"After she endured an 800 Bengazi hearing, I find it so ridiculous that a guy who whined about a 3 hour debate is attacking her stamina," she wrote Nov. 29.
"Do you have a suggested joke?" Schwerin asked.
"I think she should challenge him to an arm wrestle," she wrote back. "Or see if he could handle 11 (hours) of grilling from Dems like she did from Reps."
Tanden didn't get a response, according to the leaked emails. She sent a final message around 10 p.m.
"Humor isn't my forte," she wrote.
Over the course of the campaign, both Clinton's and Trump's health have come up as issues. The issue was spotlighted Sept. 11, when Clinton suffered from what her campaign said was heat stroke at a 9/11 memorial event in New York. She was recorded stumbling into a van.
Her doctor later said Clinton was diagnosed with pneumonia before the memorial.
Later in September, Trump discussed his health with Mehmet Oz during an appearance on "The Dr. Oz Show."
Trump and Clinton are among the the oldest people to ever run for president. If Trump is elected, he will be 70 years old when he takes office, making him the oldest person to be sworn in. Clinton would be 69 years old if she's elected; she would become the second oldest person to take office behind President Ronald Reagan.