Gender Differences in fuel metabolism

The 3-Week Ketogenic Diet

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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).

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