A walk in woodlands is said to help reduce stress levels
There's nothing new about doctors recommending their patients take more exercise. But what kind?
You could pay a fortune for gym membership, or you could trudge down to your local swimming pool and spend the rest of the day smelling faintly of chlorine.
But the best exercise of all might be the easiest and the cheapest: a stroll in the park, or a country ramble.
The secret ingredient? Greenery. Those of us who live in towns and cities, and even some who live in the countryside, don't get enough of it.
The result for most of us is highly stressful; we get irritable and depressed, and even physically ill (because high levels of stress mean higher risk of things like heart disease and diabetes).
Yet put us in contact with trees and grass and levels of stress fall away.
The notion that nature does you good is one of the themes of this year's Springwatch series on BBC 2.
Bill Oddie, one of the Springwatch presenters and an enthusiastic bird-watcher, suffers from depression. He has no doubt that contact with nature helps him.
"I know I'm really in trouble when I don't want to go outside and I can't bring myself to do it," he says.
Bill Oddie: "Get out there in the fresh air, get yourself moving"
"I've had three clinical depressions, which means going into hospital, and that's the stage where I know nothing's going to help.
"But when you get a downer, and lots of people suffer from this, there is no question, every self-help book, every doctor, every therapist will tell you: get out there in the fresh air, get yourself moving. It's to do with fitness, it's also to do with a meditational thing."
Scientific support for Bill's beliefs comes from Dr William Bird, who combines a career as a GP with a part-time role as health adviser to Natural England.
Last year he produced a report for Natural England and the RSPB arguing that contact with nature and green space has a positive effect on mental health, especially among children.
Some have gone further still. An American journalist, Richard Louv, author of Last Child in the Woods, coined the term "nature deficit disorder" (an echo of the medically-established condition, attention deficit disorder) to describe the deprivation, sometimes amounting to mental illness, of children who grow up without contact with the natural environment.
"Nature deficit disorder" is not a condition the medical profession recognises, though common sense suggests that children who take virtually no exercise and rarely get into the great outdoors are unlikely to be healthy and are missing out on a lot of pleasurable experiences.
Behind the scenes at BBC One's Springwatch programme
Dr Bird is urging his fellow GPs to prescribe regular walks and exercise in green spaces for patients suffering from heart disease, depression, obesity and the like.
Referring patients to the natural environment rather than the pharmacist is a lot cheaper than conventional pills and prescriptions and, he argues, is likely to be just as effective in many cases.
But haven't we always known that contact with nature was good for us? Yes, says Dr Bird.
"But we kind of lost it when we got clever with our science. As soon as we got antibiotics and we got technical, we felt we didn't need all that green stuff.
"Now we've realised all that technical stuff can treat you, but we also need the greenness to provide a backdrop for preventing ill-health and for healing."
There's some evidence that patients themselves are willing to go along with the idea.
The results of a Mori poll, commissioned by Natural England and released exclusively to the BBC, show that 94% of us would be happy for our GP to provide outdoor exercise instead of prescription drugs, if he or she thought it would work.
Natural England has already established jointly with the British Heart Foundation a network of "walking the way to health" initiatives. Many areas now have "walk and talk" or "health walk" schemes, run by volunteers who encourage local people to gather regularly for walks ranging from gentle rambles to more demanding hikes.
A growing numbers of doctors are recommending walks to their patients
Some GPs' practices are already prescribing exercise as an alternative to drugs.
The Culm Valley Integrated Centre for Health in Devon is one. A partnership of a dozen GPs, it occupies a splendid new architect-designed building, more like an old-fashioned cottage hospital than a conventional GP practice.
The Centre's symbol is "the Green Man", a kind of medieval nature spirit: outside the building is a herb garden; inside they offer complementary medicine as well as conventional clinical consultations.
We met one patient, Roger Cowley, who'd been suffering from obesity and depression and had been effectively confined to bed.
He's been given a "stepometer" that counts the number of paces he takes each day and receives help from Ruth Tucker, an exercise adviser working with GPs.
His eventual target is to take 10,000 steps a day, walking in the fields around his home. So far, he told me, he's up to 4,000 or 5,000.
"I think we've lost contact with our environment, and when you become de-rooted you become alienated, and that's part of becoming unhealthy," says Dr Michael Dixon, one of the partners.
"We know that the most sustainable treatment for depression is exercise, not anti-depressants: a year later people who take exercise are still improving, when with anti-depressants the effects are gone."
Other doctors may take some persuading to prescribe a walk in the country.
Natural England polled 70 GPs and nurses and found that 61% recommended that patients use green space, and 79% recommended walking informally.
But that still left a sizeable minority who didn't.
Springwatch, with Bill Oddie and Kate Humble, will be broadcast on BBC TWO for three weeks from Monday, 26 May 2008 at 2000 BST
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