Macleod did not continue to work on insulin, but he remained active as a researcher, lecturer and author. His last major contribution was a proof that the central nervous system does have an important role in maintaining carbohydrate metabolism balance, as was his original hypothesis. His theory about conversion of fats into carbohydrates remained unproven, despite his provision of several indirect proofs. He devoted his spare time to golf, motorcycling and painting. He was married to Mary W. McWalter, but they never had children. He died in 1935 in Aberdeen after several years of suffering from arthritis, despite which he remained active almost until his death. In 1933 he made a lecture tour of the US, and in 1934 he published the 7th edition of his book "Physiology and Biochemistry in Modern Medicine".