When people walk at low intensity after fasting, the energy needed is
provided mostly by oxidation of plasma fatty acids. As exercise intens
ity increases (eg, to moderate running), plasma fatty acid turnover do
es not increase and the additional energy is obtained by utilization o
f muscle glycogen, blood glucose, and intramuscular triglyceride. Furt
her increases in exercise intensity are fueled mostly by increases in
muscle glycogen utilization with some additional increase in blood glu
cose oxidation. Muscle glycogen and blood glucose contribute equally t
o carbohydrate energy production over 2-3 h of moderate-intensity exer
cise; fatigue develops when these substrates are depleted. Active peop
le can deplete muscle glycogen with 30-60 min of high intensity, inter
mittent exercise. When the ingestion of dietary carbohydrate is optima
l, it is possible to resynthesize muscle glycogen to high concentratio
ns in approximate to 24 h, which is the major factor in recovery of ex
ercise tolerance. However, this requires that a 70-kg person eat at le
ast 50 g carbohydrate per every 2 h, beginning soon after exercise, an
d ingest 500-600 g in 24 h (ie; approximate to 7-9 g/kg body wt). Carb
ohydrate foods eliciting high glycemic and insulinemic responses promo
te more rapid glycogen resynthesis than do foods eliciting lower glyce
mic responses. Therefore, foods ingested for energy before, during, or
after exercise should be classified according to their glycemic index
. Although carbohydrate ingestion before and during exercise adds exog
enous substrate to the body, it usually attenuates plasma fatty acid m
obilization and oxidation.