Anorexia nervosa is more common among people born in the spring, a new study led by Oxford University scientists has found.
Most experts already believe there is a strong genetic component to the disorder, which mostly affects girls and women.
People with anorexia have a distorted body image and refuse to maintain a minimally acceptable body weight. Bulik said anorexics are about 10 times more likely to die in a given period of time than peers the same age.
Anorexia's rarity — slightly more than 1 percent of females and well under 1 percent for males — has made it hard for scientists to gather large groups of patients for study.
The researchers writing in the British Journal of Psychiatry say their study -- which is the largest to date -- provides 'clear evidence' of a season-of-birth effect in anorexia.
The research team, led by Dr Lahiru Handunnetthi of the Wellcome Trust Centre for Human Genetics at Oxford University, compared the birth dates of 1,293 patients with anorexia to those of the general population.
They found an excess of anorexia births between March and June, and a deficit from September to October.
Although some previous studies have suggested a link between season of birth and eating disorders, these involved much smaller numbers of patients and did not reach statistical significance.
'We meta-analysed four cohorts of anorexia nervosa patients from the UK, making this the largest ever study to assess the presence of a season-of-birth effect in anorexia,' said Dr Handunnetthi. 'We found that susceptibility to anorexia nervosa is significantly influenced by a person's season of birth, being higher in those people born in the spring and lower in those born in the autumn.'
Dr Handunnetthi added: 'A number of previous studies have found that mental illnesses such as schizophrenia, bipolar disorder and major depression are more common among those born in the spring -- so this finding in anorexia is perhaps not surprising.
'However, our study only provides evidence of an association. Now we need more research to identify which factors are putting people at particular risk.'
The researchers believe that environmental factors around the time of conception or when the baby is developing in the womb may be responsible.
Dr Handunnetthi explained: 'Seasonal changes in temperature, sunlight exposure and vitamin D levels, maternal nutrition and exposure to infections are all possible risk factors. Identifying these risk factors is important in helping us understand and maybe even prevent illness in future.'