MILLIONS of malnourished people in the developing world are magnets for
infectious disease because levels of a hormone called leptin fall to low levels,
immunologists suggested this week. If they are right, the finding could lead to
a rethink of how to help save the lives of starving populations.
Experts have greeted the suggestion with optimism. 鈥淭here are 180 million
malnourished children in the world. That鈥檚 three times the child population of
the United States,鈥� says Lawrence Haddad of the International Food Policy
Research Institute in Washington DC. 鈥淪o any development that could help their
immune systems would be tremendous.鈥�
Malnutrition plays a role in the deaths of more than 6 million people, mostly
children, each year. For decades, experts have known that starvation diets cause
weakened immunity. This could help explain why diseases such as measles and
pneumonia kill so many children in developing countries.
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Lack of minerals and vitamins in diets has taken some of the blame. But
exactly why starvation makes people susceptible to infectious diseases has been
unclear.
Graham Lord and his colleagues at Imperial College鈥檚 school of medicine in
London believe they have now found a large part of the answer鈥攖he hormone
leptin, which regulates appetite. Researchers have already seen low leptin
levels in people who are underweight, and starved mice.
To find out more about the effects of low leptin levels, the Imperial College
team incubated human CD4 cells鈥攃ells that orchestrate the immune
response鈥攚ith other cells that can provoke an immune response in which the
CD4 cells multiply. They say that adding leptin in various concentrations had a
striking effect (Nature, vol 394, p 897). The more concentrated the
leptin, the higher the number of CD4 cells.
The team then tested the result in controlled experiments with mice. They
starved 24 mice for 28 hours, which lowered their leptin levels. This led to a
69 per cent reduction in the animal鈥檚 immune response to the chemical oxazolone,
which causes inflammation. But twice-daily injections of leptin during
starvation prevented the immune suppression.
Lord says that giving leptin to malnourished children at the same time as
vaccinations, for measles for example, may make immunisation more effective. 鈥淚t
seems that by keeping leptin levels up we鈥檙e telling the body to keep immune
function up,鈥� Lord told 快猫短视频. 鈥淏ut below a certain level,
leptin switches off or diverts energy expenditure from non-vital functions
including the immune system to things like the brain and the heart.鈥�
However, the results are surprising from an evolutionary perspective, says
immunologist Doug Fearon of Cambridge University. 鈥淵ou鈥檇 have to count the
cell-mediated immune system as a fairly vital system,鈥� he says. He doubts
that the immune system uses much energy, even when gearing up to fight a new
infection, so giving up that energy to organs such as the heart would seem
counterproductive. However, 鈥渢hat doesn鈥檛 make what they observed any less
interesting鈥�, Fearon says.
鈥淭his study provides a fascinating explanation for the link between
nutritional status and reduced immune responsiveness,鈥� adds Paul Trayhurn of the
Rowett Research Institute in Aberdeen. He believes it adds support to the idea
that leptin鈥檚 primary purpose is to 鈥渟ignal starvation鈥�.
Lord notes that other people with depressed immunity, such as patients with
HIV, also have low leptin levels. He believes the new findings could spur
research into whether leptin may help bring their immune systems under control.
