The United States currently has the greatest number of known centenarians of any nation, estimated at 70,490 on September 1, 2010.This corresponds to a national incidence of one centenarian per 4,400 people.
Japan has the second-largest number of centenarians, with an estimated 47,756 as of September 2011. Japan started its surveys in 1963, at which time the number of Japanese centenarians was found to be 153. This number surpassed the 10,000 mark in 1998, 20,000 in 2003, and 40,000 in 2009. According to a UN Demographic survey, Japan is expected to have 272,000 centenarians by 2050. However, some sources suggest that the number could be closer to 1 million. Many experts attribute Japan's high life expectancy to the Japanese diet, which is particularly low in refined simple carbohydrates, and to hygienic practices. In addition, the number of centenarians in relation to the total population was, in September 2010, 114% higher in Shimane Prefecture than the ratio for the whole of Japan. This ratio was also 92% higher in Okinawa Prefecture. In addition to diet, there are four other factors that have been found to increase the life expectancy for Okinawans, as noted later in the "research into centenarians" section of this article. The incidence of centenarians in Japan was one per 3,522 people in 2008, although Okinawa again exceeded this average, with one per 1,838 people 2006.