Quoted message said:The question that I said I'd look up (still haven't had the time to research it, but I shall do
so) is whether or not people on very low carbohydrate diets do, in fact, have reduced glycogen
stores.<<
This was surprisingly hard to track down. I still don't have the answer with regard to a study in
humans (perhaps not surprising; would require multiple muscle biopsies).
The following rat study was very interesting, however.
Basically, they looked at both red ("aerobic"😉 and white ("anaerobic"😉 muscles in animals on high
fat diets vs high carb diets, pre and post exercise, for glycogen content.
The only differences were in the case of the white muscle. In the high fat group, muscle glycogen
was lower than in the high carb group. In red muscle, there were no differences in glycogen, pre-or
post exercise. White muscle can burn only carbs (they are "white" because they lack mitochondria,
which are required to burn fat and lactate, which, along with glucose [from glycogen], are the main
fuels for muscle fiber contraction). It seems that with a high fat diet, there is a selective
dis-use (lack of use) of white muscle and more use of red muscle (capable of burning fat + lactate).
So the athlete burns more fat (but also takes in more fat, so the fact that the athlete burns more
fat does not necessarily mean that the net "fat balance" (intake vs burning) is skewed to promote a
greater net loss of fat).
Glycogen is probably not depleted in red muscle on high fat diets anymore than on high carb diets
because less glycogen is being burned in the red muscles on the high fat diet (red muscle can burn
either glycogen, fat, or lactate, while white muscle can burn only fat).
What is the potential disadvantage in not burning glycogen for fuel in exercising muscle? What is
the potential disadvantage in not using white muscle fibers?
Well, glycogen metabolism generates ATP faster than does fat metabolism. So the rate of ATP
production will be theoretically diminished, when glycogen isn't burned. And white muscle fibers are
the so-called "fast twitch" fibers which power explosive performance (as in sprinting). So not using
these fibers is a potential disadvantage in competition.
At the level of the ultra-endurance athlete or at the leve of the fitness swimmer, I doubt that
there would be a reproducible, measurable difference in performance.
Maybe the reason Martin's swimming performance has been just a little uneven since changing diets
has to do with the necessity of getting his muscle fibers to adjust to the changing energy sources,
which may take a little time. As I wrote some months ago (relating to my own experience), I do think
that when a swimmer loses weight that the swimmer's buoyancy changes, balance changes, and
performance may change (at least temporarily) as well.
The caveat to all of this is that this was a RAT study. Not sure that a similar study ever has been
(or will ever be) done in humans.
1: Int J Sports Med. 1998 Aug;19(6):419-24.
Glycogen depletion patterns in trained rats adapted to a high-fat or high-carbohydrate diet.
Nakamura M, Brown J, Miller WC.
Department of Kinesiology, Indiana University, Bloomington, USA.
Male Sprague-Dawley rats (n = 48, > 200 g) were progressively treadmill trained over 5 wk where they
were running 60 min/d, 5 d/wk. One-half of the group consumed a high-fat diet (HF, 78.7% of energy),
while one-half consumed a high-carbohydrate diet (HC, 68.7% of energy). On the day of the
experiment, 6 rats per diet were run at 29 m/min, 8% grade for 0, 10, 20, or 60 min. Immediately
post-exercise rats were anesthetized, and soleus (S), red vastus lateralis (RV), and white vastus
lateralis (WV) muscles were removed. There were no significant differences between diets for S
glycogen pre-, during, or post-exercise. RV glycogen (micromol x g(-1) wet wt) was lower (p < 0.05)
at rest for the HF (27.5 +/- 3.9, Mean +/- SEM) vs the HC (37.6 +/- 3.5), but similar to HC at 60
min (11.0 +/- 1.9, HF; 8.6 +/- 1.3, HC). RV glycogen use rates (nmol x g(-1) x min(-1)) were lower
for the HF (985 +/- 295, 356 +/- 61) than the HC (1593 +/- 144, 1055 +/-
2) for 0-10 and 11-20 min, respectively. Resting WV glycogen was lower for the HF (25.3 +/-
1.6) vs the HC
(3.7 +/- 5.8), while post exercise values were similar
(4. +/- 4.4, HF; 15.7 +/- 2.0, HC). WV glycogen use was negligible from 0-10 and 11-20 min in the HF
compared to the HC (280 +/- 169 and 1601 +/- 177 nmol x g(-1) x min(-1), respectively). These
data indicate that muscle glycogen is spared during the early stages of prolonged exercise in HF
adapted rats and that the sparing occurs according to expected muscle recruitment patterns.
Larry Weisenthal
Certitude is poison; curiosity is life
