The following text is taken from Holistic Guide for a Healthy Dog, by
Wendy Volhard and Kerry Brown. Their discussion of carbohydrates and the
functions they perform seem to "prove" that most dogs need additional
carbohydrates in their diet, a belief that is very pervasive in most
concepts of canine nutrition.
"In addition to providing energy, carbs maintain the health of the
thyroid, liver, heart, brain and nerve tissue. They regulate how much
starch and fat will be broken down and utilized. Once in the digestive
tract and assimilated, they are stored in the liver in the form of
glycogen, which controls energy balance. Low carb intake may cause
cardiac symptoms and angina. The central nervous system requires
carbohydrates for proper functioning as does the brain. The brain can't
store glucose and is therefore dependent on the minimum supply of
glucose from the blood. With insufficent carbs in the diet, protein and
fat are converted to energy, weakening the immune system and preventing
the body from building enough antibodies to fight disease. Poor hair
growth and constant shedding are symptoms of carbohydrate deficiency.

Thyroid function is also dependent on the correct amount of
carbohydrates in a dogs diet. B compounds found in many grains and
strach producing veggies is needed so the amino acids phenylalanine and
tyrosine can produce T3".

But do most dogs really need carbohydrates? In the Waltham Book of Dog
and Cat Nutrition (2nd edition, 1988), we read that
"There is no known minimum dietary carbohydrate requirement for either
the dog or the cat. Based on investigations in the dog and with other
species it is likely that dogs and cats can be maintained without
carbohydrates if the diet supplies enough fat or protein from which the
metabolic requirement for glucose is derived."

How can this be? Let us discuss just how the dog and cat are able to
fulfill their requirement for glucose through a diet of raw meat, bones,
and organs.

Carbohydrates do provide quick and easy energy. However, it is not
'carbs' that maintain the health of the organs listed in the quotes
above, but glucose. Glucose can be obtained from protein through a
process known as gluconeogenesis, where amino acids (not fatty acids;
those use a different cycle) are "converted" to glucose. Fat can also be
used for energy; fats are broken down into Acetyl CoA and are fed
directly into the Citric Acid Cycle, bypassing the process of glycolysis
(the first stage of carbohydrate metabolism). Thus, glucose and energy
can be obtained from other sources. However, if carbs are present they
will be converted to energy first before protein and fats because they
are easier to use. This is the reason that carbs regulate how much
starch and fat will be broken down and utilized. If there is a plethora
of carbohydrates, fat will be stored instead of used. If there are not
enough carbs to fulfill energy needs, then fat will be converted to
Acetyl CoA and used. If no carbs are present, then fat and protein are
used to fill energy needs.

Excess carbohydrates are stored in the liver and the muscles as glycogen
AND in the body as fat. However, since carboydrates are not the only
source of glycogen (which also comes from proteins and fats through a
process known as glyconeogenesis), they are not absolutely necessary.
Human athletes commonly perform 'carbo loading' techniques where they
eat huge carby meals of things like pasta to rapidly replenish their
glycogen stores in their muscles and liver before a competition. The
carbohydrates, when in excess, are more rapidly converted and stored as
glycogen compared to fat and protein. HOWEVER, once again, fat and
protein can also be stored as glycogen, which makes carbohydrates
unnecessary unless you want to perform 'carbo loading'. I believe it is
Purina that has capitalized on this and now has "energy bars" of complex
carbohydrates for the canine athlete to help them recover more quickly
between events. But, carbohydrates do not rebuild spent muscle tissue,
etc. Protein does that. Fat is also easily utilized for quick energy,
too, and provides more energy per gram that carbohydrate does (9
kcals/gram of fat compared to 4 kcals/gram of carbohydrate).

It is not low carbohydrate intake that causes things like cardiac
symptoms and angina; it is low blood glucose. If there is not enough
glucose in the blood system, then you run into many problems including
black outs, cardiac symptoms (like arrhythmia), and angina (chest pain).
Of course, it is interesting that wolves can go without food for weeks
and still survive well enough. How do they do that without eating carbs?
Simple—they use up fat reserves and may even dip into their own muscle
to get the necessary proteins and fats to provide glucose and energy for
their bodies. So carbohydrates themselves are not actually necessary;
glucose is necessary, and that can be obtained from sources other than
carbohydrates.

