It's supposed to be simple: you eat when you're hungry and stop
when you're full.
It is, of course, much more complicated than that. As the rate of
obesity demonstrates, one's appetite stems from more than an empty
stomach or a need for nutrients. Humans are built to be susceptible to
super-sizing and that second slice of pie. Studies have shown that given
the opportunity, most of us will eat when we are not hungry. Give us a
bigger portion and we'll polish it off.
This isn't surprising, given that humans evolved to pack on a few
extra pounds in preparation for future shortages. But what has surprised
scientists who study appetite are the many subtle influences on when and
what we eat. In recent years, scientists have learned the urge to eat is
governed largely by an elaborate ricochet of hormones-among the dozens
are leptin and insulin-that shuttle between the gut and brain.
"Everyone used to think it was so silly that the stomach might talk
to the brain," says John Morley, M.D., a professor of gerontology at the
St. Louis School of Medicine, whose research gives new meaning to "gut
instincts." Morley has been studying the tendency for the elderly to lose
the desire to eat. "It makes sense that if you're stomach's empty, it
wants to tell the brain."
But whether the stomach is empty of full may not even matter. One
of these chemical messengers, ghrelin (pronounced GRELL-in), discovered
only a few years ago, stimulates our "need to feed" even in cases where
the belly is full.
Give a person a shot of ghrelin and they develop a ferocious
hunger. Levels increase during fasting and before meals and fall off
after eating. Secreted by endocrine cells in the stomach and small
intestine, ghrelin circulates in the bloodstream and stimulates the brain
and the pituitary gland to release other hormones, possibly growth
hormone, affecting metabolism and the cardiovascular system.
Ghrelin levels rise in people who have lost weight and may be the
reason dieters have trouble keeping their weight down for the long term.
However, a recent study in The New England Journal of Medicine showed
that ghrelin decreases in people who have undergone gastric bypass
surgery to lose weight. These people often report feeling less hungry
after the operation.
Such findings are often heralded in the news as the breakthrough
that will lead to a cure for obesity. Several drug companies are working
on it, hoping to be the ones to capitalize on a miracle pill. But Morley
says we have a lot to learn about appetite before we can even contemplate
new therapies for people who are overweight. Researchers are still
debating the many ways that ghrelin seems to affect the body.
"People get excited because they see one effect [of a hormone] and
don't see the others until later," he says. And there are always side
effects. Ghrelin seems to affect memory formation as well, says Morley, a
mechanism that is not yet understood.
Ghrelin's first use in medicine will likely be to stimulate
appetite in anorexics and in the elderly, says Morley. "I'm tremendously
cynical about obesity drugs."
He is emphatic that people should not hope that a cure for obesity
is around the corner. "The only way to combat obesity is exercise," he
says. "It's very simple." He points to the many studies that show
exercise does more than keep a person at a healthy weight. "There's more
and more data that shows exercise helps us think better, feel better and
fight depression. Yet we are trying to find a way to avoid something
that'd good for us."
Tags:
appetite,
bloodstream,
chemical messengers,
diet,
empty stomach,
endocrine cells,
food,
gerontology,
ghrelin,
growth hormone,
gut instincts,
hunger,
insulin,
john morley,
leptin,
nutrition,
pituitary gland,
ricochet,
school of medicine,
slice of pie,
small intestine,
subtle influences
