On a February day in 2003, I walked into my GP's surgery and announced that if she didn't help me I would probably be dead by the end of the week. I wasn't sure, even then, what exactly I needed help for; only that I had reached a place of such bleakness and despair that I couldn't go on. I had lost any sense that my life was worth living, or that I would ever feel differently; simply being conscious, getting through the day, had become unbearable. I couldn't cope with the smallest decisions; often I didn't eat because the effort of deciding what I wanted and then preparing it seemed as daunting as running a marathon. I felt I had long ago used up all my inner resources; for weeks I had thought of nothing but how I could put an end to this empty pretence of a life.

I felt very stupid and ashamed in going to my doctor; I wasn't sure, even then, that my experience legitimately counted as an 'illness'. I thought she might tell me to get a grip and stop wasting her time; plenty of people had it much worse. I felt that I should have been able to pull myself out of it and piled guilt and blame on myself because I could not. Five years on, I can now see clearly that I was suffering from severe postnatal depression combined with chronic stress; having consulted psychiatrists and psychologists, I also understand that it formed part of a continuous pattern of extreme mood swings, including several previous episodes of depression, that had been part of my life since my late teens.

But at the time 'depression' was a distant concept to me; I knew it existed, because I had read about Sylvia Plath and Virginia Woolf, but I thought it was a condition that belonged to dead artists. I didn't equate it with my episodes of despair, which I suspected were due to some character flaw or weakness on my part, a failure to ride over the potholes of life with the necessary British pluck and fortitude. I certainly didn't talk to anyone about it, because I thought it would sound like a terrible admission of failure; instead I worked hard to hide it from everyone who knew me. People thought of me as strong and competent and I didn't want to disappoint them.

We are not comfortable talking about depression in our culture, which places so much emphasis on success and on the belief that achievement equals happiness. Collectively we have not made it easy to admit to depression, and as a result there are people who die each year by their own hand when they might have been helped, had they only known how to ask. According to the most recent study by the Office of National Statistics, one in six people in the UK suffers from 'significant' mental distress at any one time, of which the most common condition by far is mixed depression and anxiety. The Depression Report, a study published in 2006 by the Mental Health Policy Group at the London School of Economics, estimated that only a quarter of those who experience depression go on to receive treatment.

But it seems that this culture is gradually beginning to change. Over the past couple of years, a new openness has begun to replace the stigma that previously surrounded mental illness. In 2007, five of the leading mental health charities incorporated the findings of the Depression Report into the nationwide We Need To Talk campaign, successfully lobbying for greater investment in evidence-based talking therapies, such as cognitive behavioural therapy, for NHS patients with depression. In 2006, Stephen Fry did for manic depression what Kylie has done for breast cancer with his insightful and frank documentary series, The Secret Life of the Manic Depressive, giving many people greater confidence to talk about their illness.

When I was asked last year by Random House to write a book about my experience of depression, I had no idea that two other books on the same theme, also by women journalists, would be published at the same time, but it is not surprising. Depression is on the rise, and it seems women are especially vulnerable. Thirteen years after the publication of Elizabeth Wurtzel's Prozac Nation, the original bestselling account of depression by a young woman, 2008 sees a resurgence of the female depression memoir.

Novelist and former Elle editor Sally Brampton's Shoot the Damn Dog (Bloomsbury), which comes out later this month, recounts her experience of a severe, two-year clinical depression that included one suicide attempt, several admissions to psychiatric units and endless cocktails of antidepressant drugs before she finally began the slow climb back to normality. The Observer's Scotland editor Lorna Martin writes about her year in therapy in Woman on the Verge of a Nervous Breakdown (John Murray, April). Though she didn't experience the extreme symptoms of clinical depression, Martin felt overwhelmed by the demands and stresses of modern life and writes feelingly of the benefits of 'talking cures' even when you're not about to jump in front of a train.

My own memoir, The Devil Within (Vermilion, May), is a retrospective account of recurring depression, intermittent highs and the eventual process of learning to manage my condition without medication (at 32, I was diagnosed with a form of bipolar disorder, or manic depression, a diagnosis which has helped enormously in understanding what happens to me and how best to treat it).

Most striking about our three books is that, contrary to the current trend of 'misery memoirs', we all appeared to be exemplary career women with enviable lives. We were successful journalists, Brampton and I were both working mothers, we all had good friends, were reasonably solvent and appeared to be doing well. The same experience is echoed by former Cosmopolitan and SHE editor Linda Kelsey, who published a fictionalised account of her clinical depression last year, Fifty is Not a Four Letter Word. Common to all our stories is the refrain that we felt fraudulent in being depressed, that we were aware how little we had to complain about, that we had, as Brampton has written, 'no reason to be sad'. Coupled with this was the feeling that we had to keep up appearances, that we mustn't let people down.

