"Dogs could be a heart's best friend," reports The Times, after 2 studies showed people who own a dog seem to live longer than those who do not own one.
The possible beneficial effect – thought to be from a combination of increased physical activity and social support – seemed stronger in people who lived alone with their pet.
One study looked at length of life overall, while the other looked at length of life after a heart attack or stroke.
We know that physical activity, such as walking, can help people recover from a heart attack or stroke. Owning a dog may encourage people to be more active by taking their dog for a walk.
Social support is also important for health, especially when recovering from an illness. A dog provides companionship and may be a way of getting to know other dog owners in the neighbourhood.
However, the studies do not prove that dog ownership has a direct effect on health, only that it seems to be linked to longer life. While people who enjoy the company of dogs may benefit from exercise and companionship, the study does not show that people who are happily social and active without dogs should get one.
The story comes from 2 studies.
One is a review of previous studies, carried out by researchers at the University of Toronto and Mount Sinai Hospital in Canada, with no specific funding.
The other is a new study from Sweden, carried out by researchers at Uppsala University and the Swedish University of Agricultural Science. It was funded by the Agria Research Foundation and the Swedish Research Council for Environmental Agricultural Sciences and Spatial Planning.
The studies were reported with enthusiasm by the UK media, with The Times enjoying a pun about patients being prescribed "woof-arin" instead of warfarin. For the most part, The Times and The Telegraph presented the results accurately and in a balanced fashion.
Systematic reviews are a good way to establish the state of evidence about a subject. Cohort studies are a good way to examine links between factors (such as dog ownership) and outcomes (such as length of life), but they cannot prove that one causes the other.
The Canadian study
The systematic review looked at prospective cohort studies which included information about dog ownership at the start of the study, and information about how long participants lived.
The researchers pooled the information in a meta-analysis to see whether people who owned dogs were more or less likely to live longer than those who did not. They looked at death from any cause and at death from cardiovascular causes. They graded the studies according to their quality.
The Swedish study
They looked at whether people were alive by the end of the study, and whether they were registered as a dog owner. It has been mandatory in Sweden since 2001 for dog owners to register their pet via a dog registration scheme.
They calculated whether people with dogs were more or less likely to have survived to the end of the study. The researchers adjusted their figures to take account of the following potential confounding factors:
The Canadian study
The meta-analysis found 10 studies that fitted their criteria, including 3.8 million people with an average follow-up time of 10 years. Dog owners were less likely to have died during follow-up than people who did not own dogs:
The Swedish study
The Swedish study had similar results. After an average 4-year follow-up, it found:
For context, overall 38.1% of people died after a heart attack and 43.5% died after a stroke.
The figures were more striking for people living alone with a dog. For this group there was a 33% drop in chance of death after heart attack (aHR 0.67, 95% CI 0.61 to 0.75) and a 27% drop in chance of death after stroke (aHR 0.73, 95% CI 0.66 to 0.80).
Both sets of researchers said their results seem to show an improvement in survival time for people who own dogs.
The Canadian researchers said: "Our meta-analysis suggests that dog ownership warrants further investigation as a lifestyle intervention given the positive association with longer survival."
The Swedish researchers were more cautious, saying: "The associations found in the present study may represent a causal relationship of dog ownership with survival after acute myocardial infarction or ischaemic stroke."
Dog owners will probably not need to be told that owning a dog can be good for your physical and mental health, whether it is by encouraging you to go out for a walk, or simply by providing emotional and social support.
However, we should not get carried away with the study results. It may be that there are differences between people who do and do not own dogs, which account for some of the difference in length of life. We can see this in the differences between the results of the 2 studies.
The Swedish researchers adjusted the figures to take account of factors including the age, health and income of people who have dogs. They found dog owners tended to be younger, were more likely to have children at home, and were more likely to be higher income earners.
The Canadian study, which found stronger effects of dog ownership, did not take potentially confounding factors into account. That may be why the Canadian research showed a stronger link between length of life and dog ownership.
Plus, the Swedish study may have missed other factors that account for some of the difference. For example, dog owners may be less likely to smoke, or more likely to eat a healthy diet.
We cannot say for sure that owning a dog makes people live longer. It is also noteworthy that dog owners were younger when they had the heart attack (on average 64 versus 71) or stroke (67 versus 73) which goes against the theory that having a dog means you are healthier.
What we do know is that doing exercise and having social support and companionship are likely to help you live a longer and healthier life. Whether you do that through having a dog, or by other means, may be less important.