Women who are married or live with a partner are less likely to die of heart disease than those who live alone, according to a major study which suggests that the health of women, as well as men, may benefit from living as a couple.

It has been known for some time that being in a long term relationship is good for men's health, but the impact on women has been unclear. The Million Women Study, based at Oxford University, set out to examine this from its massive database on the health and lifestyle of 1.3 million middle-aged women, recruited between 1996 and 2001 and followed up since. Among its other past achievements, it has identified the main lifestyle factors behind breast cancer.

The researchers, whose new paper is published in the journal BMC Medicine, set out to discover whether being married or living with a partner was as good for women as it is for men.

They looked at more than 730,000 women from the study who did not have any previous heart disease, stroke or cancer when they enrolled in the study. Over the next eight or nine years, more than 30,000 of them developed heart disease and 2,000 died from a heart attack or heart failure as a result of coronary heart disease.

Living with a partner or living alone made no difference to whether women developed heart disease. But it did make a difference to their chances of dying from it. Three in every 100 married women with coronary heart disease died, compared with four in every 100 of those who lived alone, whether they were single, divorced or widowed. That amounts to a 28% lower risk for women living in a couple.

Most women do live with a partner – 81% of those involved in the study. There were some differences in their lifestyles. Those who were married or with a partner were less likely to live in deprived areas and less likely to smoke or be physically active. They were more likely to drink more heavily, though.

But once those things had all been taken into account, there was still an unexplained benefit for women who were part of a couple. "It doesn't appear to be due to a lower risk of developing heart disease," said Sarah Floud, lead author of the paper. Instead, it could be the support they receive from their spouse or partner.

The researchers were not able to investigate in the study what form this support might take . One possibility is that women might be urged by their partner to go to the doctor sooner if they have symptoms of illness. Another is that a partner's support might encourage women to improve their lifestyle, by for instance taking more exercise, and stick to any treatment the doctor put them on. "They may have a spouse who will come with them to their appointments or encourage them to stick to their regime, but they weren't asked about that," said Floud.