British psychologists say that skinny people are more likely to be unhappy and commit suicide than those who are overweight.
For many, women in particular, being slim has long been promoted as a key element in sexual attraction, success and happiness.
But now research at Bristol University say they found that as a person's body mass index (BMI) rose, so the risk of serious depression fell.
The team examined, over a 16-year period, the ups and downs of more than a million lives and the results of their study will deliver a blow to the lucrative dieting business.
The study also lends credibility to the theory that the overweight really are a jollier bunch.
Their research showed that 3,000 individuals who committed suicide had a BMI significantly lower than those who did not kill themselves.
Professor David Gunnell, of Bristol University, says they were quite surprised as there is a view that people who are overweight may be stigmatised and made to feel depressed.
Professor Gunnell says their findings provide some support for the idea that fatter people are at a reduced risk of problems that lead to suicide.
The psychologists said the study took into account factors that could distort the results, such as socio-economic status.
Anti-diet campaigners are applauding the study and are claiming the research gives scientific backing to their views that dieting causes misery.
Joanne Roper of Hugs International says that dieting can bring people down and make them obsessed with their body image.
She claims the dieting rollercoaster brings great unhappiness to millions with a variety of sizes and metabolisms who will never be able to attain an industry-invented 'ideal' size.
Geri Halliwell, 33, a former Spice girl, who lost a huge amount of weight on a regime of exercise and starvation after leaving the band, has admitted to suffering years of unhappiness before coming to terms with her figure and regaining her curves.
She says she has learnt healthier ways of coping.
Sharp criticism was directed at UK television presenter Fern Britton recently when she declared she was 'a jolly size 16' and said diets don't make anyone happy.
This pan-European study by Bristol University has revealed that for each five point increase in BMI, the risk of suicide decreased by 15 per cent.
The scientists say they are unsure why why this happens, though some research has suggested that individuals with insulin resistance, a condition associated with being overweight, may have a reduced risk of depression and suicidal behaviour.
Another consideration is that the feel-good hormone, Seratonin, is associated with insulin and one of the main ways of treating depression is by increasing the amount of this hormone.
But the scientists warn those who suffer from depression not to intentionally increase their weight to combat it.