In a nutshell...

1.)
When you eat carbohydrates (sugars and starches) your body must
maintain it's blood sugar balance quickly.  Your body produces
insulin which causes the sugars to quickly be stored as fat.

Conversely, when you have low blood sugar levels your
body burns fat, either from blood fats, or stored fats, usually
both at the same time.

Thus if you eat little or no carbohydrates you will
be burning fats.

2.) Most people overproduce insulin so not only does
all the carbs. go to fat, but about an hour later you
get VERY hungry since too much carbs are
taken out of the blood stream.  Thus willpower
breaks down an a low calorie, low fat diet,
and the person gives up and gains even more weight.

3.) After about 30 years of abusing your pancreas
by making it produce too much insulin, it gives out
and you become diabetic.

4.) The whole idea of a low carb diet is to get your body
running on fats instead of carbs.  It still amazes me how much
I can eat, as long as there are few carbs, and still loose
weight!

Then there is Paleo....

1.) Modern agriculture has introduces many new proteins
into the human diet, which evolution has not yet adapted to.
These proteins come from grains, dairy, and legumes.
These 'foreign' proteins cause obesity, arthritis, and
heart disease.

Thesis: If you eat a cave-man diet, you will be very healthy,
fit, and not overweight.

The overall premise of NeanderThin is that anything you can
acquire naked, with a sharp stick and eat raw (like a neanderthal)
is what your genetics are adapted to.
So next time you go to the store, take a sharp stick and leave your
clothes behind, oops, better not do that, anyway...
This results in a diet that is vegetables, meats, berries, and fruits,
with a little
nuts.  No dairy, grain or legumes.
No, the cholesterol won't kill you.  That is a great myth.

I started doing the Paleo thing to loose weight.  The completely
unexpected side efect has been the dissipation of
"brain fog" and depression.

So, search the net for sites that discuss this more...
http://www.panix.com/~paleodiet/
http://www.socsci.uci.edu/mbs/personnel/devany/EvolutionaryFitness.html

http://www.sofdesign.com/neander/nmain.html

R. Keene