Based on his medical work in Kenya and Ugana (1929-1958), he identified a condition known as protein-energy malnutrition. From 1935 until his retirement in 1959, he worked as a consultant physician and paediatrician at Mulago Hospital and Medical School in Kampala, Uganda.
Trowell was internationally acknowledged as an authority on Kwashiorkor. He was the first to discover that serum albumin concentration in children with kwashiorkor was below normal which was used to support the hypothesis that a protein deficient diet was responsible for the condition.
With his colleague Denis P. Burkitt, Trowell was influential in promoting dietary fiber. He developed the fiber hypothesis in a series of books and papers with Burkitt in the 1970s and 80s. Their research showed that diets low in fiber increase the risk of coronary heart disease, diabetes, obesity and large bowel conditions such as colorectal cancer, appendicitis and diverticulosis. The British Medical Journal describes Trowell as "probably the first person to link diets deficient in fibre with obesity, diabetes and coronary heart disease."