Walter Willett

University of Dar es Salaam (1974–1977)

Following his residency at Boston City Hospital, Willett took up a position at the University of Dar es Salaam's Faculty of Medicine in Tanzania. He served as Lecturer in Medicine from 1974 to 1975, and from 1975 to 1977 as Lecturer in Community Medicine and Head of the Community Health Department.^[18] Work from this period included a randomised trial examining the relationship between ascaris infection and child growth rates, published in the American Journal of Public Health in 1979.^[19]

Harvard School of Public Health (1980–present)

Willett joined the Harvard School of Public Health (now Harvard T.H. Chan School of Public Health) as Assistant Professor of Epidemiology in 1980, was promoted to Associate Professor in 1984, and to Professor of Epidemiology and Nutrition in 1987 and has held this position continuously.^[17]

From 1991 to 2016 he served as Chairman of the Department of Nutrition and the Fredrick John Stare Professorship of Epidemiology and Nutrition.^[20][21] He was also appointed Lecturer in Medicine at Harvard Medical School from 1986 to 1992, and Professor of Medicine from 1992 to 2021.^[17]

From 2010 to 2016, Willett held a Foreign Adjunct Professorship in Nutritional Epidemiology at the Karolinska Institute of Environmental Medicine in Stockholm.^[18]

He held hospital appointments at Boston City Hospital (1977–84) and Brigham and Women's Hospital, where he was Associate Physician from 1982 to 2021.^[17]

EAT-Lancet Commission (2017–2025)

Willett served as co-chair of the EAT-Lancet Commission on Food, Planet, Health from 2017 to 2025.^[22] The Commission brought together scientists in nutrition, sustainability, and food systems to develop scientifically grounded targets for healthy diets from sustainable food systems. Its 2019 report, Food in the Anthropocene, set global benchmarks for healthy and environmentally sustainable diets and was published simultaneously with a flagship article in The Lancet.^[23]   An update Commission report was published in 2025.^[24]

Prospective cohort studies

The principal vehicle for Willett's research has been a set of large, long-running prospective observational cohorts. He is principal investigator of Nurses' Health Study II (NHS II), which enrolled more than 116,000 younger female nurses beginning in 1989 and examines diet, lifestyle, and the risk of chronic disease. He is also the principal investigator of the Health Professionals Follow-up Study (HPFS), a cohort of more than 51,000 male health professionals enrolled in 1986. In both studies, participants complete detailed dietary questionnaires every two to four years, allowing analysis of diet as it changes over time.^[25]

He is a co-investigator on Nurses' Health Study I (NHS I), the original cohort established in 1976, which enrolled more than 121,000 nurses and has followed them for five decades.^[25] Dietary assessment methodology.

Dietary assessment methodology

A substantial part of Willett's methodological contributions concerns the measurement an analysis of dietary intake in large epidemiological studies. His group developed and validated the semiquantitative food frequency questionnaire (FFQ) as a practical tool for capturing habitual diet in large cohorts. A foundational paper on the reproducibility and validity of the FFQ was published in the American Journal of Epidemiology in 1985.^[26] The textbook Nutritional Epidemiology (Oxford University Press, 1990; 2nd ed. 1998; 3rd ed. 2013) documents these methodological approaches and is widely used in graduate instruction.^[17]

Diet, cancer, and cardiovascular disease

Willett's research group has produced a large body of work examining associations between specific dietary patterns, nutrients, and foods and the risk of cancer (particularly breast and colorectal cancer), cardiovascular disease, type 2 diabetes, and other chronic conditions. Early papers from the Nurses' Health Study examined the relationships between dietary fat, red meat intake, and colorectal cancer risk. Later work has investigated the role of specific fatty acids, refined carbohydrates, glycaemic index, whole grains, and plant-based dietary patterns in risk of chronic disease.^[17]  His research on industrial trans fatty acids^[27] provided key evidence that ultimately led to their elimination in the United States and many other countries.^[28]

Obesity and body weight

Willett and colleagues have published extensively on biases in studies of body weight and mortality.^[29][30] He has been a prominent critic of research associated with the so-called "obesity paradox," the hypothesis that overweight or mild obesity reduces mortality. He described work on this topic by Katherine Flegal and colleagues at the Centers for Disease Control and Prevention's National Center for Health Statistics as a "pile of rubbish."^[31] In 2013 the journal Nature published an editorial rebuking Willett for the manner of his criticism, noting that it misrepresented the complexity of the evidence and employed inappropriate language.^[32]

In 2021, Flegal published an article in Progress in Cardiovascular Diseases describing what she characterised as a sustained campaign by Willett and associates to undermine and discredit her research, including "insults, errors, misinformation, social media posts, behind-the-scenes gossip and maneuvers, and complaints to her employer."^[33] Flegal also questioned Willett's statistical qualifications to criticise her team's work on mortality estimation.^[33]  Notably, multiple committees have reviewed the evidence on body weight and the Centers for Disease Control and the National Institutes of Health have not changed weight guidelines because of the “obesity paradox” hypothesis.^[34] The clinical definition of obesity remains a subject of debate, though the discussion has centred on whether to broaden the definition rather than to narrow it as the “obesity paradox” would imply.

Dietary guidelines and public communication

Willett has been critic of official dietary guidelines, in particular those issued by the United States Department of Agriculture, which until recently have promoted reduction in all types of fat and high intakes of carbohydrates. His 2001 book Eat, Drink, and Be Healthy presented an alternative "Healthy Eating Pyramid" grounded in epidemiological evidence, arguing that the USDA food pyramid gave insufficient weight to the distinction between saturated and unsaturated fats and between refined and whole-grain carbohydrates.^[35][36]

Willett has been frequently cited by news media on nutrition topics.^[37][38] He has worked with the Culinary Institute of America and Harvard University dining services to align the university's food offerings with current nutritional evidence.^[39][40]