Gender Differences in fuel metabolism
There are differences between men and women in terms of the physiological response to
aerobic exercise. While the exact implication of these differences are unclear, they have one
major consequence with regards to the ketogenic diet, especially the CKD.
At any aerobic intensity, women use more fat and less carbohydrate and protein during
exercise (42-45). Studies also show that women do not respond to carb loading the same as men
do, most likely because they deplete less muscle glycogen less during aerobic exercise (42). This
has two important ramifications for women wishing to follow a ketogenic diet. First and foremost,
less dietary protein is required during the week as less protein is used during exercise and at rest
(43). As discussed in chapter 9, if ketosis can not be established and all other facets of the diet
are in order, protein should be reduced gradually until trace ketosis is established. Additionally,
since less glycogen is depleted during aerobic exercise (42,44) carbohydrate requirements for
women on a ketogenic diet are affected.
Anecdotally, some women report excessive fat regain during the carb-load portion of a
CKD, especially if they are not weight training during the week. This may be related to the
physiological differences outlined above. Since glycogen levels are depleted less in women than in
men, the chance for extra carbohydrate calories to ‘spill over’ during the carb-up and be stored as
fat is more likely.
If a woman is only performing aerobic exercise, a CKD will not be appropriate and the TKD
is the better choice. If a woman is weight training and following the CKD, but finds that fat
regain is occurring during the carb-up, the carb-up can be shortened or only performed once every
two weeks.
The exact cause of these gender differences is unknown but is probably related to one or
more of the following factors. Women have higher growth hormone levels than men at rest and
have a greater increase in GH during exercise (46). Additionally, women show a higher adrenaline
release to exercise than men as well as having lower baseline insulin levels (42,44,45). Finally,
women have a greater capacity for beta-oxidation (fat burning) than do men (47).
These differences only appear to occur during the luteal phase of the menstrual cycle,
which is the time period between ovulation and menstruation (42). Higher levels of estradiol (one
of the estrogens) also appear to be involved (44). Interestingly, this difference in substrate
utilization occurs only in untrained women and well trained women show roughly the same fuel
utilization pattern as men (48).
Section 3: The hormonal response
to aerobic exerciseSeveral hormones are affected by aerobic exercise depending on exercise intensity and
duration. Overall, the hormonal response to aerobic exercise is very similar to what is seen
during a ketogenic diet. Levels of anabolic hormones, such as insulin, go down while levels of