The researchers base their findings on the exercise patterns of 61 junior doctors, working at two hospitals in Southern England, one of which had an on-site gym.
Half the doctors were women, and the specialties in which they worked were equally divided between medicine and surgery. Their average age was 27.
The doctors were quizzed about their levels of physical exercise before and after graduating, as well as lifestyle factors likely to influence their general health, including smoking and drinking.
The Department of Health recommends that every adult should undertake 30 minutes of moderate exercise at least five times a week, to curb weight gain and stave off the risks of serious disease.
The evidence shows that death from all causes is almost halved in men who take regular exercise.
The doctors weighed less and smoked less than the national average, and only 7% of them drank more than the recommended number of units.
But only one in five (21%) met the recommended exercise levels, which is much lower than the national average of 44%, say the authors.
Of those that did too little exercise, most worked at the hospital with an on-site gym, although a third of all the doctors working there didn't know about its existence.
Nevertheless, among the 35 doctors who either used the on-site gym or had gym membership elsewhere, only three took enough exercise.
But this was not always the case. As medical students, almost two thirds of the total sample had met the Department of Health recommendations.
The biggest single reason given for not exercising enough after graduating was lack of time (58%). But a significant proportion (29%) said they weren't motivated to exercise, or were too tired after work to do so.
When the doctors were asked what would boost their participation in exercise, many suggested health promotion at work and the availability of a Trust exercise class or sports team.
The authors comment that Trusts should do more to promote physical activity among their medical staff.
"This is very important, not only for the doctors' own health, but also for the health of the patients. Numerous studies have shown that students and consultant equivalent doctors who exercise are more likely to counsel their patients to exercise too," they say.