Doctor Denham Harman: Scientist who developed the free-radical theory of aging and who himself carried on working into his nineties

Denham Harman, who has died at the age of 98, was a renowned scientist who developed the most widely accepted theory on aging that is now used to study cancer, Alzheimer's disease and other illnesses. He worked into his mid-nineties at the University of Nebraska Medical Centre.

Harman developed the free-radical theory of aging in 1954, though it took years for additional research to prove its importance. In simple terms, the theory holds that one of the by-products of oxygen utilisation is adverse chemical reactions in cells. The result is aging and, ultimately, death.

The medical community initially scoffed at Harman's theory. But by the 1980s free radicals had become part of research into cancer, cardiovascular disease and strokes and have since been linked to Alzheimer's disease.

David Sinclair, a professor at Harvard Medical School, said Harman's research inspired thousands of young scientists to work on aging research. "Dr Harman is one of the most influential scientists of the past 50 years, bringing world-class science to what was once a backwater of biology," he said, adding that Harman's theory "is a cornerstone of the aging field."

Harman believed the aging process could be slowed by reducing the production of free radicals with a healthy diet, regular exercise and taking certain vitamins. He also recommended not smoking and limiting alcohol intake– advice to which he adhered himself. Although he officially retired in 1986, he worked for more than another decade on his research before retiring again in 2010.

Denham Harman, biogerontologist: born San Francisco 14 February 1916; married (four children); died Omaha, Nebraska 25 November 2014.