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 causing water retention, progesterone causes women to be less likely to retain water during the first half of the luteal phase.
As progesterone drops during the late luteal phase/PMS week, there is a rebound effect which can cause water retention. As with the surge in estrogen before ovulation, this drop in progesterone changes how the body handles sodium and women on a high-sodium diet during this time may show extreme amounts of water retention.
The above are the potentially good effects of progesterone on a woman's body weight and body fat levels. Now let me look at the large number of bad effects. Like estrogen, progesterone increases LPL activity in women's lower body fat, breaking fatty acids off of chylomicrons for storage. This is compounded by the fact that progesterone also activates ASP which is not only one of the key enzymes in storing body fat but has been described as the most potent enzyme for the fat storage process.
ASP is found preferentially in subcutaneous fat (which women have more of to begin with) and, for all these reasons, one researcher has called ASP "A female fat storing factor". All of which is important as, unlike estrogen, progesterone potently activates ASP (ensuring storage of the fatty acids made available by its effects on LPL). Within the context of the menstrual cycle this sets up a sequence of events where the surge in estrogen before ovulation not only sensitizes the progesterone receptor but also increases the number of the fat loss inhibiting alpha-2 receptors. Increasing progesterone levels then activates LPL and ASP ensuring that excess calorie intake (driven by increased hunger) is stored as body fat.
As well, progesterone opposes estrogen in that it impairs insulin sensitivity meaning that the body doesn't utilize carbohydrates as well. This can be good or bad depending on the situation. In the context of a high-carbohydrate, low-fat diet, insulin resistance is not a good thing as there will be an overproduction of insulin. In contrast, insulin resistance can be beneficial on a lowered or low-carbohydrate diet. The practical implications of this, is that a higher carbohydrate/lower fat diet will be superior in the follicular phase while a lowered carbohydrate/higher fat diet will be superior in the luteal phase. As mentioned above, this insulin resistance also causes blood glucose levels to become more unstable, affecting energy levels, mood and potentially hunger.
Perhaps confusingly, the insulin resistance that develops during the luteal phase causes a decrease in carbohydrate use and an increase in fat utilization both at rest and during aerobic exercise. While this sounds beneficial, most of the additional fat being burned is from the IMTG stored within the muscle so this does not impact the visible subcutaneous fat stores directly. The combined effect is that progesterone increases the storage of fat in a woman's fat cells while increasing the use of fat stored in her muscles.
Taken as a whole, the effects of progesterone are to ensure and enhance not only general fat storage but fat storage specifically in a woman's lower body. Following ovulation, in preparation for pregnancy, the increase in progesterone will not only block estrogen's beneficial effects but will cause her to store more fat in her fat cells while burning more in her muscles. Although metabolic rate may be up slightly, hunger and cravings will increase and this tends to increase calorie intake far above the small increase in metabolic rate. If calorie and fat intake is too high during this phase, there will be increased fat storage. In contrast, if calories can be controlled, the changes in metabolic rate can be harnessed to potentially increase fat loss.
Looking briefly at training, progesterone's overall effects are quite negative. First and foremost, progesterone binds to the androgen receptor which would normally bind testosterone. This not only prevents testosterone from binding and having a positive effect on muscle but progesterone acts as a receptor antagonist, inhibiting any effect that might be seen. Due to this, progesterone tends to decrease tendon strength and decrease the ability to build muscle.
For endurance athletes primarily, the increase in body temperature during the luteal phase can cause problems with thermoregulation during exercise in hot or humid conditions. High-intensity endurance performance may also be impaired as the use of carbs for fuel is lowered because of the insulin resistance that develops.
In the next blog we will summarize the cycles of the menstrual cycle.
The above information is taken from the The Woman's Book by Lyle Mcdonald with Eric Elms