The key finding: Young couples who marry or move in together are more likely to report gaining weight than those who stay single.
"The results suggest that sharing a household environment with a romantic partner may predispose individuals to become at risk for obesity and obesity-promoting behaviors," write Natalie The and Penny Gordon-Larsen, PhD, of the University of North Carolina at Chapel Hill.
They presented their findings this week in New Orleans at The Obesity Society's 2007 annual scientific meeting.
Data came from nearly 8,000 men and women who were studied from 1995 to 2002, as they matured from teens to young adults.
Participants reported their height, weight, physical activity, screen time (time watching TV, using a computer, or playing video games), and relationship status in the mid-1990s and in 2001-2002.
(How has your weight gain affected your relationship? Tell us on WebMD's Couples Coping: Support Group message board.)
Weighty Relationship Shifts
In the mid-1990s, none of the participants was married or living with a romantic partner.
By 2001-2002, 16% were living with a romantic partner and 14% were married.
Women who married or moved in with a partner by 2001-2002 were more likely to be obese than women who were still single. The same wasn't true of men.
Married or cohabiting men and women were more sedentary than their peers who were single or dating.
For instance, married or cohabiting couples were more likely than people who kept dating to exercise less than five times per week and to get more than 15 hours of weekly screen time.
But that sedentary shift doesn't have to be "'till death do you part." Previous research has shown that when one spouse upgrades his or her health habits, the other often follows.