Loneliness can increase cancer risk, at least in rats, according to a new study published in Proceedings of the National Academy of Sciences.

Researchers found that social isolation of normally gregarious rats (and the resulting stress) led to a huge increase in both the number and the nastiness of tumours. They kept lab rats either alone or in groups of five. Those kept alone had a 135% increase in the number of mammary tumours, a 8,391% increase in the size of tumours and a 3.3-fold increase in the relative risk of malignancy.

“There is a growing interest in relationships between the environment, emotion and disease,” says Gretchen Hermes, of the Yale Department of Psychiatry (press release). “This study offers insight into how the social world gets under the skin.”

Other researchers have also shown that social isolation increases the growth of tumours. Earlier this year Suzanne Conzen, of the University of Chicago, and colleagues reported in the journal Cancer Prevention Research that keeping rats alone increased tumour growth and altered mammary gland gene expression.

However the link between stress and breast cancer in humans has not been conclusively established.

Meg Macarthur, from Breakthrough Breast Cancer, says (via the Daily Telegraph), “This extremely early stage research is based on a sample of 40 rats – 20 of which were isolated to identify the impact of stress on developing breast cancer. This very small study suggests that stress due to isolation might increase the number of breast cancer tumours; however, these findings cannot be directly translated to humans.”

When it is published, the paper will be available here.