Titles and Affiliations

Professor of Epidemiology and Nutrition
Department of Nutrition
Harvard T.H. Chan School of Public Health
Boston, Massachusetts

Research area

Understanding the effects of diet and gut microbial health on breast cancer risk. 


Lifestyle choices, including diet, can play a pivotal role in breast cancer prevention. Diets rich in vegetables, fruits, whole grains, fish, and olive oil have consistently been associated with a decreased risk of breast cancer. Recent studies have suggested that the microbiome—the collection of millions of intestinal bacteria—plays an important role in modulating risk of several chronic diseases, including breast cancer, by altering estrogen metabolism. At the same time, it is now understood that diet plays a significant role in shaping the microbiome and its overall health. Understanding the connections between diet, the microbiome, and breast cancer risk could reveal significant opportunities in altering breast cancer risk through diet.

Progress thus far

Dr. Willet and his team have collected about 20,000 stool samples from participants in the Nurses’ Health Study (NHS) II and are evaluating how hormone levels are affected by gut microbiota, which ultimately may provide additional strategies to reduce breast cancer incidence and improve survival. 

What’s next

In the coming year, Dr. Willett will continue building upon resources of NHS/NHS II, and the Pooling Project of Prospective Studies of Diet and Cancer (DCPP), which has been supported previously by BCRF. Dr. Willett is nearing completion of a fecal sample biorepository to evaluate the microbiome in relation to breast cancer risk. The data come from 20,000 women in the NHS II, mainly white women. Because the NHS II cohort is comprised primarily of white women, Dr. Willett plans to combine this resource with samples from women in the Southern Community Cohort Study (SCCS), a cohort of predominantly Black women, to evaluate the microbiome in relation to breast cancer risk. Combining cohorts will greatly enhance the racial diversity of the study and increase the sample size. Dr. Willet is proposing a critical pilot study to document the long-term variability of the microbiome in fecal samples because until now, all large epidemiologic studies have only collected a single sample. He and his team also propose testing plasma samples for estradiol, estrone sulfate, and testosterone to develop a microbiome signature of plasma estrogens, which may also contribute to breast cancer risk.


Dr. Walter Willett is Professor of Epidemiology and Nutrition at the Harvard T.H. Chan School of Public Health and Professor of Medicine at Harvard Medical School. Dr. Willett was born in Hart, Michigan and grew up in Madison, Wisconsin, studied food science at Michigan State University, and graduated from the University of Michigan Medical School before obtaining a Doctorate in Public Health from Harvard School of Public Health.

Dr. Willett has focused much of his work over the last 40 years on the development of methods, using both questionnaire and biochemical approaches, to study the effects of diet on the occurrence of major diseases. He has applied these methods starting in 1980 in the Nurses' Health Studies I and II and the Health Professionals Follow-up Study. Together, these cohorts that include nearly 300,000 men and women with repeated dietary assessments are providing the most detailed information on the long-term health consequences of food choices.

Dr. Willett has published over 2,000 articles, primarily on lifestyle risk factors for heart disease and cancer, and has written the textbook, Nutritional Epidemiology, published by Oxford University Press. He also has four books book for the general public, Eat, Drink and Be Healthy: The Harvard Medical School Guide to Healthy Eating, which has appeared on most major bestseller lists, Eat, Drink, and Weigh Less, co-authored with Mollie Katzen, The Fertility Diet, co-authored with Jorge Chavarro and Pat Skerrett, and most recently, Thinfluence, co-authored with Malissa Wood, emphasizing the powerful and surprising effect friends, family, work, and environment have on weight. Dr. Willett is the most cited nutritionist internationally and is among the five most cited persons in all fields of clinical science. He is a member of the National Academy of Sciences and the recipient of many national and international awards for his research.

BCRF Investigator Since


Donor Recognition

The Hale Family Award

Areas of Focus