Carbohydrate loading for exercise -- myth?
The classic study leading to the practice of carbohydrate loading was published in 1939 by
Christensen and Hansen [1]. Their observation: the more carbohydrate in the diet, the longer
the subjects could perform exercise. Later on, Bergstrom et al.[2] showed a direct correlation
between the amount of glycogen in a muscle and the duration of exercise that muscle could
sustain. However, the duration of Christensen's study was 7 days and that of Bergstrom's
was 7-10 days. In 1980, Phinney et al. showed that one week did not allow enough time for
the subjects to adapt to the low-carb diet, and their exercise times decreased. After 6
weeks, however, there was a major increase in exercise time exceeding the pre-diet
performance[3]. Another study showed that after 4 weeks on a low-carb high fat diet, trained
athletes can perform at 60% of their maximum aerobic power for the same duration as baseline
high-carb diet[4]. An important finding in this study was decrease in glycogen content
(about 50% drop) in thigh muscles of these athletes. It proved that there is not a direct
correlation between muscle glycogen content and work performance (provided there is enough
time to adapt to low-carb diet).
Therefore, the key principal of the carbohydrate-loading concept (correlation between muscle
glycogen content and performance) is valid only in the context of short term changes in diet
(less than 2 weeks).
Reference
[1] Christensen EH and Hanson O: Arbeitsf'~ihigkeit und Ern~ihrun. Skand Arch Physio 1939;81:160-172.
[2] Bergstrom J, et al.: Diet, muscle glycogen and physical performance. Acta Physiol Scand 1967;71:140-150.
[3] Phinney SD, et al.: Capacity for moderate exercise in obese subjects after adaptation to a hypocaloric, ketogenic
diet. J Clin Invest 1980;66:1152-1161.
[4] Phinney SD, et al.: The human metabolic response to chronic ketosis without caloric restriction: preservation of
submaximal exercise capability with reduced carbohydrate oxidation. Metabolism 1983; 32:769-776.
Christensen and Hansen [1]. Their observation: the more carbohydrate in the diet, the longer
the subjects could perform exercise. Later on, Bergstrom et al.[2] showed a direct correlation
between the amount of glycogen in a muscle and the duration of exercise that muscle could
sustain. However, the duration of Christensen's study was 7 days and that of Bergstrom's
was 7-10 days. In 1980, Phinney et al. showed that one week did not allow enough time for
the subjects to adapt to the low-carb diet, and their exercise times decreased. After 6
weeks, however, there was a major increase in exercise time exceeding the pre-diet
performance[3]. Another study showed that after 4 weeks on a low-carb high fat diet, trained
athletes can perform at 60% of their maximum aerobic power for the same duration as baseline
high-carb diet[4]. An important finding in this study was decrease in glycogen content
(about 50% drop) in thigh muscles of these athletes. It proved that there is not a direct
correlation between muscle glycogen content and work performance (provided there is enough
time to adapt to low-carb diet).
Therefore, the key principal of the carbohydrate-loading concept (correlation between muscle
glycogen content and performance) is valid only in the context of short term changes in diet
(less than 2 weeks).
Reference
[1] Christensen EH and Hanson O: Arbeitsf'~ihigkeit und Ern~ihrun. Skand Arch Physio 1939;81:160-172.
[2] Bergstrom J, et al.: Diet, muscle glycogen and physical performance. Acta Physiol Scand 1967;71:140-150.
[3] Phinney SD, et al.: Capacity for moderate exercise in obese subjects after adaptation to a hypocaloric, ketogenic
diet. J Clin Invest 1980;66:1152-1161.
[4] Phinney SD, et al.: The human metabolic response to chronic ketosis without caloric restriction: preservation of
submaximal exercise capability with reduced carbohydrate oxidation. Metabolism 1983; 32:769-776.