On the other hand, a man's diet tends to become healthier when he starts cohabiting with a female partner - and her influence has a long-term positive impact.
The reason for the change in dietary habits, say experts, is that both partners try to please each other during the 'honeymoon period' at the start of a cohabiting relationship, by adjusting their routine to suit their partner and eating food that he or she likes.
However, women have the strongest long-term influence over the couple's diet and lifestyle, mainly because the majority of female partners still assume the traditional role of food shopper and cook.
The report, by Newcastle University's Human Nutrition Research Centre, is published in the health professional title Complete Nutrition.
It reviews the findings of a variety of research projects from the UK, North America and Australia which looked at the eating and lifestyle habits of cohabiting heterosexual couples, including married couples.
The research shows that women are more likely to put on weight and increase their consumption of foods high in fat and sugar when they move in with their partner. Men, on the other hand, report a reduction in 'bad foods' when they begin to cohabit, reducing fat and sugar and increasing consumption of vegetables.
Women are also more likely than men to turn to food to deal with emotional stress in their relationship. Women have been found to gain weight when they quit their relationship, but the same finding has not been observed in men.
Yet the report also highlights one study where both men and women were found to put on weight after they started living together, which experts suggest could be due to changes in eating patterns and a tendency to make less time for exercise.