He edged out Australian favourite Cameron McEvoy, clocking 47.85 seconds in the blue riband event, four years after he tested positive for the banned stimulant clenbuterol.
The swimmer with boyish good looks is a household name in China and goes by the nicknames "Baozi" or "steamed bun".
"There has suddenly been news recently about the Steamed Bun that has caused concern among many leaders, media, friends, family and Bun fans who have come one after another to show concern and seek proof," Ning said in a statement.
He said he and his team mates had been preparing hard for the Olympics, adding that two days ago leaders from China's General Administration (of sports) visited their training camp to lift morale.
"With just over a month to go before the Olympics, striving for glory for the nation at the Olympics is without a doubt the biggest dream of a lifetime for all athletes and is my greatest motivation in training hard and swimming forward each day."
Dropping an athlete from the team due to endorsements and television commercials is not unprecedented in China which holds its national athletes to strict standards of conduct.
Olympic diving champion Tian Liang was kicked out of the national squad in 2005 for taking part in too many commercial activities, despite having won a gold and a bronze at the 2004 Athens Games and the 10 metres platform title in Sydney in 2000.
"As for some rumours, the Steamed Bun does not want to respond because it would only create more noise amid preparations for the Olympics," Ning added.
"Now is a critical time in the sprint of preparations for us athletes... People all around the country are paying attention, it's once in four years, and there can be no mistakes!"
(Reporting by Sudipto Ganguly in Mumbai; additional reporting by John Ruwitch in Shanghai; editing by Ed Osmond)
