When your partner orders an extra side of fries or hits snooze on your alarm for the gym, it's easy to do the same. On the flip side, having a partner who enjoys a healthy lifestyle might give you the extra boost of motivation that you need to make a change for better health.

This is the thinking behind new research that investigated heart health in more than 5,300 couples across the US, finding that married couples and partners who live together commonly share risk factors and healthy habits influencing their heart health.

"We expected to see some shared risk factors," said Samia Mora, a cardiologist at the Brigham and Women's Hospital, and Harvard Medical School, in Boston.

"But it was a surprise to see that the vast majority of couples were in a non-ideal category for [their] overall cardiovascular health."

People and their partners, aged between 39 to 57 years old, had their heart health assessed as part of a wellness program offered by their employer, Quest Diagnostics, a laboratory based in the US.

Seven risk factors for heart disease, defined by the American Heart Association as Life's Simple 7, were evaluated or measured: smoking status, daily physical activity, body mass index, diet, blood sugar levels, total cholesterol, and blood pressure.

These risk factors – which can all be improved through lifestyle changes – influence your chances of developing a range of heart conditions, such as clogged arteries, that can lead to heart failure, heart attack, or stroke.

Data was collected in questionnaires, physical examinations, and lab tests. Each person in the study was scored individually as poor, intermediate, or ideal for each LS7 category, and these scores were summed together to give an overall rating of their heart health.

The analysis showed that most couples in the study were either heart-healthy together or shared poor heart health.

When one person was in the ideal category for a particular risk factor or behaviour (all except cholesterol), their partner was more likely to have a healthy score as well.

Amongst close to 2,190 couples who participated in the program every year for five years, if one person had quit smoking, lost weight, stepped up their exercise or improved their diet, their partner had likely done the same the year before.

But generally, there was little change in couples' risk factors, behaviour, and heart health over 5 years.

In almost 80 percent of the couples studied, people shared poor or 'non-ideal' heart health with their partner, with most sharing unhealthy diets and a lack of exercise.

That's not to say that your relationship is necessarily bad for your heart health. An observational study like this can only provide associations between one person's health and another; it cannot point to a direct cause.

The study also relied on self-reported data, collected in a questionnaire, a limitation which the study authors acknowledged. Asking people to answer questions about what food they eat and how much exercise they do can introduce inaccuracies, whether through mistakes or less-than-truthful answers.

It's also possible that people who opted to join the company-sponsored wellness program were more concerned about their health in the first place, which could also skew the findings.

That said, the study involved people from all 50 states of the US, and the findings seem to hold true elsewhere in the world too.

Previous studies have reported that married couples in Italy, Belgium, and England (randomly recruited through doctors' clinics) and over 3,000 Korean couples (assessed in a nationwide survey) shared many risk factors for heart disease.

However, there are other factors affecting heart health – such as financial stress and lack of sleep – not captured in the US study. A family history of heart problems raises your risk, too.

Also, the study failed to capture any information about how long the couples had been together, whether it be five weeks or five years. You would think the more time couples spend together, the more opportunity someone might have to adopt the other person's lifestyle – for better or for worse.

Still, the study authors said in their paper that the findings could help design public health programs promoting healthy lifestyles for couples, given the extra social support partners might provide. Previous research suggests couple‐focused interventions may be more effective than individual interventions.

"Our data suggest that risk factors and behaviours track together for couples," Mora said. "Rather than thinking about interventions for individuals, it may be helpful to think about interventions for couples or whole families."

"It's important for people to think about how their health and behaviours may influence the health of the person(s) they are living with," she added.

"Improving our own health may help others."

The research has been published in JAMA Network Open.