FW: Leptin and bone health

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Article from cnn.com

http://www.cnn.com/2002/HEALTH/diet.fitness/10/21/appetite.leptin.ap/index.

html

Appetite hormone linked to bone health

Monday, October 21, 2002 Posted: 2:04 PM EDT (1804 GMT)

WASHINGTON (AP) -- Researchers have spotted a likely new reason for bone

loss that afflicts women and teenage girls who exercise so much or eat so

little that they wind up starving themselves.

The focus is on the appetite hormone leptin, which is produced by fat

cells and influences brain chemistry that regulates appetite. When fat

cells are plump, leptin levels are high. When fat cells are lean, leptin

levels are low.

But scientists believe leptin's role gets more complicated in people who

eat far less than they require for normal metabolism. In these cases, the

body reduces activities that are not vital, and building bone is among

them.

Researchers say very low leptin levels prevent bone cells from doing their

normal job of building mineral in bone. " The bone goes to sleep, " said Dr.

P. Warren of Columbia-Presbyterian Medical Center in New York

City. " It's almost a hibernation, to conserve energy. "

Warren and colleagues at Columbia-Presbyterian reviewed research,

including her own, in the October issue of a medical journal, The

Physician and Sportsmedicine.

They focused on girls and women whose energy deficit can lead to a

exercise-associated amenorrhea -- the loss of their regular monthly

periods. Over time, women with this condition can lose bone, setting them

up for increasingly fragile bones, including osteoporosis, as they age.

A low-calorie, high-energy-output imbalance can happen because of the

demands of physical activities -- thinness is a valued characteristic in

gymnasts and ballet dancers, for instance. It also can happen to those

with anorexia, the compulsion to be thin.

" Low leptin levels have been reported in amenorrheic women who exercise

regularly at high levels, " the journal said.

Previous research had focused on estrogen, which declines in amenorrheic

females. That was partly blamed for the shutdown of the reproductive

system and loss of bone.

The new research indicates it's not so simple. Estrogen and leptin seem to

have different ways of doing damage, Warren said.

While leptin loss retards new bone-building, estrogen loss seems to take

the brakes off other cells, called osteoclasts, which strip out old bone

mineral, Warren said. The result would be double the bone vulnerability --

growth is inhibited and loss is accelerated.

Exercising this much also reverses what would otherwise be a healthy

trend. " Physical activity is important in maintaining bone mass; however,

many women tend to exercise excessively, causing hormonal changes that

predispose them to an increased risk of fractures, " the journal article

said.

Warren said the leptin link is worthy of further study. But scientists say

there is no sign now that leptin supplements would solve the bone-loss

problem.

Warren said the idea sounds promising but research to show it would spur

new bone growth has not been done. In addition, the supplement might harm

appetite in people who have calorie-balance problems, she said.

It's also too soon for doctors to use leptin levels as an early warning

sign of inadequate bone mass, said Dr. Carol Otis of Kerlan-Jobe

Orthopaedic Clinic in Los Angeles, who did not work on the review article.

Leptin is more properly " a marker of inadequate nutrition and weight

loss, " she said.

And even use of low leptin levels as an indicator of calorie-balance

problems could be misleading, Otis said. Leptin levels vary during the

day, so several blood samples would be required, and normal reference

levels of leptin in the blood have not been determined, she said.