This is where the physiology of the Laron patients links up with the longevity studies that researchers have been pursuing with laboratory animals. IGF-1 is part of an ancient signaling pathway that exists in the laboratory roundworm as well as in people. The gene that makes the receptor for IGF-1 in the roundworm is called DAF-2. And worms in which this gene is knocked out live twice as long as normal.

The Laron patients have the equivalent defect  their cells make very little IGF-1, so very little IGF-1 signaling takes place, just as in the DAF-2-ablated worms. So the Laron patients might be expected to live much longer.

Because of their striking freedom from cancer and diabetes, they probably could live much longer if they did not have a much higher than usual death rate from causes unrelated to age, like alcoholism and accidents.

Dr. Longo said he believed that having very low levels of IGF-1 was the critical feature of the Laron patients’ freedom from age-related diseases. In collaboration with Dr. Guevara-Aguirre, he exposed human cells growing in a laboratory dish to serum from the Laron patients. The cells were then damaged with a chemical that disrupts their DNA. The Laron serum had two significant effects, the two physicians reported on Wednesday in Science Translational Medicine.

First, the serum protected the cells from genetic damage. Second, it spurred the cells that were damaged to destroy themselves, a mechanism the body uses to prevent damaged cells from becoming cancerous. Both these effects were reversed when small amounts of IGF-1 were added to the serum.

Dr. Longo said that some level of IGF-1 was necessary to protect against heart disease, but that lowering the level might be beneficial. A drug that does this is already on the market for treatment of acromegaly, a thickening of the bones caused by excessive growth hormone. “Our underlying hypothesis is that this drug would prolong life span,” Dr. Longo said. He said he was not taking the drug, called pegvisomant or Somavert, which is very hard to obtain.