Progesterone is the second primary reproductive hormone in women and is released from the corpus luteum that develops after release of the egg at ovulation. While it has a tremendous number of roles in the body most of them aren't that relevant to this blog and I will once again focus on fat loss, fat gain while briefly addressing training.

As a steroid hormone, progesterone has a structure similar to many other hormones in the body and can actually be converted to aldosterone (involved in water balance), cortisol (a stress hormone), and the androgens. Due to that structural similarity, progesterone can bind to four different receptors. The first is the progesterone receptor itself where a normal signal will be sent. At the cortisol receptor, progesterone only sends a weak signal, weaker than cortisol itself. Progesterone is also an antagonist at the androgen and aldosterone receptor, not only blocking the effects of the hormones that would normally bind there but sending an actual negative signal. This cross reactivity not only explains many of progesterone's effects but is critical for the discussion of birth control, in which we will focus in later blogs.

As I described in the previous blog, progesterone remains low during the follicular phase of the menstrual cycle and has little to no effects at that time. During the luteal phase, following ovulation, progesterone starts to increase gradually, reaching a peak halfway through the cycle before decreasing again prior to menstruation. As much as estrogen tends to get the blame for so many aspects of women's fat loss issue, progesterone is of far more importance. Not only does it have its own profoundly negative direct effects, it also acts to cancel out estrogen's many positive effects, essentially doubling its negative effects in this regard.

Before discussing progesterone's negative effects, let's examine its one potential benefit in terms of fat loss. As previously mentioned progesterone is the cause of an increase in women's body temperature after ovulation and that Basal Body Temperature (BBT) has long been used to indicate when ovulation has occurred. Along with changes in vaginal mucus (due to estrogen), this can be used to determine a woman's peak fertility. Along with this increase in body temperature comes an increase in energy expenditure and resting metabolic rate.

This is counteracted by the fact that, during this time, both hunger and cravings are increased, adding to the fact that women are more prone to food cravings already. Women also show an increased attention of tasty, high-calorie foods. This can make controlling food intake more difficult and, on average, women's calorie intake increases more than their metabolic rate. The increase in hunger during the luteal phase occurs for several reasons. One of the primary reasons is the drop in estrogen from before ovulation which causes both serotonin and dopamine levels to go down.

While progesterone by itself does not appear to increase hunger, it does so in the presence of estrogen, which describes is the hormonal profile at this time. Progesterone causes some degree of insulin resistance and one consequence of this is that blood sugar may become unstable. This can cause blood sugar levels to fall, also stimulating hunger. Finally, although leptin levels go up during the luteal phase (which should help to control hunger), leptin resistance also develops so that this effect is blunted.

Overall, the above effects result in increased hunger and cravings, especially for high-sugar/high-fat foods (chocolate is the most commonly reported craving although other foods are often craved). When diet is uncontrolled, an increase in food intake of 90-500 calories has been observed during the luteal phase and this can readily exceed any small increase in metabolic rate that occurs.

So long as food intake can be controlled during this time, the increase in metabolic rate during the luteal phase can be used to enhance fat loss. If a woman is able to avoid an increase in calorie intake, the 100-300 calorie/day increase in metabolic rate with an average duration of 10 of the 14 days of the luteal phase would burn ~1000-3000 extra calories. This amounts to somewhere between 1/3rd to just under one pound of extra fat lost. Alternately, food intake could be increased slightly during this phase which could increase diet adherence for those women seeking fat loss.

I mentioned above that progesterone can bind to the aldosterone receptor, which is involved in water retention in the body. Since it blocks aldosterone from binding and