, and no negative associations were reported. All studies (n = 16) evaluated physical outcomes, the most common being blood pressure (n = 8). Other outcomes included heart rate or heart rate variability (n = 3), insulin or glucose measures (n = 3), and blood lipids (n = 1). | Low (+) |
| Other (n = 33) | 5 | 7 | 19 | 2 | n = 17 studies found positive associations, n = 15 found mixed/null results, and n = 1 negative association was reported. Nearly all studies (n = 13) evaluated residential proximity to green space; many of the gardening studies (n = 6) were found in this category as well. Some of the more novel and compelling recent experimental studies were reported here (eg, gut microbiome, immune function, etc). | N/A |
Note that n = 343 (greater than original study n = 296) is used because studies were included more than once if they included health outcomes in >1 category. Exp, experimental study; HOMA-IR, homeostatic model assessment for insulin resistance; Long, longitudinal study; N/A, not applicable; XS, cross-sectional.
Strength of evidence as rated by author consensus on a 7-point scale: low, low+, moderate-, moderate, moderate+, high-, high (see Methods, Strength of Evidence).
Other category not summarized because of high variability in nature exposures.