We often refer to dogs as being part of the family, but that statement doesn't fully do them justice - not only are they part of the family, they're a unique, unwaveringly wonderful, part of the family.

Always happy to see you, there with a reassuring wag when you need it, they never judge or answer back, and somehow just being near them can lift the spirits.

The power of man's (and woman's) best friend to boost our mood and general wellbeing should not be underestimated, and they can be a lifeline for people experiencing serious depression - an illness currently in the spotlight following Robin Williams' recent tragic death.

"The research evidence is pretty clear that pets in general, and in particular dogs, seem to work in a number of ways to be helpful to human health," says Samaritans trustee Steve Platt.

"One is their actual involvement in therapeutic activity, whether that's helping children with autism or people with dementia. They also seem very sensitive to ill health in others and can help alert in situations like epileptic seizures or even cancer, being able to detect them at an early stage. And there have been many examples where dogs help people with depression, and the elderly. They can have a profound effect."

Heartwarming tails

This profound effect is at the heart of new book, My Dog, My Friend: Heart-Warming Tales Of Canine Companionship From Celebrities And Other Extraordinary People.

In it, a number of people, including some high-profile names, share personal tales about the roles their beloved pooches have played in their lives.

TV chef Antony Worrall Thompson writes: "It is well documented that in 2012 I had what can only be described as a mental breakdown. During that time I was grateful for the support of my family. And classed as family are my two faithful dogs.

"Walking them was a much-needed source of "strength and solace", he reveals.

Esther Rantzen talks about the joy and comfort a string of cherished mutts have brought her family over the years, from perking her up after a stressful day at work to helping them all through the dark days of grief following the death of her husband, Desmond, in 2000.

Also featured is Nanette Mutrie, professor of physical activity for health at the University of Edinburgh, who is a firm believer in the power of exercise for keeping us well, inside and out - but that doesn't have to mean pricey gym memberships and gruelling triathlons, regular dog-walking will work wonders too.

The brilliance of dogs doesn't end there either. This book - supported by Samaritans with all author donations being donated to the charity - is also a reminder that owning a pooch can make a difference to your all-important social life too.

"Dogs are a fantastic aid for socialisation," says Platt, who has written the book's foreword. "They encourage people to get out, and they also attract attention and help you get into conversation with others. For some people, taking their dog for a walk and having a brief chat with a fellow dog-walker might be the only interaction they have.

[Related story: How to stop the signs of clinical depression]

"They help structure the day, too. We all need structure and one of the problems that arises for people who are lonely, or living in isolation, is the collapse of structure which can have severe consequences. A dog can help provide that structure, in terms of feeding times and walking times."

No discriminator

Robin Williams had a history of depression and his suicide earlier this month is a reminder that mental illness really can impact anybody, and indeed it does, affecting around one in four people in the UK alone, with anxiety and depression the most common form.

"Not everyone with a mental illness will kill themselves," points out Platt, "but there is an elevated risk."

This risk can be as high as 15% for people with more severe forms of depression and bipolar disorder, but in the majority of cases, people who take their own lives will have had undiagnosed mental illness, or would have received inadequate treatment.

While this is obviously sad, it does also reassure us that treatments can be extremely effective - if they're given in time.

"But we've got to identify problems and then engage that person with treatment, that's the challenge," notes Platt.

My Dog, My Friend

Another challenge is raising awareness of mental health both within the health services and in the general public.

"I've been reading some of the comments made about Robin Williams' death and I've been struck by how much ignorance there is, and how many callous and insensitive remarks have been made," says Platt, who recently retired as professor of health policy research at the University of Edinburgh and is also Chair of Befriending Networks (www.befriending.co.uk), a charity set up to help combat loneliness. "Comments along the lines of, 'He should have pulled himself together and realised he had everything', and, 'Depression doesn't happen to tough guys'.

"Nothing could be further from the truth. Depression is very widespread and it would be a mistake to think anybody is immune."

Its burden, though, is "strongest and most felt in youth and old age". People are most likely to experience a first episode of depression during their teens, but this is also a time when the illness is most likely to be overlooked, perhaps dismissed as hormone-fuelled mood swings and teenage angst. The same applies to older age groups, whose health is often in danger of being ignored, deliberately or otherwise.

Depression can also creep up on people, making it tricky to spot. "With an acute psychiatric illness, where it's sudden and dramatic with clear symptoms, then it's clearer. But where it's a much slower onset, more insidious, it's not as easy to spot. People often tend to adjust and the family also adjust around it and then fail to see it," Platt explains.

Another key at-risk group are young men, who are also the group most likely to take their own lives. All men aged in their 30s, 40s and 50s are a concern, but those in lower socio-economic groups, or living in deprivation, tend to be most at risk. The 'tough guys don't get depression' myth may be part of the problem, contributing to the stigma around talking about mental health and seeking support.

"There are many kinds of treatments that can really help people though," Platt stresses. "But we've got to intervene in time to give those treatments a chance."