(CNN) — Former President Jimmy Carter was hospitalized Tuesday in Cleveland, Ohio, for an upset stomach, the Carter Center said in a statement.

He was taken to MetroHealth Hospital, where he was under observation and resting comfortably, the Carter Center said.

Emergency personnel were called to an incoming Delta Air Lines flight for a passenger who felt sick, reported CNN affiliate WEWS. Upon arrival, they learned that the passenger was the former president.

Airport emergency crews performed an initial checkup and then transported Carter to the hospital.

Carter, who will turn 86 on Friday, was in Cleveland to sign copies of his new book, “White House Diary,” at Joseph-Beth Booksellers at noon. About 400 people were still waiting at the store for Carter’s appearance.

He is expected to resume his book tour this week, the Carter Center said.

He and former first lady Rosalynn Carter are known as fitness buffs who maintain active lifestyles.

Carter recently returned from a trip to North Korea, where he secured the freedom of U.S. citizen Mahli Gomes.

During a September 20 appearance on CNN’s “Larry King Live,” Carter said he planned to stay busy in the future.

“I’ll stay involved in the Carter Center as long as I’m physically and mentally able,” he said. “I’m still a professor at Emory University. This is my 29th year as a professor. And I’ll still write a book every now and then.”