At 99, the late Henry Kissinger still worked 15 hours a day and flew around the world on business trips

• Even in his old age, Henry Kissinger was still working around the clock.
• The controversial statesman told CBS News that at 99, he was  clocking 15-hour work days.
• Former Google CEO Eric Schmidt says Kissinger '"works harder than a 40-year-old."

Back in May, the late Henry Kissinger was on the cusp of becoming a centenarian, but he was still working around the clock.

"I work about 15 hours a day," he told CBS News in an interview that aired on May 7, several weeks before his 100th birthday.

Kissinger, the former Secretary of State who served in both the Nixon and Ford administrations, turned 100 on May 27. The controversial scholar and diplomat passed away on Wednesday.

Friends and family of Kissinger believe that his longevity stemmed in part from his dedication to work.

Kissinger's son, David wrote in an op-ed in The Washington Post in May that his "father's longevity is especially miraculous when one considers the health regimen he has followed throughout his adult life."

According to his son, Kissinger consumed "a diet heavy on bratwurst and Wiener schnitzel." The younger Kissinger said that his father didn't play any sports and pursued "a career of relentlessly stressful decision-making."

"How then to account for his enduring mental and physical vitality? He has an unquenchable curiosity that keeps him dynamically engaged with the world," the younger Kissinger wrote, noting his father's abiding interest in global challenges such as nuclear weapons and artificial intelligence.

'The other secret to my father's endurance is his sense of mission. Although he has been caricatured as a cold realist, he is anything but dispassionate," Kissinger's son wrote in The Post. "He believes deeply in such arcane concepts as patriotism, loyalty and bipartisanship."

Eric Schmidt, who co-authored the book "The Age of AI" with the late Kissinger was equally impressed with the latter's work ethic.

"He works harder than a 40-year-old. I can tell you that he gets up in the morning and he works all day. He has dinner with his wife and his family and he works at night," the former Google CEO said in an appearance on the Time Ferriss Podcast in 2021.

"I am convinced that the secret to longevity is being a workaholic," Schmidt added.