Weight loss usually tops the New Year’s resolution list every year after many of us have given ourselves the license to eat a little more during the holidays. Losing weight is a challenging feat all on its own, so we start to think who better than our significant other to join us in the battle of the bulge. This plan may backfire, however, because, according to a recent study published in the journal Eating Behaviors, a couple that diets together actually fails separately.

“In my case, this was true, as I felt much less confident about being able to limit my portions than did the Consultant over the holiday, particularly during meals when my mother was present,” wrote author of the study Jennifer Jill Harman about her couple's dieting experience in the blog Science of Relationships. Harman admitted she generally does well controlling her food portions, but she tended to consume larger portion sizes of leftovers while her family was still visiting. She believes the portion size, not the holiday meal, is the root of the problem.

To observe the eating behaviors of individuals and couples, and how much self-efficacy they have in controlling their food portions, Hill and her colleagues from Colorado State University conducted a brief survey. They asked people how much they agreed with statements like, “When eating out with friends, they influence how much I eat” and “I feel confident that I can leave food on my plate if I think a serving size is too large,” according to her blog. This survey was able to measure not only someone’s belief that they could control their food portions while alone, but whether they were able to in the presence of others.

The findings for this part of the study revealed women are generally less confident than men in their ability to control their portions, and women judge their food portions to be larger than men do. Hill suspects this has to do with the fact that women are more concerned with their weight than men are.

A Couple Who Diets Together...

The researchers, who also assessed 50 overweight romantic couples who made New Year’s resolutions to lose weight, found those who diet together generally fail separately. The more successful a partner was at regulating his or her diet and eating healthier, the less confident the other partner felt in controlling his or her own food portions. It seems when people strive to reach a goal with a close someone who is successful at it, they begin to feel less confident in their own efforts to reach the goal. According to Hill, it all comes down to “People feel less confident achieving their goals when they see others succeeding at the same goals.”

Moreover, in heterosexual couples, comparing weight loss can be frustrating, especially for women. Men tend to lose weight and keep it off easier than women. This is because men have more muscle, which helps burn more calories and speed up their metabolism, according to the Mayo Clinic.

One partner’s success in weight loss could also take a toll on the relationship. A 2013 study published in the journal Health Communication found weight loss may drive a wedge between couples and test the bond between them. In the study, the partner who had lost weight was more likely to insist his or her significant other to follow a new healthy lifestyle, while the partner who hadn't lost weight was not supportive and felt threatened and insecure. This led to critical comments, less interest in sex, or even trying to sabotage their partner with unhealthy food to derail weight loss efforts to prevent both the partner and relationship from changing.

The change in one partner’s lifestyle influences the dynamic of a couple through their interactions and even has the potential to tip the scale. Whether significant others are trying to lose weight individually or as a couple, it’s important to be supportive without feeling threatened by his or her health changes. Couples who do decide to diet together should make mutual decisions about what foods to buy and prepare to help each other lose weight while also acknowledging the differences bound to arise.

Sources: Burnette J., Dreith F., Fast L. C., Harman J. J., Maertens J. A., L. Creating a measure of portion control self-efficacy. Eating Behaviors. 2015.

Dailey RM and Romo LK. Weighty Dynamics: Exploring Couples’ Perceptions of Post-Weight-Loss Interaction. Health Communication. 2013.