Fairly recently, isulin resistance (IR) has been recognized in horses and ponies (ponies being more prone to it). The IR horses were originally diagnosed as cushings horses, but
they didn't really fit the general profile of being older, laminitic, with curly unshedding coats.
They tend to get cresty with unusual fat deposits on their sides or tail
head. They were labeled "omental cushings" because of the omental-type fat,
but now are being called IR (insulin resistant) horses.
Here's a picture... look at the side of the horse and in the area of the
middle of the ribs... do you see the round blob of unusual fat?
My horse previously had been normal sized, receiving 3 large flakes per day of bermuda. Due to our move, and having to be boarded, she received 2 smaller flakes of oat hay per day. Seems like she might have lost weight, but she gained! The culprit was the oat hay.
For more information, please see the EquineCushings email list: EquineCushings. The files section has several important documents, as well as some in the archives of the GaitedHorse list.
Dr. Evelyn Kellon is a wonderful resource for information on this subject.
In SOME overweight people with metabolic syndrome / insulin
resistance / type II diabetes, a change has been found in the way the
fat cells in the abdomen handle cortisol (the omentum is the tissue
that covers the abdominal organs. It contains fat). The result is
an increased cortisol effect, a result of which is insulin resistance.
Whether this chance occurs after these people get overweight or is
the cause of their becoming overweight is not clear. There is
THEORY that insulin resistance in horses may have the same mechanism.
However, in people with this problem they classically only have heavy
fat deposits around the abdomen, not arms, legs, neck, etc.. The
horses get fat deposits all over. Also, this is far from the only
cause of insulin resistance in people and other animals so even if it
is eventually found to also occur in horses it probably won't be
accurate to suggest every horse with insulin resistance has omental
Cushings. For example, it's been known for close to 30 years that
ponies are more insulin resistant than horses - and this includes
normal ponies too, not just fat laminitic ones. It may very well be
just part of their normal metabolic make up, not a "disease" per se.
Cushings by the way just means an overproduction of active cortisol.
In the horse, the only proven cause of this is a pituitary tumor. It
could also happen from an adrenal gland tumor or problem with how the
cortisol is metabolized.
It would be nice if we could fit every horse into a little box, be
able to say that if it's not a pituitary tumor producing insulin
resistance it must be peripheral Cushings. Probably won't turn out
to be that simple though. The good news is that regardless of what
is causing the insulin resistance the management changes to control
it are the same - regular exercise, low NFC diet, avoidance of
extremes of fat or protein intake and mineral balancing. These
things are absolutely essential for good control, regardless of
whether it's just the individual's basic metabolism or is caused by a
pituitary tumor. Even the drugs for controlling Cushings/pituitary
tumor hormonal outputs won't completely control it without management
One of the genetic researchers
who is investigating breed associated disease tendencies, comments:
The first concept is "multi-generational environmental conditioning
(MGEC)." This may be related to a possible "sugar / fat
intolerance." MGEC can become significant when you have a constant
environment for hundreds of years. The baby in the womb is
being "genetically programmed" for its outside world. Is there a lot
of food out there, or is food a little grass and weeds growing along
the canal and a few dried corn stalks every once in a while? For
hundreds of years this breed may have been conceived, born,
reproduced and died in the latter environment.
When viewed from a multi-generational aspect, in an austere
environment, an easy keeping breed can turn into a "rich diet"
intolerant breed. Some portions of the population can handle rich
diets, some portions of the population will be intolerant of rich
diets and some portions of the population will develop and transmit
a genetic INABILITY to metabolize a rich diet. The spectrum of
response is ability, tendency, intolerance, and inability. The
inability manifests as a disease which has a genetic component.
However a significant comment is that the degree of expression is
influenced by both genetic and environmental factors. He went on to
name numerous documentated cases of this factor in the human