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Active teens are significantly less likely to suffer heart attacks in 30 or 40 years later. A new study found a strong link between a person's fitness as a teenager and their risk of heart attack in later life.

The latest study involved 43,498 Swedish men who had medical examinations at the age of 18 when they were conscripted into the army between 1969-1984. The findings revealed that the more aerobically fit men were in late adolescence, the less their risk of suffering a heart attack 30 or 40 years later. The findings held true even after accounting for the men's body mass index when they were teenagers. However, fit but overweight or obese men were found to have a significantly higher risk of suffering a heart attack compared to unfit, lean men.

"Our findings suggest that high aerobic fitness in late adolescence may reduce the risk of heart attack later in life. However, being very fit does not appear to fully compensate for being overweight or obese in respect to this risk. Our study suggests that it's more important not to be overweight or obese than to be fit, but that it's even better to be both fit and a normal weight," lead researcher Professor Peter Nordström, of Umeå University, Umeå, Sweden, said in a news release.

For the study, aerobic fitness was measured by a cycle test where the resistance was gradually increased until participants were too exhausted to continue.

The study revealed that every 15 percent increase in aerobic fitness was linked to an approximately 18 percent reduced risk of a heart attack (myocardial infarction or MI) 30 years later after adjusting for various confounding factors including socioeconomic background and BMI. The findings also independently linked regular cardiovascular training in late adolescence to an approximately 35 percent-reduced risk of an early heart attack in later life.

However, researchers warned that the study found association and not causation.

"The relationship between aerobic fitness and heart disease is complex and may well be influenced by confounding factors that were not investigated in this study. For instance, some people may have a genetic predisposition to both high physical fitness and a low risk of heart disease. In a recent study of twins, we found that 78 percent of the variation in aerobic fitness at the time of conscription is related to genetic factors," Nordström said.

"As far as we know, this is the first study to investigate the links between an objective measure of physical fitness in teenagers and risk of heart attack in the general population. Further studies are needed to investigate the clinical relevance of these findings, but given the strong association that we have found, the low cost and easy accessibility of cardiovascular training, and the role of heart disease as a major cause of illness and death worldwide, these results are important with respect to public health," he concluded.

The findings are published in the European Heart Journal.