Dr. George F. Cahill Jr., a diabetes expert who made pivotal discoveries about the role of insulin in metabolism by studying research subjects on starvation diets, and who testified for the prosecution at the trials of Claus von Bülow on charges of trying to murder his wife with insulin, died on July 30 in Peterborough, N.H. He was 85.

The cause was complications of pneumonia, his daughter Elizabeth Cahill Tiedemann said.

Dr. Cahill and generations of researchers he trained “wrote a lot of what have become the textbooks of physiology,” explaining glucose and protein metabolism both in normal health and in diabetes, said Dr. C. Ronald Kahn, the chief academic officer at the Joslin Diabetes Center in Boston, where Dr. Cahill was research director from 1962 to 1978.

Among his research subjects were divinity students who were paid $300 to fast for a week and hibernating bears. Some of his most important research, in the 1960s, involved tracking the blood chemistry of people who were trying out an experimental treatment for severe obesity: total starvation, for up to six weeks, in the hospital.

A crucial finding was that in the first few days without food, the liver starts breaking down protein to make glucose to feed the brain. But using protein as fuel can be perilous, because it is the stuff of vital organs and muscle.