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A SMALL body may mean a long life, say geneticists in Europe and the US. But
it鈥檚 not being small itself that makes you live longer. Rather, it seems that
key genes controlling growth and metabolism can also have a dramatic effect on
lifespan.

One team led by Marc Tatar at Brown University in Rhode Island studied flies
with mutations in INR, the fly equivalent of the receptor for insulin and
insulin-related growth factor (IGF-I) in mammals. Mutations in these genes in
worms can triple adult lifespan, and the team wanted to see if the same was true
of flies.

The flies lived 85 per cent longer, the team found鈥攂ut they were dwarfs
compared with normal flies. They also failed to make a key hormone called
juvenile hormone that controls reproduction. If the mutants were given the
hormone, their lifespans returned to normal. 鈥淭here鈥檚 a trade-off between
reproduction and survival,鈥� Tatar says.

Another team led by Linda Partridge and David Gems of University College
London found that flies with mutations in chico, a protein that is activated
when hormones bind to the InR receptor, also produced dwarf flies that lived
nearly 50 per cent longer. But flies with only one copy of the mutant gene were
normal in size, yet still lived longer.

The findings might apply to humans. In mammals, the equivalent of chico
interacts with the hormone IGF-I, which controls growth and body size. Dwarf
mice with low IGF-I levels live 55 per cent longer. So factors that are known to
influence longevity, such as a very low-calorie diet, may work by reducing IGF-I
levels.

The work may also explain why a tiny Chihuahua can live for 16 years, while a
Great Dane keels over after six or seven. But size isn鈥檛 everything, says Tom
Kirkwood, an expert on ageing at the University of Newcastle upon Tyne. 鈥淭he
relationship between body size and longevity is something that has to be teased
out a lot further.鈥�

Kirkwood says the link between insulin, metabolism and ageing is consistent
with theories that ageing is the result of an evolutionary trade-off between
allocating resources to reproduction and growth, or to maintenance and repair.
But he is sceptical about there being a universal mechanism of ageing across
species. 鈥淟ooking at evolutionary conservation will inform our thinking, but
there may be important differences as well.鈥�

The research is also a cautionary tale for anyone hoping to defy the ravages
of time by popping pills. The human growth hormone taken by some Hollywood
celebrities to keep their youthful looks also boosts the production of IGF-I,
says Gems. 鈥淵ou could be rejuvenating yourself, but shortening your lifespan,鈥�
he warns.