The authors were Jo Barton and Jules Pretty (personal page at University of Essex here) and the study was published in the Environmental Science and Technology journal.
There are some bold claims, such as:
- "The biggest effect was seen within just five minutes."
- "A bigger effect was seen with exercise in an area that also contained water - such as a lake or river."
The authors state: "The research used meta-analysis methodology to analyze 10 UK studies involving 1252 participants." It doesn't say that it was a meta-analysis, only that it used the methodology (sic) of meta-analysis. Of course, you can only really meta-analyse studies with a degree of homogeneity and the conclusions one can draw from such reviews depend greatly on the quality of the underlying studies. Poorly-controlled and heterogenous studies with wild estimates of effect sizes don't lend themselves to robust systematic review.
Another alarm bell starts ringing when one reads, "Outcomes were identified through a priori subgroup analyses". Okay, the sub-groups were determined a priori, but subgroup analysis is often used as a way of mining the data for some significant finding. The BMJ has a recent article on the credibility of subgroup analyses.
It's surprising that the study also reported, "Dose responses for both intensity and duration showed large benefits from short engagements in green exercise, and then diminishing but still positive returns." This is counter-intuitive. If exercise is so good, why does it get less effective the more you do of it? I doubt the included studies were looking at elite athletes who may have been overtraining - the authors are reporting that more exercise is less beneficial for most of the reported outcomes (e.g. self-esteem).
It's clear that Jules Pretty is favourably disposed to exercise in green environments (who wouldn't be?), but previous 'reviews' would indicate that reported benefits in different studies are taken at face value, and not critically appraised in the way that they should be. For example, many studies in exercise for depression look at those with mild depressive illness which is, for many people, a self-limiting condition. The fact that someone feels better after a six-week course of exercise may have little to do with the exercise per se, and more to do with the natural course of the illness.
The 2009 Cochrane Review of Exercise for Depression concluded: "Exercise seems to improve depressive symptoms in people with a diagnosis of depression, but when only methodologically robust trials are included, the effect sizes are only moderate and not statistically significant." Essentially, when you exclude poorly-conducted trials, the effects of exercise are not statistically different from those that you would see with chance alone.
