That's why the idea of dieting with a partner can be so appealing. There is someone to share that kale salad with, and it won't be hard to skip desert if you know your partner is skipping it too.

But dieting with someone you're involved with romantically might not be as simple as that.

Associate Professor at Colorado State University Dr Jennifer Harman suggests the dieting success of one person in the relationship could be affected  if the other partner does better.

Dr Harman was studying people's ability to restrict portions while alone and while with others. The spoiler was that most people found it harder while with friends.

She uncovered anecdotal evidence that trying to diet with a partner could be counter-productive.

"We also found that among 50 overweight, romantic couples who made New Year's resolutions to lose weight, the more successful a partner was at restricting his or her diet and eating healthier, the less confident the other partner was in controlling their own food portions," Dr Harman writes in The Science of Relationships.

She says there are many things that lead people astray from their weight-loss goals, and portion control is a critical piece of the weight-loss puzzle.

"When people strive to reach a goal, being close - in this case romantically - with someone who is successfully reaching the same goal can make the other partner less confident in their own efforts to reach the goal," Dr Harman writes.

"You heard that right: people feel less confident achieving their goals when they see others succeeding at the same goals."

Considering men lose weight easier than women, it could be easy to assume that this could be a serious problem for heterosexual couples wanting to shed a few kilos together.

However, University of Western Sydney lecturer Dr Rebecca Pinkus, who studies comparisons in romantic relationships, has a much more positive view of the idea.

"It hasn't been examined specifically for dieting, but it seems likely people who are in a good, strong relationship and see one another as teammates should be able to diet together, and do well together," Dr Pinkus says.

"I think this idea of couples dieting together, the fact that they're entering into that diet together has a positive affect on their relationship.

"We know that things like social support really help people stick to their plan, so having a partner who is supportive and encouraging is probably more beneficial."

It is possible for couples to be envious of one another's successes, including dieting, but they can also be happy for the success of the other person, she says.

While theories around couples and dieting need to be tested scientifically, anecdotal evidence and other relationship studies suggest couples should be able to diet together, she says.

"New literature suggests people in strong relationships are able to take on that comparison, and they can be happy as a couple for the success."

Dr Harman suggests it might be better to try to avoid comparison.

"The take-home lesson is to set my own goals and not compare my progress with my partner," she says.

"Easier said than done when we eat together as a family regularly, but nonetheless important to at least consider, so that I do not let his progress undermine my own efforts."