What about the brain? The brain is preferentially given glucose above
all other organs. Glucose in its ready form, at that. But does this mean
carbohydrates are necessary? Since glucose can be had from protein and
Acetyl CoA from fat, then no.

What about the claim of protein and fat—when converted to
energy—weakening the immune system? This seems to be taken from human
research where athletes in intensive training had suppressed immune
systems which could be improved by consuming proper amounts of
carbohydrate. Additionally, white blood cell production in humans seems
linked to glucose production. More glucose present means the body is
better able to mount an immune response—until there is "too much"
glucose around and insulin spikes and starts suppressing all other
pathways in the body except for those needed to force the glucose into
cells (fat cells). High amounts of simple carbohydrates and sugars are
known to suppress the immune system. If this is the case, though, one
could wonder how a diet high in grain affects our pets—overstimulation
of the immune system due to high concentrations of glucose from the
grain? Perhaps this is why many pets suffer "allergies" while on grain!

One other comment I have here is that as long as the animal is receiving
appropriate fat and protein, glucose production will not be an issue.
And for carnivorous animals like dogs, I cannot help but wonder if their
white blood cells are more sensitive to glucose than ours--meaning, less
glucose is needed to "stimulate" canine white blood cell (WBC)
production compared to human WBC production.

Using protein and fat for energy does not weaken the immune system
unless there is not enough to go around, so to speak. If someone is
starving, then using protein and fats for energy—while necessary—is a
little 'cost-intensive' on the body. But it is not the lack of carbs
that is hurting them; it is the simple lack of enough food. Similarly, a
human athlete in intensive training may overwork their body to the point
that using protein and fats for fuel becomes too cost-intensive to their
body.

What about poor hair growth and constant shedding resulting from a lack
of carbohydrates? Can these indicate a 'need' for carbs? Maybe, but more
likely it indicates a need for better overall nutrition. I personally
have NEVER heard of 'carbohydrate deficiency' in any animal. Why?
Because there is NO SUCH THING as a "necessary carbohydrate," just
necessary glucose. Our bodies, and our dogs' bodies, can do without
carbohydrates (although I would say our dogs would fare better than
humans, since we are omnivores who do well with fresh vegetables in our
diet—except for some cultures that eat mostly meat!). Fats and proteins
can be converted easily to Acetyl CoA and glucose, respectively. Poor
hair growth and constant shedding are linked to an overall poor diet,
poor consumption of essential fatty acids, biotin deficiencies, some
vitamin and mineral deficiencies, AND a lack of good fats and proteins
in the diet. PROTEIN, not carbohydrate, is the building block for hair
and skin and all other parts of the body. Carbohydrates do nothing for
building and maintaining the body structures except provide easy glucose
to fuel the rebuilding process.

What about thyroid function? Thyroid function is dependent upon the
correct amount of GLUCOSE produced by the dog's body, not by the correct
amount of carbohydrates in the diet. Too much glucose from easily
available carbohydrate energy sources can cause just as many problems as
not enough glucose. Since we have already established that glucose can
be produced from protein (or even skipped during fat metabolism, where
Acetyl CoA is used instead), then it would again seem that carbohydrates
are actually unnecessary provided that there is enough protein and fat
to go around (and a raw diet has PLENTY!).

B compounds, or B vitamins, are found not only in the dog's own
intestine (bacteria produce some B vitamins) but also in the meat and
organs of prey animals. Feeding a variety of organ meats as part of a
proper raw diet will cover the B-vitamin requirement quite easily. One
has to wonder--how much of the B compounds in grain and starch and
veggies is actually available to the dog? Compared to something more
bioavailable like liver, then I would say 'not much.'

http://rawfed.com/myths/carbs.html