Upon receiving her PhD, Dewey joined the faculty at the University of California, Davis in 1980.[3] As a professor of nutrition, she led two studies in Honduras which concluded that infants should receive only breast milk for their first six months of life.[4] Her research team also found that the growth rate of breast-fed babies differs significantly from that of formula-fed babies, thus prompting a change to the World Health Organization's (WHO) recommendations for infants.[5] In 1999, Dewey co-authored a book titled Complementary Feeding of Young Children in Developing Countries which was aimed at WHO and UNICEF to "stimulate and guide creation of national programs to improve child-feeding practices in low-income countries."[6] In the same year, Dewey was also the lead researcher on a study which found that diet and aerobic exercise could help breast feeding mothers lose weight after pregnancy.[7] As a result of her research, Dewey was appointed director of a WHO-led research project to revise growth charts for children under the age of 5 years.[8] The results of the project were released by WHO in 2006.[9] Following this, Dewey led another research project focusing on delayed umbilical cord clamping which also contributed to WHO policy.[10]
In 2010, Dewey led a research team in Bangladesh and Guatemala to test whether Nutributter could prevent stunting and abnormal motor development in malnutritioned children.[11][12] As a result of her research, Dewey was recipient of UC Davis' Award of Distinction[3] and McCollum International Lectureship in Nutrition Award.[13] In 2017, Dewey was appointed to sit on the Food and Nutrition Board of the National Academies of Sciences, Engineering, and Medicine for a three-year term.[14] The following year, she was elected a Fellow of the American Society for Nutrition[15] and named to sit on the 2020 Dietary Guidelines Advisory Committee.[16]
