From 1988 to 2005 he held various positions at the National Institute for Public Health and the Environment (RIVM) in Bilthoven, for example head of the Center for Epidemiology, director of the Public Health Research Division and director of the Nutrition and Consumer Safety Division. He was RIVM professor of Public Health Research at Wageningen University from 1994 to 2005 and Nutrition Foundation professor of Nutrition and epidemiology at Leiden University from 1984 to 1994. He was assistant and associate professor at the Institute of Social Medicine of Leiden University from 1977 to 1988.
Daan (1950) graduated as human nutrition expert from Wageningen Universtity in 1974, and obtained his PhD there in 1978. His thesis was about risk factors of cardiovascular disease in schoolchildren. In 1981 he became Master of Public Health from the University of Minnesota, Minneapolis, USA.