Generally, losing weight is beneficial for our health, but when only one partner in a romantic relationship loses weight, it doesn’t always result in a positive outcome for the relationship. According to new research from North Carolina State University and the University of Texas at Austin, there can be also be a “dark side” to weight loss when both partners are not willing to make healthy lifestyle changes.

“People need to be aware that weight loss can change a relationship for better or worse, and that communication plays an important role in maintaining a healthy relationship,” says Dr. Lynsey Romo, an assistant professor of communication at NC State and lead author of a paper on the research.

Researchers surveyed 21 couples from across the country for this study. One partner in each couple grouping had lost 30 pounds or more  in less than two years, with the average weight loss being about 60 pounds. Reasons for the weight loss ranged from changes in diet and exercise to medical procedures. Questionnaires were used to ask each member of the couple grouping about the impact of the weight loss on their relationship.

Researchers discovered that, after weight loss, communication generally changed for the better. The partner who lost weight was more likely to talk about healthy behaviors and inspire their partner to live a healthy lifestyle. Couples in which both partners were receptive to these healthy changes reported more positive interactions and increased physical and emotional intimacy.

However, in some cases, weight loss resulted in negative communication. Some partners who lost weight nagged their significant other to do the same, which caused tension between them. Other partners who hadn’t lost weight reported feeling threatened and insecure by their partner’s weight loss. These participants were resistant to change in their relationships. They were more likely to make critical comments toward their significant other, be less interested in sex, or try to sabotage their partner with unhealthy food in order to derail their partner’s efforts to change themselves or the relationship.

“This study found that one partner’s lifestyle change influenced the dynamic of couples’ interaction in a variety of positive or negative ways, tipping the scale of romantic relationships in a potentially upward or downward direction,” Romo says. “When both partners bought into the idea of healthy changes and were supportive of one another, weight loss appeared to bring people closer. When significant others resisted healthy changes and were not supportive of their partner’s weight loss, the relationship suffered.

“This study should not dissuade anyone from losing excess weight, but it should encourage people to be aware of the potential pros and cons of weight loss on their relationship,” Romo adds. “It is really important for the partner of someone trying to lose weight to be supportive of their significant other without feeling threatened by their health changes. This approach will help people lose weight without jeopardizing the quality of their relationship.”

“Weighty Dynamics: Exploring Couples’ Perceptions of Post-Weight-Loss Interaction” was published online  in the journal Health Communication.