It is the Holy Grail of health research, discovering the key to help people live longer.
Now scientists believe they may be one step closer.
A team at Yale School of Medicine have identified a hormone, produced by the thymus glad [sic], extends lifespan by 40 per cent.
Their findings reveal increased levels of the hormone, known as FGF21, protects the immune system against the ravages of age.
Researchers said the study could have implications in the future for improving immune function in the elderly, for obesity, and for diseases such as cancer and type 2 diabetes.
When it is functioning normally, the thymus produces new T cells for the immune system.
But with age, the gland becomes fatty and loses its ability to produce the vital cells.
This loss of new T cells in the body is one cause of increased risk of infections and certain cancers in the elderly.
Researchers led by Vishwa Deep Dixit, professor of comparative medicine and immunobiology at Yale, studied transgenic mice with elevated levels of FGF21.