This is the explanation from the Leangains site:
Stubborn body fat refers to areas of
the body that hold on to fat the
longest. Generally speaking, these
areas include the lower abs and lower
back in men, and the lower body in
women. These areas are damn hard to
get lean.
How come these areas are stubborn in
the first place? To understand this,
let's look at how fat is mobilized
(the very short version).
After you eat, insulin and fatty acids
are elevated. You are in the fed state
and there's zero fat burning going on.
Your body is relying completely on
glucose oxidation during the hours
following the meal.
One way of measuring this is via the
respiratory quotient (RQ). An RQ of
1.0 denotes pure carbohydrate metabolism ("storage mode"), while 0.7
denotes pure fat metabolism. To put
this into perspective, consider that
RQ is 0.95-1.0 for about 1.5-2 hours
after a meal, 0.82-0.85 after
overnight fasting and 0.72-0.8 after
16 hours of fasting.
As the hours go by and the nutrients
from the meal are done being absorbed,
RQ drops in conjunction with insulin.
There's a shift towards fat burning
and mobilization of stored fat. This
process is mediated by insulin and
blood-borne fatty acids; when levels
drop, an energy deficit is "sensed"
and catecholamines (adrenaline and
nordrenaline) increase.
The catecholamines travel through the
blood and bind to receptors on fat
cells. A receptor can be thought of as
a "lock." Hormones and
neurotransmitters are keys that fit
into that lock and make something
happen. In this case catecholamines
trigger fat mobilization by activating
hormone sensitive lipase (HSL), which
then shuttles the fat out of the cell
to be burned off.
Now here's the critical difference
between regular fat and stubborn fat:
regular fat have a lot of beta-2
receptors in proportion to alpha-2
receptors.
In The Stubborn Fat Solution Lyle
McDonald used the analogy of
b2-receptors being "accelerators" for
fat loss and a2-receptors acting as
"breaks" for fat loss. That's the
easiest way to think of them without
getting too deep into the physiology.
The ratio between b2-receptors and
a2-receptors determines how easy it is
to facilitate fat loss from one region
of the body. "Easy" fat has a high
ratio of b2-receptors to a2-receptors,
while stubborn fat has a high ratio of
a2-receptors to b2-receptors.
One notorious example that Lyle brings
up in his book is that women have up
to nine times (!) as many a2-receptors
as b2-receptors in their hip and thigh
fat. Though I can't recall if similar
numbers are available for lower ab and
lower back fat for men, you can be
sure that the a2-receptors outnumber
the b2-receptors in these areas as
well.
