Abstract
Carbohydrate and
fat are the two primary fuel sources oxidized by skeletal muscle tissue
during prolonged (endurance-type) exercise. The relative contribution of
these fuel sources largely depends on the exercise intensity and
duration, with a greater contribution from carbohydrate as exercise
intensity is increased. Consequently, endurance performance and
endurance capacity are largely dictated by endogenous carbohydrate
availability. As such, improving carbohydrate availability during
prolonged exercise through carbohydrate ingestion has dominated the
field of sports nutrition research. As a result, it has been
well-established that carbohydrate ingestion during prolonged (>2 h)
moderate-to-high intensity exercise can significantly improve endurance
performance. Although the precise mechanism(s) responsible for the
ergogenic effects are still unclear, they are likely related to the
sparing of skeletal muscle glycogen, prevention of liver glycogen
depletion and subsequent development of hypoglycemia, and/or allowing
high rates of carbohydrate oxidation. Currently, for prolonged exercise
lasting 2-3 h, athletes are advised to ingest carbohydrates at a rate of
60 g·h⁻¹ (~1.0-1.1 g·min⁻¹) to allow for maximal exogenous glucose
oxidation rates. However, well-trained endurance athletes competing
longer than 2.5 h can metabolize carbohydrate up to 90 g·h⁻¹ (~1.5-1.8
g·min⁻¹) provided that multiple transportable carbohydrates are ingested
(e.g. 1.2 g·min⁻¹ glucose plus 0.6 g·min⁻¹ of fructose). Surprisingly,
small amounts of carbohydrate ingestion during exercise may also enhance
the performance of shorter (45-60 min), more intense (>75 % peak
oxygen uptake; VO(₂peak)) exercise bouts, despite the fact that
endogenous carbohydrate stores are unlikely to be limiting. The
mechanism(s) responsible for such ergogenic properties of carbohydrate
ingestion during short, more intense exercise bouts has been suggested
to reside in the central nervous system. Carbohydrate ingestion during
exercise also benefits athletes involved in intermittent/team sports.
These athletes are advised to follow similar carbohydrate feeding
strategies as the endurance athletes, but need to modify exogenous
carbohydrate intake based upon the intensity and duration of the game
and the available endogenous carbohydrate stores. Ample carbohydrate
intake is also important for those athletes who need to compete twice
within 24 h, when rapid repletion of endogenous glycogen stores is
required to prevent a decline in performance. To support rapid
post-exercise glycogen repletion, large amounts of exogenous
carbohydrate (1.2 g·kg⁻¹·h⁻¹) should be provided during the acute
recovery phase from exhaustive exercise. For those athletes with a lower
gastrointestinal threshold for carbohydrate ingestion immediately
post-exercise, and/or to support muscle re-conditioning, co-ingesting a
small amount of protein (0.2-0.4 g·kg⁻¹·h⁻¹) with less carbohydrate (0.8
g·kg⁻¹·h⁻¹) may provide a feasible option to achieve similar muscle
glycogen repletion rates.