“Fat adaptation” for athletic performance: the nail in the coffin?
ENDURANCE ATHLETES have a high capacity for the oxidation
of fat during exercise as a legacy of their training. There-
fore, it is intriguing that this capacity can be easily upregu-
lated by the chronic consumption of a low-carbohydrate
(⬍2.5 g䡠 kg
⫺1
䡠 day
⫺1
), high-fat (⬃65–70% of energy) diet. For
example, 2– 4 wk of exposure to such a diet in trained indi-
viduals has been shown to markedly increase fat oxidation and
reduce the utilization of muscle glycogen during subsequent
submaximal exercise (10, 11). Despite the promise of an
enhanced ability to “tap into your body fat,” fat loading per se
does not seem to lead to a clear enhancement of exercise
capacity or performance (for review, see Ref. 8). In fact, there
is at least a short-term increase in the perceived effort of
training (2, 3) and an impairment of the response to training
when the high-fat, low-carbohydrate eating continues for pe-
riods longer than 4 wk, based on data from previously un-
trained individuals (7).
Several more recent studies reignited the interest in fat
loading for athletes. Goedecke and colleagues (5) provided a
practical option with their observations that an increased fat
utilization during submaximal exercise could be achieved in as
little as 5 days of training on a high-fat (69% of energy),
low-carbohydrate diet. These adaptations were subsequently
shown to be consistent and robust, persisting in the face of
protocols to increase carbohydrate availability by subsequent
restoration of muscle glycogen content with 1 day of rest and
the intake of a high-carbohydrate intake (10 g䡠 kg
⫺1
䡠 day
⫺1
) (1,
3, 4) or the consumption of carbohydrate before and during a
bout of prolonged exercise (3, 4). Such a combination of
dietary strategies would seem the perfect competition prepara-
tion for an endurance or ultraendurance athlete, simultaneously
restoring carbohydrate stores while maximizing the capacity
for fat oxidation during submaximal exercise. Interestingly,
when carbohydrate loading after dietary fat adaptation is ex-
tended beyond 3 days, muscle glycogen stores are supercom-
pensated, and a high-carbohydrate utilization during exercise is
achieved (8). Nevertheless, the effect of various “dietary peri-
odization” on exercise performance has remained unclear, with
studies reporting benefits (9), no change (1, 3, 4), or impair-
ment (7, 8) to various endurance and ultraendurance protocols.
A variety of explanations has been offered to explain the
apparent lack of transfer between metabolic changes and per-
formance outcomes (2). They include the failure of scientists to
detect small changes in performance that might be worthwhile
in real-life sports and the existence of “responders” and “non-
responders” to fat-adaptation strategies (1, 11). In addition,
adaptations to a fat-rich diet have been shown to increase
plasma norepinephrine concentrations and heart rate during
submaximal exercise (7), possibly leading to increased per-
ceived effort of exercise training (2, 3). The paper by Have-
mann and colleagues (6) in the present issue of Journal of
Applied Physiology adds weight to this possibility.
Previous studies have focused on the metabolic changes
occurring with dietary fat adaptation strategies as an indication
of the upregulation of fat metabolism. Mechanisms have in-
cluded increases in putative fatty acid transporters as well as
enzymes of lipid metabolism (for reviews, see Refs. 2, 8).
However, there is now evidence that what was initially viewed
as “glycogen sparing” after adaptations to a fat-rich diet may
be, in fact, a downregulation of carbohydrate metabolism or
“glycogen impairment.” One study (12) has reported that fat
adaptation/carbohydrate restoration strategies are associated
with a reduction in the activity of pyruvate dehydrogenase; this
change would act to impair rates of glycogenolysis at a time
when muscle carbohydrate requirements are high. The present
study of Havemann et al. (6) furthers our knowledge by
applying the fat adaptation/carbohydrate restoration model to
an endurance cycling protocol that involves several features of
a real-life race: self-pacing and the interspersing of high-
intensity bouts of cycling with more moderate-intensity seg-
ments. The results show that the dietary strategy has no effect
on overall performance of a 100-km time trial but compromises
the ability of well-trained cyclists to performance high-inten-
sity sprints.
It is tempting to classify endurance and ultraendurance
sports as submaximal exercise, which might benefit from
increased fat utilization and a conservation of limited endog-
enous carbohydrate stores. However, the strategic activities
that occur in such sports, the breakaway, the surge during an
uphill stage, or the sprint to the finish line, are all dependent on
the athlete’s ability to work at high intensities. With growing
evidence that this critical ability is impaired by dietary fat
adaptation strategies and a failure to find clear evidence of
benefits to prolonged exercise involving self-pacing, it seems
that we are near to closing the door on one application of this
dietary protocol. Scientists may remain interested in the body’s
response to different dietary stimuli and may hunt for the
mechanisms that underpin the observed changes in metabolism
and function. However, those at the coal-face of sports nutri-
tion can delete fat loading and high-fat diets from their list of
genuine ergogenic aids for conventional endurance and ultra-
endurance sports.
https://www.researchgate.net/public...r_athletic_performance_The_nail_in_the_coffin
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