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.  But people do survive weeks of starvation. Dr. Cahill’s study helped explain why. After about a week, the body makes another shift, and instead of cannibalizing its own proteins it starts breaking down fat into substances called ketones, which can feed the brain in place of glucose. A natural drop in insulin is what drives the shift to ketones, Dr. Cahill determined.  The studies transformed scientists’ understanding of starvation and the way insulin regulates metabolism, said Dr. Joseph Avruch, a professor of medicine at the Harvard Medical School.  On a practical level, the findings helped establish the importance of dietary protein to prevent organ damage in people who are on very low-calorie diets or who are ill or injured and not able to eat normally, said Dr. Rudolph Leibel, co-director of the Naomi Berrie Diabetes Center at Columbia University Medical Center.  In addition, Dr. Kahn said, Dr. Cahill helped the military formulate high-energy bars for emergency rations.  In 1982 and 1985, Dr. Cahill was called as an expert witness for the prosecution in the two trials of Mr. von Bülow for the attempted murder of his wife, Martha (also known as Sunny). Mrs. von Bülow was in a deep coma, possibly caused by extremely low blood sugar. Her husband was accused of trying to kill her with insulin, which lowers blood sugar.  Dr. Cahill reviewed more than two years of her medical records and testified at the first trial that her coma, and a prior one from which she had recovered, were caused by insulin. The insulin level in her blood, he told the court, was “out of the ballpark.”  In the first trial, Mr. von Bülow was found guilty and sentenced to 30 years in prison. He appealed, the conviction was overturned, and he was tried again.  At the second trial, Dr. Cahill showed up with a pocket full of sugar cubes, which he lined up on the witness stand to help explain his points. He testified that insulin was the only plausible explanation for Mrs. von Bülow’s coma, but the defense lawyer challenged him repeatedly.  Finally, Dr. Cahill said, “It is so difficult for me to answer inane questions.”  Mr. von Bülow was acquitted at the second trial.  Dr. Cahill was disappointed, Dr. Kahn said, adding, “I think he felt justice had not been done.”  Mrs. von Bülow died in 2008, at 76, after nearly 28 years in a coma.  George Francis Cahill Jr. was born in Manhattan on July 7, 1927. His father was a urologic surgeon at the Columbia College of Physicians and Surgeons. George Jr. attended the Hotchkiss School in Lakeville, Conn., and entered Yale at 16.  At 17 he enlisted in the Navy as a pharmacist’s mate and was scheduled to take part in the invasion of Japan, but the atomic bombing of Hiroshima and Nagasaki changed that.  After the war he finished college at Yale, and in 1953 he graduated from medical school at Columbia.  He went on to become a professor of medicine at Harvard, and taught there until retiring in 1990. From 1958 until 1978 he also did research at the Joslin center, and from 1962 until 1990 he was a researcher and administrator for the Howard Hughes Medical Institute, which supports medical research.  Besides Ms. Tiedemann, Dr. Cahill is survived by three other daughters, Colleen Cahill Remley, Sarah Rhett Cahill and Eva Wagner Cahill; two sons, Peter duPont Cahill and George F. Cahill III; and 15 grandchildren. His wife of 60 years, the former Sarah duPont, died in 2010.  In an online article, Dr. Kahn called Dr. Cahill a “consummate teacher” and said that when he retired from medicine he began teaching a biology course for nonscientists at Dartmouth College that was an overnight success.  Within days of Dr. Cahill’s first lecture, the class had to be moved from a room that held 100 to an auditorium that seated more than 400.
Dr. Cahill clarified insulinâs role in metabolism by studying research subjects on starvation diets, and he testified at Claus von BÃ¼lowâs trials that he had tried to murder his wife with insulin.