On Sun, 13 Jan 2002, matesz wrote:

> Todd Moody <[log in to unmask]> wrote:
> >According to Walsh (http://www.zonehome.com/met/met.htm), we use about
> >180g of glucose per day, mostly by the brain and red blood cells.  That
> >would vary somewhat by body weight, because of differences in blood
> >volume, but not all that much.  Utilization of glucose by the brain
> >should not vary much either.  So I take this to be a ceiling value:
> >anything more than 180g/day is converted to SFA anyway.
>
> My reply:
> That sounds arbitrary.  Whether carbohydrates consumbed above 180 grams per
> day are turned to fat or burned for energy would depend on several factors:
> 1) the person's daily energy (kcalorie) needs; 2) activity level, body
> weight, and muscle mass; 3) whether muscle and liver glycogen stores are
> full or empty; 4) what else the person eats that day.... 5) total calorie
> intake for the day, and perhaps other factors not mentioned.

If one is routinely eating more than 180g of carbohydrate per
day, glycogen stores will be pretty much continuously full,
except in the morning.  Energy needs are not as important as
*glucose demand*, and glucose demand comes mainly from brain
tissue, red blood cells, and fast-twitch muscle.  So activity
levels would affect it somewhat, especially high intensity
activity levels.  Conditioned muscles tend to use *less* glucose
and more fat.

Note that I am not saying that carbs above 180g must be turned to
fat and then stored in adipose tissue.  Your "turned to fat or
burned for energy" leaves out the third option, in which glucose
is turned to fat *and* burned for energy in that form.  Even a
eucaloric high-carb diet will result in a lot of glucose being
converted to (saturated) fat, and then burned for energy in that
form, although there would be no net gain in body fat.

> If a person stays with his or her daily calorie requirements but eats 200 or
> even 300 grams of carbs per day, there is no reason the body would convert
> it to fat if the body can use those calories for energy.

Yes there is.  The reason is that blood glucose must be kept
within narrow bounds, and incoming glucose can only have three
fates: (1) It is taken up by tissues immediately and used for
energy; (2) It is stored as glycogen; (3) It is converted to
saturated fat as circulating triglycerides, which either get used
for energy or eventually stored in adipose tissue.  If the person
stays in energy balance, storage in adipose tissue should be
minimal.  Concerning (1), the tissues that can use glucose
immediately can only do so at a rather steady rate, which Walsh
estimates to be 7.5g/hour.  Lutz estimates 8g/hour.  As stated
above, these tissues are mainly brain, red blood cell, and
fast-twitch muscle.  High-intensity exercise definitely increases
the glucose utilization rate somewhat, although it probably
doesn't affect the daily net utilization that much since
high-intensity exercise is of short duration.  But I grant that a
person who engages in a form of exercise with plenty of short
bursts of extreme exertion, such as tennis or karate, is going to
burn up more glucose.

Concerning (2), a person eating 200g to 300g of carbs per day is
going to have full glycogen stores most of the time precisely
because of the rate-limited nature of option (1).  But there will
always be some clearance in the morning.  If you eat dinner at
6:00pm, by 10:00pm your blood glucose will start to fall, and you
will have to start drawing glycogen out of storage to keep it in
range.  If you don't eat again until, say, 7:00am, then that's 9
hours of continuous glycogen draw, using about 65-70g of glucose.
So there will then be that much "clearance" for new glycogen
storage.  That means that if you ate 60g of carbs at breakfast,
whatever was not soon used by tissues (in three or four hours)
would go straight into glycogen storage, without any insulin
spike.  But this wouldn't work at lunch and dinner, because the
glycogen stores would not then be able to accept much more
glucose.

This, incidentally, appears to be the key to the Hellers'
"carbohydrate addict's" diet.  By allowing a single high-carb
"reward meal" per day, the diet creates a large glycogen
clearance, so that the extra carbs go right into the glycogen
bank without causing an insulin spike.  It doesn't have to be in
the morning, since saving it until evening only increases the
glycogen clearance.

Certainly the 180g/day rate is not absolute, for reasons that we
both mentioned, but the point is that it doesn't vary all over
the place.  A person wanting to reduce SFA load will not want to
eat much more than 180g/day of carbs, in order to avoid forcing
the conversion of glucose to fat.

Todd Moody
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