The untold story about leptin and weight loss
Have you ever found yourself stuck at a certain weight? And no
matter how hard you try, you can't seem to shift those last
stubborn 10 pounds of fat?
Dieters know the phenomenon all too well. They cut calories,
exercise, and lose some weight. Then — like the flow of traffic
at junction eight of the M1 on a Friday afternoon — everything
grinds to a halt. And you can't figure out why.
It's easy to think the problem lies with you. Is it because your
metabolism is slow? Is it in your genes? It worked before, so why
isn't it working now? You stick to the program, and still you
don't lose any weight.
What's going on?
There are plenty of reasons why weight loss slows down the closer
you get to your target weight. But there's also some exciting
research to show that the struggle to lose weight is linked to
the amount of leptin in your body.
Your body has a set point — a level of fat that it wants to
maintain. When you try to reduce your body fat below this set
point, your body fights back by reducing the number of calories
your body burns each day (your metabolic rate) and turning on the
appetite "switch" in your brain.
The result is that weight loss slows down and you get very
hungry!
One of the hormones responsible for the increase in hunger is
known as leptin. Leptin (the Greek term for thin) is a hormone
released by your fat cells. When you lose fat, leptin levels
drop. When you gain fat, leptin levels rise.
Before the discovery of leptin, body fat was seen as a lifeless
source of energy. However, exciting work by researchers at the
Rockefeller University in the 1990's led to a radical change in
perspective.
Leptin made the news recently following animal research published
in the April 2004 issue of Science. Rockefeller scientists
report that leptin acts by changing the way brain cells
communicate with each other.
"This is a very dynamic effect that's quite dramatic and somewhat
surprising," says Dr. Jeffrey Friedman. "In response to leptin,
the cells create new connections. The malleability of these
feeding circuits by leptin suggest the possibility that the
brain's wiring may be different in lean versus obese
individuals."
One of the ways leptin regulates weight is to increase your
appetite as your body fat levels drop. Some direct evidence
linking leptin and hunger in humans comes from a trial carried in
the American Journal of Clinical Nutrition.
Twelve overweight but otherwise healthy women followed a
low-calorie diet for three months. The researchers found that the
desire to eat doubled in response to the diet. More interesting
still, the volunteers reporting the greatest increase in hunger
were those with the largest drop in leptin.
Once leptin has been secreted by your fat cells, it travels to
the hypothalamus. This is the part of your brain that controls
eating behavior. In the medial hypothalamus, leptin activates
anorectic nerve cells, which suppress your appetite. At the same
time, leptin prevents orexigenic cells from stimulating your
appetite.
Although it might seem counterintuitive, many obese individuals
actually have high levels of leptin. On the face of it, this
appears to blow all of the theories regarding leptin and weight
loss out of the water.
After all, if obese people have high levels of leptin, surely
they shouldn't be obese in the first place.
While obese people do have high levels of leptin circulating in
their body, the leptin isn't active where it needs to be — the
hypothalamus. Instead, they seem to have developed a form of
leptin resistance, where leptin is unable to make the "jump" from
the blood to the brain.
Not only do leptin levels drop in response to a reduction in body
fat, they also decline in response to exercise. That's why
it's important to choose foods with a high satiety index when
you're trying to lose weight with a combination of diet and
exercise.
(continued next week)
To view this update on-line, including the full list of
references and a printable version, please visit:
http://thefactsaboutfitness.com/research/leptin.htm