Despite increases in the muscle’s ability to utilize fat for fuel, exhaustion during exercise is
still related to a depletion of glycogen in Type I muscle fibers (73). As discussed previously, the
exact cause of fatigue is unknown in this case but may relate to a decrease in Krebs cycle
intermediates such as citrate which are necessary for oxidation of FFA (79). This means that
endurance athletes following a ketogenic diet will still require carbohydrates for optimal
performance.
As with short-term carbohydrate depletion, there is a drop in RQ both at rest and during
exercise indicating greater reliance on fat for fuel (73-76). At least part of the long-term
adaptation to a ketogenic diet is an increase in the carnitine palmityl transferase 1 (CPT-1)
system allowing for greater utilization of fats during exercise (79). This may in part explain the
difference between short-term glycogen depletion and long-term adaptation and it has been
suggested that “These adaptations to a chronic exposure to high-fat or low CHO feeding may
‘retool’ the working muscle mitochondria and increase their capacity for fat oxidation.” and that
adaptation to a ketogenic diet are similar to that seen with endurance training (58)
Why the discrepancy between short and long-term studies?
As noted, there is a discrepancy in the capacity for endurance performance between
studies of short-term (1-5 day) glycogen depletion compared to longer term (2-6 weeks)
adaptation to a ketogenic diet. It appears, as with many aspects of human adaptation, the
impact on skeletal muscle metabolism of a ketogenic diet may take several weeks or more to
occur (58). Thus during the first few 3-4 weeks of a ketogenic or CKD, aerobic exercise
performance will most likely decrease. With chronic carbohydrate depletion, the muscles adapt
by improving their ability to use fat for fuel and performance may improve again.
Summary
Long-term adaptation to a ketogenic diet appears to improve the ability of the muscle to
use fat for fuel, although the exact mechanisms are not known. Endurance during low-intensity
exercise (below 75% of maximum heart rate) can be maintained or improved with a total lack of
dietary carbohydrates.
As exercise intensity increases, glycogen plays a much greater role in performance. And
the limited studies available suggest that performance at moderate-intensity (75-85% of
maximum heart rate) is decreased with a ketogenic diet. At higher intensities (90% VO2 max.
and above), fatigue is generally caused by factors other than glycogen availability and is
discussed in a later section.
The primary point of this section is this: individuals on a long-term ketogenic diet are
limited in the types of activity that they may comfortably perform. It appears that either low-
intensity activity or high-intensity activity is tolerated and only moderate-intensity activity
(near the lactate threshold where glycogen availability is the primary determinant of
performance) is compromised and should be avoided.