Exercising outdoors can be good for people's overall wellbeing, it has been suggested.
A study published in BMC Public Health looked at 25 previous research projects analysing the effect of exercising outdoors compared with working out indoors.
Overall the results suggested that natural environments can have a direct and positive impact on wellbeing.
However, there were calls for further research to be conducted before official guidance was issued to the public.
Peter Hier, editor of Running 4 Women, commented: "Physically [I don't think running in the gym] is as good as outdoors, and mentally there is no comparison with running indoors and outdoors.
"Obviously, if the weather is grossly adverse and you are training for a race then a treadmill is much, much better than nothing."
Mr Hier added that people considering running as a form of outdoor exercise should be careful to build up their ability gradually.
"Don't say 'I'm going to start with two miles' because you might find that two miles will take you 25 minutes, whereas ten minutes is probably enough to start with," he explained.
Independent advice on private healthcare