According to the most recent studies, women in the UK are twice as likely as men to suffer from depression, and figures from the National Institute of Mental Health in the US show the same ratio. One recent study showed the same ratio across nine other countries, regardless of ethnic background or economic status (though bipolar disorder, thought to have a stronger genetic link, shows fairly equal incidence between the sexes).

Various explanations have been suggested, most commonly that women are more used to talking about their emotions and more likely to seek help for depression, while men hide theirs in alcohol abuse or overwork, so the statistics are inherently inaccurate. But this theory seems counterintuitive when the accounts above suggest that modern pressures on women to prove that they can juggle careers, relationships, motherhood and still look 10 years younger make it less likely that we will admit to depression, something most of us still privately regard as shameful, a failure to cope with life.

New research suggests that the answer may lie in brain chemistry. Evidence suggests that serotonin, the neurotransmitter that regulates our moods and emotional processing, operates differently in men and women. (Low levels of serotonin are closely associated with mood disorders.) According to Catherine Harmer, research fellow at the Department of Psychiatry at Oxford University, it seems that the serotonin system in women is especially vulnerable. 'There are definite biological contributors to low serotonin in women,' she says. 'The serotonin system appears to be more sensitive in females, perhaps because of differences in responses to stress hormones and other hormonal fluctuations.'

The brain makes serotonin from the amino acid tryptophan, which we derive in small doses from food; when women diet, they experience a much greater drop in serotonin levels than men. The low-carbohydrate diets so popular among young women are especially dangerous. It seems women can't win - we're miserable because advertising and fashion tell us we're not thin enough, so we diet, our serotonin levels plummet and we end up properly depressed.

The good news is that research also shows that women respond well to tryptophan supplements; a study from Oxford's Department of Psychiatry showed that they 'induced a positive bias in emotional processing' among healthy female volunteers.

In other words, the supplements reversed the negative thought processes associated with depression, a similar effect to that of selective serotonin re-uptake inhibitor (SSRI) antidepressants such as Prozac or citalopram. Harmer stresses, though, that the Oxford study was carried out on non-depressed volunteers and that there is not yet sufficient evidence from clinical trials on patients to draw conclusions, but that this is an area for further research.

I have been taking 5-HTP, a tryptophan supplement available from health food shops, for almost two years as part of a programme of nutritional medicine and in that time I have not experienced a return of that severe depression (though I still have fluctuations of mood, they have been nothing like as extreme). Nor have I suffered the distressing side-effects I had with SSRI antidepressants, as 5-HTP is a natural substance.

Antidepressant medication is undoubtedly highly effective for many people, but for others it can cause as many problems as it solves and there is concern among some experts that it is vastly over-prescribed - the most recent figures show that in 2006, 31 million prescriptions were issued for antidepressants in the UK, a rise of 6 per cent on the previous year. Nutritional medicine, by contrast, is safe and effective when prescribed by a qualified nutritionist, but the absence of large-scale clinical trials means that most evidence remains anecdotal or restricted to small samples and it is likely to be some time before it finds its way into mainstream treatments.

If the purpose of increased public acknowledgement and discussion of depression is to encourage more people to seek help before they find themselves, as I did, in real danger of irreversible harm, it should not be simply so they all end up on medication. By contributing to that conversation I hope that my book, and those of my fellow sufferers, will encourage people to approach their doctor, to ask questions, to try different treatments and, above all, to understand that their depression is an illness to be cared for and treated, not a personal failing to be ashamed of and concealed.

Facts and figures

11.2 Percentage of women who suffer from depression

300,000 Number of under-35s claiming incapacity benefit because of 'mental and behavioural disorders'

20 Percentage of people with depression during their life

£100bn Cost of mental health problems each year

31m Prescriptions for antidepressants in the UK in 2006

1 in 6 People with 'significant' mental distress at any one time

3 Number of times more likely it is for a man to commit suicide than a woman

40 Percentage of those suffering with depression who never consult their GP

121m People affected by depression worldwide

2 in 5 Elderly people in care homes suffer depression

400 per 100,000 population - rate of self-harm in the UK

2 Percentage of under-12s affected by depression

5 Percentage of teenagers affected by depression

5,206,044 Calls to the Samaritans in 2006

70 Percentage of the prison population that has two or more mental health disorders

Caroline White

Sources: Depression Alliance; World Health Organisation; Samaritans; Office of National Statistics; Mind