Castro's prolonged silence after months of prolific column writing had contributed to speculation that the ailing 82-year-old was on his death bed.
He did not disclose the reason that he had not written a column, or ‘reflection’ as he calls them, since December 15, after averaging nine a month in 2008.
Castro wrote that he had met with Argentine President Christine Fernandez on Wednesday near the end of her three-day visit to Havana and told her the revolution that put him in power on January 1, 1959, had outlasted 10 US presidents.
He spoke of his admiration for Obama, who took office on Tuesday, replacing George W. Bush, and is the United States' first black president.
"I expressed that personally I had not the least doubt of the honesty with which Obama, the 11th president since January 1, 1959, expressed his ideas, but in spite of his noble intentions there remained many questions to answer," he said.
One question, Castro said, was "how can a wasteful and consumerist system par excellence preserve the environment?"
Obama has said he wants to move toward normalization of US-Cuba relations but would not eliminate the 46-year-old US trade embargo against the communist-led island.
Castro's column, published on state-run Internet site www.cubadebate.cu, came out a few hours after his younger brother, President Raul Castro, denied rumors that Fidel Castro's health was worsening.
"He is exercising, thinking a lot, reading a lot, assisting me and helping," Raul Castro said. "Soon I'm going to make a trip to Europe. Do you think I could leave from here if Fidel was gravely ill?"
The elder Castro has been seen only in a few videos and photos since undergoing intestinal surgery in July 2006 from which he never fully recovered.
But he has maintained a public profile through his writings, and the frequency of his columns has become an informal barometer of his health among Castro watchers.
Raul Castro provisionally took power after the surgery, then officially became president in February when his brother said he was not well enough to continue.
Fernandez said at the Havana airport that Castro wore a blue jogging suit during their meeting and told her he had followed Obama's inauguration on television.
"With much passion, with much conviction, he told me he's a sincere man, believes absolutely in everything he's saying, he has many good ideas and a very good history," Fernandez said.