Exercise may lead to a hardening of the arteries that is linked to a higher risk of heart attack - but the benefits of being active still outweigh the risk, research suggests.

A new study published in the journal Heart found that people who are very physically active appear to have high levels of calcium deposits in their coronary arteries.

How much calcium people have in their arteries is often thought to be one of the strongest indicators of the risk of having a heart attack in the future.

Calcium is a sign of how much fatty build-up (plaque) is contained within the arteries, because this build-up contains calcium.

Experts said the new study may mean that exercise increases the risk of a heart attack, or it may be that calcium build-up is not a good measure of heart attack risk.

To look at the relationship between calcium build-up and exercise, researchers studied healthy adults who underwent regular check-ups at two major health centres in Seoul and Suwon, South Korea, between March 2011 and December 2017, as part of the Kangbuk Samsung Health Study.

At each health check, people were asked questions on medical and family history, lifestyle and educational attainment.

Weight (BMI), blood pressure, and blood fats were also measured.

People were marked at the first check-up as either inactive, moderately active or "health-enhancing" (intensely) physically active (the equivalent of running 6.5 km/day), based on a questionnaire.

Scans were also carried out to track the development and/or progression of coronary artery calcification (which was translated into a coronary artery calcium, or CAC, score) over an average of three years.

Overall, 25,485 people (22,741 men and 2744 women), aged at least 30, and with at least two CAC scores, were included in the final analysis.

The study found that people were more physically active tended to be older and less likely to smoke than those who took less exercise.

They also had lower total cholesterol, more high blood pressure, and existing evidence of calcium deposits in their coronary arteries.

The study found that, regardless of CAC scores at the start of the research, the more exercise people did the more calcium they tended to have in their arteries.

Indeed, higher physical activity was associated with faster progression of CAC scores both in those with no calcium deposits and in those who already had a CAC score at the start of the study.

The researchers suggested that exercise may lead to narrowing of the arteries through mechanical stress and vessel wall injury, alongside responses in the body such as increased blood pressure.

Physical activity may also modify the effect of diet, vitamins, and minerals; or exercise may increase CAC scores without increasing heart attack risk.

The researchers concluded that the cardiovascular benefits of physical activity "are unquestionable" and said people should take 150 to 300 minutes per week of moderate intensity exercise, or 75 to 150 minutes per week at vigorous intensity.

In a linked editorial, Drs Gaurav Gulsin and Alastair James Moss, from the department of cardiovascular science at the University of Leicester, said: "Do these findings mean that we should stop using coronary artery calcium scores to assess coronary artery disease?"

They said the study highlights the complexity of interpreting CAC scores in patients who have upped their physical activity or started taking statins, which are also associated with higher CAC scores.

They argued that "clinicians should be cautious regarding the overuse of this test in otherwise healthy individuals".

Dr Moss added: "It may be the target we need to look for is non-calcified plaque rather than calcified plaque."