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.
A key reason for the change in dietary habits is the symbolic nature that food assumes in a relationship. Many cohabiting couples reported food as being central to their partnership, and eating together in the evening was an ideal sought by many.
The report's author, Dr Amelia Lake, a research fellow with Newcastle University's Human Nutrition Research Centre and a registered dietitian, said the research findings highlighted major health issues which couples needed to address as a team early on in the relationship.
The research findings confirm work by Dr Lake which found more men than women found their partner to be a positive influence on their diet, in terms of encouraging them to eat more fruit and vegetables, eating regular meals and taking control of their food shopping and preparation.
Dr Lake said: "You can't just blame an unhealthy lifestyle or diet on your partner, as there are many other things that affect what you eat and do. However, research has shown that your partner is a strong influence on lifestyle and people who are trying to live healthier lives should take this factor into consideration.
"Couples who move into together should use the opportunity of the honeymoon period to make positive changes to their diet and lifestyle by working together and supporting each other.
"But couples who have been in their relationships for longer should remember that it is never too late to make changes and again this needs team work."
INTERVIEWS: Dr Amelia Lake, research fellow, Newcastle University's Human Nutrition Research Centre.
Tel: +44(0) 191 222 3828. Email: Amelia.firstname.lastname@example.org
Personal web page: http://www.
Complete Nutrition: Lynne Garton, editor and state registered dietitian, +44(0) 1992 538 001 email@example.com
Copies of the full report from Complete Nutrition can be obtained in pdf format, via email, from Newcastle University press office.
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Human Nutrition Research Centre: http://www.
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