A new study suggests that many young recruits for fire and ambulance services are overweight and already have risk factors for heart disease — a problem that poses a threat to public safety, researchers say.

In a study of 370 candidates for Massachusetts firefighter and ambulance services, the researchers found that 77 percent of the young recruits were either overweight or obese, based on body mass index (BMI).

BMI is a measure of weight in relation to height, and it is possible for men who are muscular, but not fat, to fall into the "overweight" BMI category.

However, the study found, overweight recruits were not a generally fit bunch. Compared with their normal-weight peers, they had higher blood pressure, higher cholesterol, higher triglycerides (a type of blood fat) and performed less well on fitness tests.

"These findings are strong evidence against the common misconception in the emergency responder community that many of their members have BMIs in the overweight and obese ranges simply on the basis of increased muscle mass," senior researcher Dr. Stefanos N. Kales noted in a written statement.

"Even in these young recruits," he added, "we documented a very strong association between excess BMI and an increased cardiovascular risk profile."

Kales and his colleagues at Harvard University and Boston University School of Medicine report the findings in the journal Obesity.

For the study, the researchers reviewed medical exam records for 370 mostly male recruits for emergency responder services. All had passed their services' minimum requirements, so the study group included those recruits most likely to get a job, according to the researchers.

Overall, 44 percent of the recruits were overweight and one-third were obese. While all of the normal-weight recruits met the minimum fitness level on exercise tests, 42 percent of obese recruits did not. Obese and overweight candidates also tended to have higher heart rates after a 3-minute step test.

These results — coupled with the higher cholesterol, triglyceride and blood pressure levels — all suggest that overweight emergency responders face greater personal health risks on the job. On top of that, their health problems could put their coworkers and the public at risk, Kales warns.

"These professionals perform highly psychologically and physically stressful work and are therefore at high risk for cardiovascular events," he said. "Sudden incapacitation during duty puts these emergency responders, as well as their colleagues and the public, in danger."

Kales and his colleagues suggest making BMI a "vital sign" on emergency responders' medical exams. Keeping emergency personnel in shape, they write, may require multiple approaches — including nutrition education, mandatory exercise during work hours and periodic "consequential" health and fitness testing.