wayne hammons wrote:

> So, the heart of my question is this.  We will eat fresh fruits and
> vegetables every day and some days more than others.  Is there a
> threshold
> of carbs that can not be exceeded before you stop your ketosis metabolism
> and revert to some kind of glycogen metabolism.  It seems like I read
> that
> the brain prefers ketones to glycogens but the muscles will burn glucose
> reserves before processing fatty acids and therefore will not be creating
> ketones if carbohydrate consumption rises to high.


Some parts of the brain can and will use ketones for energy, while
others absolutely require glucose.  In addition, red blood cells and
fast-twitch muscle cells require glucose.  As for whether the brain
"prefers" ketones, it depends on how you look at it.  If glucose is
available, the brain will use that instead of ketones, so I suppose you
could say the brain prefers glucose.  On the other hand, you could also
say that when glucose is abundant, the body tries to deal with it, not
as a "preference" but as a way of regulating glucose levels.

I've seen different estimates as to the absolute minimum daily glucose
requirement for those tissues that absolutely need it.  For example,
I've seen it estimated that the brain burns about 500 kcal/day, which
would be about 125g of glucose, but that is when glucose is abundant, so
that figure could be treated as a ceiling value.  If you go into
ketosis, some of those 125g will be replaced by ketones, but not all.
How much?  Estimates vary but I've seen the claim that it's cut in half,
or less.  So, for the sake of argument, suppose the brain's absolute
minimum glucose requirement is 60g/day.  Red blood cells cannot use
ketones so they will require additional glucose, but I don't know how
much.  As for fast-twitch muscle fibers, it all depends on
activity/exercise patterns.  The kind of exercise that works the
fast-twitch fibers the most is short-duration, maximum intensity,
anaerobic effort, like power lifting, short sprints, shotputting, etc.
Those muscle fibers need glucose, but since this kind of exercise is
short duration, the total amount used isn't that much.  Moreover, if an
athlete trains in ketosis, the other muscle fibers, which can use either
glucose or fat, become more efficient at using fat.

The adaptation to ketosis by the brain and muscles can take a few days
to a week or so, which is why it's not unusual for people to complain of
feeling fogged in and sluggish at first.  Interestingly, some people
don't experience this at all (anecdotally), but feel energetic
immediately when in ketosis.  I don't know if this difference in
response to ketosis has been studied, but it would be interesting.  My
guess is that those who immediately feel good in ketosis are those who
are diabetic or pre-diabetic, with chronically elevated BG and insulin
making them feel not so great, so they experience ketosis as relief from
that.  Maybe those with normal BG and insulin feel a bit lagged when
first entering ketosis.  Just a guess.

Todd Moody
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