Some amino acids cannot be used to make glucose; those that can are
called glucogenic.  A smaller number can only be used to make ketones
and are called ketogenic; these can also be used to make energy.  Four
amino acids are both glucogenic and ketogenic; one of these is
tryptophan, a precursor for both seratonin and melatonin.  Only two
amino acids, leucine and lysine (not so coincidentally are essential
amino acids), cannot be converted to TCA cycle intermediates and used as
an energy source.  Animals cannot convert fat directly back into glucose,
instead making fatty acids and glycerols.  Both fatty acids and glycerols
are eventually converted into acetyl-coenzyme A (acetyl-coA), which are
used for energy in the TCA cycle.  Glycerols can also be converted to
glucose through several intermediate steps.  When non-carbohydrate
sources such as protein or fat are used to make glucose, it is called
gluconeogenesis.  It is clear from this discussion that ferrets can
create energy from four different sources: carbohydrate, fat, alcohol,
and protein, but the same is true of any mammal--even humans.  The
difference is that ferrets are superbly adept at converting protein into
energy and do not suffer the same problems as humans, or many other
mammals for that matter.  Rather than suffering rabbit starvation as
would a human on a lean prey diet, ferrets would thrive and become fat
little carnivores on a diet of nothing but Mr. McGregor's vegetable
patch nemesis, Peter.
 
In terms of the energy requirements during exercise, they of course would
be higher in exercising animals.  Muscles require energy to work, so the
more you work them, the more energy you need.  This is one reason why all
carnivores lounge around or sleep so much; it is to conserve energy.  The
need for additional energy would be much greater in an exercising ferret
than their corresponding need for nutrients.  In the wild, because
polecats and New Zealand feral ferrets use protein for energy, what this
means is they simply eat more food--that is, prey.
 
All mammals, humans included, eat to meet caloric needs, but will adjust
food preference -- whenever possible -- to satisfy particular dietary
cravings.  The catch is, eating to meet caloric needs is a misnomer, as
is the idea that ferrets are trying to regulate blood sugar.  In reality,
even when fasting, blood sugar levels are remarkably stable.  If they
weren't, then a human or a ferret sleeping for six hours would have
extreme hypoglycemia because the amount of glucose in their blood would
be used up in less than an hour.  Two hormones, insulin and glucagon,
regulate blood sugar by reacting to increased or decreased levels in the
blood.  Simplistically, insulin makes muscles and other cells store blood
sugar, so it is removed from the blood and stored in the liver, muscles
and other tissues.  Glucagon makes those same cells give up blood sugar,
increasing their levels in the blood.  Because both mechanisms are always
in the on position , they react very rapidly to changes in blood sugar
levels, so, unless some disease or pathological condition is in play, or
if there is a heavy workload while fasting, the blood sugar level is
remarkably stable--even during long fasts.
 
Ferrets, as well as humans, essentially eat to maintain insulin levels;
when they drop (and glucagon increases), you get hungry.  Consuming
carbohydrate, which is essentially a long chain of sugar, inundates the
blood stream with glucose, so insulin levels are increased to regulate
the flood.  When the blood glucose levels drop, so does the amount of
insulin, which is part of the hunger trigger .  So, ferrets on a diet
of carbohydrates tend to eat frequent, small diets in their attempt
to regulate insulin levels.  They do it NOT because they need to, but
because it is an adaptation to a diet the ferret is not designed to eat.
The ferret pancreas is not adapted to such a diet and the long-term
effect is that a large number of them develop insulinoma.  If asked, at
a future time I can post about the carbohydrate-pancreatic connection to
insulinoma, how they relate to stress, and how both have possible
influences on adrenal disease.
 
Ferrets are designed to eat a high protein diet and to use that nutrient
source to provide them with adequate sources of energy, so why do some
people insist ferrets require carbohydrates?  Part of the problem is most
studies are on humans and extrapolated to other species.  In humans, the
amount of glucose that can be made from protein is limited because the
process primarily takes place in the liver (and about 10% in the
kidneys).  The liver requires oxygen for gluconeogenesis, but most of
its oxygen for that purpose comes from oxygen-poor venous blood in the
hepatic portal system.  Between this oxygen poor system and the cost in
energy to convert protein to glucose, humans can only make enough glucose
to barely supply basal metabolic needs; that is around 1400-1500 kcal per
day (good luck if you are a large guy).  This is not enough calories for
active humans and they experience energy malnourishment; that is, they
suffer from rabbit starvation.
 
Ferrets, however, evolved eating a high protein, moderate fat, low
carbohydrate diet.  It is clear that ferrets have no carbohydrate
requirements, so their primary source of energy MUST be proteins
and fats; the amounts of carbohydrates consumed are essentially
scientifically insignificant.  If you assume the ferret is no more
efficient than a human in converting protein to glucose, how do they meet
their energy needs if they do not require carbohydrates?  One reason is
that ferrets have a brain far smaller in proportion to their body size
than a human, and the brain sucks up a lot of energy.  Another reason is
because the ferret has a liver that is proportionately larger than those
in humans, so they have proportionately more area for gluconeogenesis.
There are indications that more gluconeogenesis takes place in the ferret
kidneys, and that ferrets are slightly more efficient at recycling
nitrogen, so not as much is lost as urea.  Finally, ferrets are just a
lot smaller than people, so the internal volume of body tissues that
require energy are proportionately smaller.  All these reasons, and
likely more, allow the ferret to live a carbohydrate-free life, using
nothing more than protein to create the glucose necessary for life.
 
Bob C
Communication?  [log in to unmask]
Questions?  [log in to unmask]
[Posted in FML issue 4595]