“Loneliness can make cancer ‘more likely and deadly’”, according to BBC news. The news service says new research found that isolation and stress trebled the risk of breast cancer in the “naturally sociable” Norway rats.
This story is based on research conducted at the University of Chicago. It took rats that were predisposed to developing breast cancer tumours and looked at the effect of rearing them in groups or in isolation. The scientists then measured the rats’ stress levels plus the number and severity of any tumours they developed.
As this was an animal study it cannot be directly related to humans. Also, isolation in rats is likely to be very different to human loneliness, and the complex effects it may have on the human body. This study cannot show that human loneliness increases the likelihood of cancer or that it causes cancer to be more severe.
Where did the story come from?
This research was conducted by Dr Gretchen Hermes and colleagues from the University of Chicago. The research was funded by a number of US institutions including the National Institute of Environmental and Health Sciences, the National Cancer Institute and the Department of Psychiatry, Yale University. The study was published in the peer-reviewed medical journal Proceedings of the National Academy of Sciences.
The media did report that this research was in animals, but generally exaggerated its direct relevance to humans.
What kind of research was this?
This was an animal study comparing the development of tumours in rats reared in groups or kept in isolation. The research aimed to test the theory that social isolation is associated with abnormal hormonal and behavioural responses to stress, and that these responses may contribute to the development of tumours.
This was a relatively small animal study and its direct relevance to humans is limited.
What did the research involve?
The researchers selected a strain of rats that was genetically predisposed to developing tumours. At 21 days old the rats were split into two groups: 20 rats to be reared alone in separate cages and 20 rats to be reared in four groups of five rats per cage.
The researchers checked the rats twice a month for 15 months, assessing the location of any tumours, counting the number of tumours and estimating the weight of the tumours by feeling their size (tumour burden).
The rats’ behaviour was assessed on two occasions: firstly when they were young (one to three months) and had not yet developed tumours, and once again when the rats were middle aged (13 months) and had already developed tumours. As part of the behavioural testing the researchers observed how the rats responded to stress by exposing them to the smell of fox urine. They then measured the amount of stress hormone corticosterone they produced in response to the predator’s scent.
The rats died at 18 months on average. The researchers then performed post-mortem analyses of the types of tumours that the rats had developed. They specifically looked at the number of hormone receptors in the tumours, which is a measure of the amount of hormone present in a tumour.
What were the basic results?
By middle age (13 months) 74% of rats from both groups had developed a tumour, meaning the risk of developing at least one tumour did not differ between the groups. However, the socially isolated rats had an 84% greater tumour burden than the rats reared in groups. Their tumours were also more widespread.
When the researchers assessed whether the tumours were carcinomas (cancerous tumours) they found that the risk of having at least one carcinoma was around three times greater in the isolated animals. Overall, isolated animals had a 50% incidence of cancerous tumours compared to 15.4% in the grouped animals.
The researchers found that the isolated rats had a greater increase in corticosterone stress hormone levels than the group-reared rats when exposed to the stress-inducing fox scent. This was the case both before tumours had developed and at 13 months. The researchers found that the receptor for the corticosterone stress hormone was present in the carcinomas. The researchers also found that the isolated rats showed more typical traits of stressed behaviour than the group of reared rats.
How did the researchers interpret the results?
The researchers conclude that their study shows that “female rats living in social isolation from puberty through late middle age become progressively more reactive to superimposed stress”. They also say social isolation increased the size, number, distribution and malignancy of spontaneous mammary tumours, illustrating how psychological factors can change the way inherited genetic risks are expressed.
The research found an association between isolated rearing, increased stress responses and increased tumour burden in rats that were genetically predisposed to tumours.
Although the animal study was well conducted it shows an association rather than a direct causal relationship between corticosterone levels and increased likelihood of malignant tumours.
Also, while rats and humans are both social animals their social dynamics clearly differ. The stress factors used in this experimental work are not relevant to modelling how human social interactions may affect risk of breast cancer, and it is not clear how relevant changes in corticosterone hormones are to the development of cancer in humans.