About three months after Sen. Bernie Sanders had a heart attack, doctors said the 78-year-old presidential hopeful is fit to serve in the White House.

"At this point, I see no reason why he cannot continue campaigning without limitation and, should he be elected, I am confident he has the mental and physical stamina to fully undertake the rigors of the Presidency," wrote Martin LeWinter, an attending cardiologist at the University of Vermont Medical Center, in one of three doctors' assessments the Sanders campaign released on Monday.

Questions about age and health have swirled around the 2020 Democratic presidential primary field as three septuagenarians run near the front of the pack. The campaigns of 77-year-old former Vice President Joe Biden and 70-year-old Sen. Elizabeth Warren, D-Mass., have also released medical summaries saying the candidates are fit to serve as president.

The campaign of another primary hopeful, 77-year-old Mike Bloomberg, also released a medical summary saying the former New York City mayor is in "outstanding health."

Sanders, who had stents inserted in October following a partial artery blockage, has resumed normal campaign activities. The independent senator from Vermont is "in good health" and has "been engaging vigorously in the rigors of [his] campaign, travel and other scheduled activities without any limitation," wrote Brian Monahan, who has been Sanders' doctor in Washington for 29 years.

Sanders saw cardiologists in Vermont in November and December following the heart attack, Monahan said. The senator has not had symptoms of congestive heart failure and tests show no "significant" heart rhythm abnormality, according to the physician.

Monahan wrote that Sanders' heart muscle strength "has improved." Sanders also went through a test on a graded treadmill in December and performed better than average for his age, the doctors' letters said.

In his letter, the UVM Medical Center's LeWinter added that the senator suffered "modest heart muscle damage" but "has been doing very well since."

Sanders has been treated for gout, high cholesterol and laryngitis caused by esophageal reflux, among other ailments. He exercises regularly, does not use tobacco and "seldom" drinks, according to Monahan.

If elected, Sanders would become the oldest U.S. president ever. President Donald Trump, 73, was the oldest man to start a first term in the White House. He was 70 at the time of his inauguration in January 2017.

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