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1 Exercise Metabolism Group, Department of Human Biology and Movement Science, RMIT University, Bundoora, Victoria 3083; and 2 Sports Science and Sports Medicine, Australian Institute of Sport, Belconnen, Australian Capital Territory 2616, Australia
We determined the effect of fat adaptation
on metabolism and performance during 5 h of cycling in seven
competitive athletes who consumed a standard carbohydrate (CHO) diet
for 1 day and then either a high-CHO diet (11 g · kg
1 · day1 CHO, 1 g · kg1 · day1 fat; HCHO)
or an isoenergetic high-fat diet (2.6 g · kg1 · day1 CHO, 4.6 g · kg1 · day1 fat;
fat-adapt) for 6 days. On day 8, subjects consumed a
high-CHO diet and rested. On day 9, subjects consumed a
preexercise meal and then cycled for 4 h at 65% peak
O2 uptake, followed by a 1-h time trial (TT). Compared with
baseline, 6 days of fat-adapt reduced respiratory exchange ratio (RER)
with cycling at 65% peak O2 uptake [0.78 ± 0.01 (SE) vs. 0.85 ± 0.02; P < 0.05]. However, RER
was restored by 1 day of high-CHO diet, preexercise meal, and CHO ingestion (0.88 ± 0.01; P < 0.05). RER was
higher after HCHO than fat-adapt (0.85 ± 0.01, 0.89 ± 0.01, and 0.93 ± 0.01 for days 2, 8, and
9, respectively; P < 0.05). Fat oxidation
during the 4-h ride was greater (171 ± 32 vs. 119 ± 38 g; P < 0.05) and CHO oxidation lower (597 ± 41 vs. 719 ± 46 g; P < 0.05) after fat-adapt. Power output was 11% higher during the TT after fat-adapt than after
HCHO (312 ± 15 vs. 279 ± 20 W; P = 0.11).
In conclusion, compared with a high-CHO diet, fat oxidation during
exercise increased after fat-adapt and remained elevated above baseline
even after 1 day of a high-CHO diet and increased CHO availability.
However, this study failed to detect a significant benefit of fat
adaptation to performance of a 1-h TT undertaken after 4 h of cycling.
time trial; metabolism
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