Dr. Underwood's clinical activities are in the care of premature and sick newborn infants. His research interests include necrotizing enterocolitis, development of intestinal innate immunity, and mechanisms of probiotics and prebiotics in pre-term infants. Necrotizing enterocolitis is a gastrointestinal disease that affects premature infants and involves infection and inflammation that causes destruction of the intestinal epithelium. It is the most common and serious gastrointestinal disorder among preterm infants, and can cause life-threatening systemic infection by pathogenic bacteria that penetrate the wall of the intestine. The premature infant's organs and tissues are immature, and even with immediate treatment there may be serious complications from necrotizing enterocolitis. Babies who are breastfed have a lower risk of developing the disease, which led to the investigation of the functions of breastmilk in the development of the infant gut and in the prevention of the disease.

Dr. Underwood's research projects include clinical trials of probiotics and prebiotics in premature infants, assessment of Paneth cell antimicrobial expression in infants with necrotizing enterocolitis and evaluation of Paneth cell expression in an animal model.