Moving in with a boyfriend causes women to eat more unhealthily and put on weight, according to a study.
But the opposite is true for men, whose long term health benefits when they move in with a female partner.
Dieticians at Newcastle University said both part-ners try to please and impress one another during the "honeymoon period", and so change their dietary habits to suit their other half.
It leads men to eat more light meals, such as salads, fruit and vegetables, while women chose to make creamier, heavier dishes like curry or rich pasta sauces, which may please their partner.
Women still have the strongest long-term influence over the couple's diet and lifestyle, as they still have the traditional role of shopper and cook in most households.
The report, by Newcastle University's Human Nutrition Research Centre, is published in the health professional title Complete Nutrition.
It reviewed 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.
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 also use food as a comfort when dealing with emotional stress and have been found to gain weight when a relationship ends.
The researchers also found one study which showed both men and women put on weight when they began living together, suggesting both partners made less time for exercise.
A key reason for the change in dietary habits is the symbolic nature that food assumes in a relation-ship, the study said.
Many couples reported food as being central to their partnership, and eating together in the evening was particularly important.
Report author and regis-tered dietician Dr Amelia Lake said it highlighted major health issues which couples needed to address as a team early on in the relationship.
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.
"Research has shown that your partner is a strong influence on lifestyle and people who are trying to live healthier lives should take this into consideration.
"Couples who move in to 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 teamwork." ..SUPL: