On Tue, 28 Dec 1999, Kenny Brown wrote:

> All I wanted to know is if there is a metabolic need
> for increased carbs with increased activity or if the
> fat would be enough and fast acting enough.  Have you
> ever been so low on blood sugar that you could not
> hear, barely see, have fits of diziness along with
> slurred speech while working at the bookstore?

The simple answer is that for increased aerobic activity you
should not require more carbs.  Studies by Phinney suggest that
it may take a few weeks to adjust to lower carb levels, during
which time performance may suffer, but after that you should do
as well or better.  Note that this would not apply to anaerobic
training such as weight lifting.

The study below pertains to extreme low-carb diet (ketogenic),
but should help to answer your question.

Todd Moody
[log in to unmask]


Metabolism 1983 Aug;32(8):769-76


The human metabolic response to chronic ketosis without caloric restriction:
preservation of submaximal exercise capability with reduced carbohydrate
oxidation.

Phinney SD, Bistrian BR, Evans WJ, Gervino E, Blackburn GL

To study the effect of chronic ketosis on exercise performance in
endurance-trained humans, five well-trained cyclists were fed a eucaloric
balanced diet (EBD) for one week providing 35-50 kcal/kg/d, 1.75 g protein/kg/d
and the remainder of kilocalories as two-thirds carbohydrate (CHO) and
one-third fat. This was followed by four weeks of a eucaloric ketogenic diet
(EKD), isocaloric and isonitrogenous with the EBD but providing less than 20 g
CHO daily. Both diets were appropriately supplemented to meet the recommended
daily allowances for vitamins and minerals. Pedal ergometer testing of maximal
oxygen uptake (VO2max) was unchanged between the control week (EBD-1) and week
3 of the ketogenic diet (EKD-3). The mean ergometer endurance time for
continuous exercise to exhaustion (ENDUR) at 62%-64% of VO2max was 147 minutes
at EBD-1 and 151 minutes at EKD-4. The ENDUR steady-state RQ dropped from 0.83
to 0.72 (P less than 0.01) from EBD-1 to EKD-4. In agreement with this were a
three-fold drop in glucose oxidation (from 15.1 to 5.1 mg/kg/min, P less than
0.05) and a four-fold reduction in muscle glycogen use (0.61 to 0.13
mmol/kg/min, P less than 0.01). Neither clinical nor biochemical evidence of
hypoglycemia was observed during ENDUR at EKD-4. These results indicate that
aerobic endurance exercise by well-trained cyclists was not compromised by four
weeks of ketosis. This was accomplished by a dramatic physiologic adaptation
that conserved limited carbohydrate stores (both glucose and muscle glycogen)
and made fat the predominant muscle substrate at this submaximal power level.

PMID: 6865776, UI: 83244130

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