A third of women said they ended up eating more after moving in with their partner, often putting on weight as a result.
A study of 1,300 men and women in the UK carried out by Centrum Multivitamins found that a third of men became thinner after moving in with their partners as they tended to reduce their food intake.
Many said they had switched to foods they considered more 'womanly', such as yoghurt and salad, the Daily Mail reported.
Nutritionists say couples who prepare food together often do not take into account their different dietary needs, dishing out equal portions of the same meals.
The survey found men and women often wrongly fed each other according to gender stereotypes, with 90 per cent of women saying they were most likely to purchase meat when shopping for their partner.
Nutritionist Fiona Hunter said a 'one-size-fits all approach' can be detrimental to both partners' health, partly because the sexes need different vitamins.
"Men need more of the different B vitamins than women but their knowledge of these vitamins, and where to get them, is also very low," she said.
The study found that overall nutritional awareness among UK men and women is poor with only half of women recognising that folic acid played a crucial part in pregnancy while few men were able to say which food contained vital vitamins.