It's official! Marriage makes you gain weight

Married couples tend to eat better than singles but they weigh more and are less likely to do sport, according to a new study conducted in nine European countries.

Numerous studies have shown that marriage is good for health. However, researchers from the University of Basel in Switzerland and the Max Planck Institute for Human Development in Berlin, have found that does not apply to all health indicators.

They found that married couples on average eat better than singles, but that they also weigh significantly more and do less sport. The researchers compared the relationship between marital status and body mass index, which relates body weight to height. A high body mass index can be a risk factor for chronic illnesses such as diabetes or cardiovascular disease.

The researchers drew on representative cross-sectional data from 10,226 respondents in Austria, France, Germany, Italy, the Netherlands, Poland, Russia, Spain, and the UK.

Beyond their focus on married couples, the researchers conducted additional analyses with cohabiting couples. They also examined possible reasons for weight gain by asking respondents about their eating and exercise behaviours. Findings from all nine countries showed that couples have a higher body mass index than singles – whether men or women.

The differences between countries were surprisingly small. "Our findings show how social factors can impact health.

In this case, that the institution of marriage and certain changes in behaviour within that context are directly related to nutrition and body weight," said Ralph Hertwig, Director of the Center for Adaptive Rationality at the Max Planck Institute for Human Development.

In the study, couples reported buying more regional and unprocessed products and less convenience food. Moreover, married men were more likely than single men to buy organic and fair trade food.

"That indicates that particularly men in long-term relationships are more likely to eat more consciously and, in turn, probably more healthily," said lead author Jutta Mata, assistant professor of Health Psychology at the University of Basel. The study shows that married men are less involved in sports than singles. "Our findings indicate that couples are not healthier in every respect, as has previously been assumed," said Mata.

The study was published in the journal Social Science and Medicine.