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Figure 4: Schematic of study design for short-term glycogen depletion studies

Day 1 2 5 5-8 8

Baseline Deplete glycogen Repeat exercise No Repeat

Exercise Exercise test with exhaustive test exercise exercise

exercise test

Diet Normal Low carb, Low carb, Carb load

high fat high fat

With decreased glycogen levels, there is a decrease in blood lactate levels both at rest (61-
65,67) and during exercise (35, 59-68). This reflects the decreased use of carbohydrate and
greater use of FFA for fuel.

The increased use of FFA for fuel occurs both at rest (35,60-62,69) and during exercise
(35,59,60,62,64,67,69), as indicated by a decreased respiratory quotient (RQ). RQ is a measure
of the proportion of fat and carbohydrate being burned. Lower RQ values indicate greater fat
utilization and higher RQ values greater carbohydrate utilization.

Blood levels of FFA acids increase at rest (60,61-63) and during exercise (60,62). This
occurs due to the drop in insulin and blood glucose (60,62,63) as well as the increase in levels of
adrenaline and noradrenaline (70). The increase in FFA levels provides ample substrate for
ketogenesis in the liver. In the short-term there is no change in blood glucose uptake during
exercise (60,61,64).

Despite the increased use of fat for fuel and the ‘glycogen sparing’ effect, exercise
performance still suffers. Time to exhaustion decreases significantly with short-term
carbohydrate depletion at both low and moderate aerobic intensities (59,60,65,71).

Additionally, there is higher oxygen uptake at rest (61) and during exercise (29,60,62
67,69) as well as a higher heart rate at rest (61,69) and during exercise (60,62,67,69). The
oxidation of fat requires more oxygen than the oxidation of the same amount of carbohydrate so
this is to be expected. From a practical standpoint this means that, at any given workload,
exercise will feel subjectively harder under conditions of glycogen depletion. It should be noted
that not all studies have shown a change in heart rate or oxygen uptake during exercise (64)
when carbohydrates are restricted. Finally, there is increased activity of the enzyme lipoprotein
lipase, an enzyme involved in fat utilization in the muscle (66).

Effects of glycogen super compensation on aerobic exercise

In general, studies that compare glycogen loading to normal or depleted glycogen levels find
the opposite of the above results. In a glycogen compensated state, some studies find increased
levels of lactate during exercise (35,68) while others have found no change (66). There is also a
higher RQ during exercise indicating greater use of glycogen (35). Despite increased reliance on
carbohydrate, there is still a greater time to exhaustion (65) and higher peak power output (67)
in a carb-loaded state. Overall performance capacity increases.