Frank Dikotter, a historian who teaches at the University of Hong Kong has studied the period and reached these conclusions. The work appears in his new book entitled: Mao's Great Famine; The Story of China's Most Devastating Catastrophe.
My source is this article in The Independent, a U.K. newspaper. Speaking of the period from 1958 to 1962, the article summarizes Dikotter's findings:
At least 45 million people were worked, starved or beaten to death in China over these four years.