Rumors have been swirling for about a week that the former communist dictator, who turned 86 in August, is dead or close to it.
Castro was officially succeeded by his brother, Raul, as president of the country in 2008, two years after surgery for an undisclosed intestinal illness. Since then Castro has written frequent newspaper columns, which suddenly halted in June, prompting opponents to speculate about his health, CNN reported.
Castro's letter mentioned the Bay of Pigs, nuclear weapons and his distrust of mass media. He also said he's so healthy, he doesn't even remember what a headache feels like.
"I stopped posting [articles] because it certainly is not my role to occupy the pages of our newspaper, devoted to other tasks required by the country," Castro wrote.
Meanwhile, a former Venezuelan vice president, Elias Jaua, said he spent time speaking with Castro Sunday, The New York Times reported. Jaua showed journalists a photo of himself with the former leader.
"Fidel is very well," Jaua said.