Washington, Dec 31 (IANS): The campaign team of Democratic presidential candidate Bernie Sanders has released letters by his doctors claiming that the Party's oldest contender was in good health and "fit and ready to serve as President".

"You are in good health currently and you have been engaging vigorously in the rigors of your campaign, travel and other scheduled activities without any limitation," Brian Monahan, the attending physician for members of Congress and Sanders' long-time primary physician, said in one of the three letters on Monday.

Sanders, 78, briefly left the campaign trail in early October as he suffered a heart attack, but has recovered and engaged himself in a robust campaign, reports Xinhua news agency.

An examination in December found that the Vermont Senator "was able to exercise to a level that is approximately 50 percent higher than other men his age with a similar diagnosis", Philip Ades, director of cardiac rehabilitation at the University of Vermont, said in a separate letter.

"While he did suffer modest heart muscle damage, he has been doing very well since," said Martin LeWinter, the attending cardiologist at the University of Vermont Medical Center.

"I am confident he has the mental and physical stamina to fully undertake the rigors of the presidency."

Sanders is currently among the top three favourites in the Democratic primary race, along with former Vice President Joe Biden -- who is leading in polls -- and Massachusetts Senator Elizabeth Warren.