[2] He studied medicine at St Mary's Hospital Medical School, graduating on 5 July 1948, the day that the National Health Service (NHS) was established.
[3] He travelled to Bethesda, Maryland, in 1960 for a year-long research fellowship at the National Institutes of Health,[2] where he experimented with insulin assays and early attempts to isolate pancreatic islets.
[4] When Keen returned to London from the United States in 1961, he was hired as a lecturer by Guy's Hospital and its associated medical school, where he would spend the rest of his career.
[1] The study led to the first definition of prediabetes, which Keen called "borderline diabetes", and demonstrated the relationship between glucose intolerance and cardiovascular disease at a population level.
[1] With the London School of Hygiene & Tropical Medicine, he conducted the Whitehall Survey in 1969,[5] which led to the creation of different glucose thresholds for microvascular and macrovascular disease.