One of the primary factors that separate women and men is the presence of the menstrual cycle, the roughly 28 day cycle during which her primary sex hormones estrogen and progesterone change in a fairly “standard” pattern. During this time, nearly every aspect of her physiology changes. Specific to today’s article I want to look at the impact of the menstrual cycle on energy balance (i.e. calorie intake vs. calorie expenditure). In doing so I will be primarily looking at the following paper.
Women and Body Composition
As I discuss in extreme detail in The Women’s Book women get the short of the end of the stick when it comes to body composition. Their bodies fight back harder, they lose both weight and fat slower (even given an identical intervention), they tend to gain fat more easily, they gain muscle more slowly, etc. Then again, they do get that whole multiple orgasm thing so there is at least some good that comes along with the bad.
In any case, there are a lot of potential reasons for things to be this way and it’s been theorized that the importance of women in keeping humanity alive (by raising children) during famines is a huge part of the gender discrepancy. For example, women are more likely to be in the super-obese category and far more likely to survive famines than men. While even men’s bodies fight back, the simple fact is that women’s pretty much always fight back more.
Of course, the biggest potential impact on all of this is hormones which differ drastically between men and women. It’s been known for a while that women’s fuel utilization changes during their menstrual cycle, as does appetite and potentially energy expenditure.
In this vein, it’s been suggested that dieting (and training) might or should be modified during the menstrual cycle to match up physiologically with what is going on in a woman’s body. I’ll come back to this a bit below.
That’s what today’s research review is about, a look at how things such as energy intake, appetite, energy expenditure and body weight change throughout a woman’s cycle, as well the impact of birth control is briefly examined along with some issues related to PMS and food cravings.
And Overview of the Menstrual Cycle
The first section of the paper is simply a review of the hormonal changes which occur during a normal menstrual cycle. Although there is variability, the typical woman’s full cycle is 28 days (this is an average) which is typically divided up into 4 distinct phases. With menstruation taken as day 1, we can define early follicular phase (day 1-4), late follicular (days 5-11), periovulation (day 12-15), and luteal phase (days 16-28).
A number of hormones change during the cycle but the two that I’m going to focus on are estrogen and progesterone. During the early follicular phase, both estrogen and progesterone are relatively low. Estrogen (estradiol in the graphic below) shows a peak in the late follicular phase followed by a drop. Progesterone starts a slow increase through ovulation and both estrogen/progesterone peak in the middle of the luteal phase before returning to baseline.
This pattern of hormonal change between estrogen (estradiol) and progesterone is shown in the graphic below. Estradiol is the blue line, progesterone the black line. The top lines show changes in a woman’s body temperature with the increase occurring just after ovulation before dropping back to normal halfway through the luteal phase.
The Menstrual Cycle and Food Intake
The paper then examined research on energy (food) intake during different parts of the menstrual cycle. In animals, energy intake is reduced at ovulation (when estrogen peaks) and increases after ovulation when progesterone is peaking; this has long been interpreted as indicating that progesterone drove food intake.
Research in humans has generally borne out that pattern. Appetite tends to be the least right before ovulation, increasing as ovulation. Pointing out the often enormous variability between women, the increase in calorie intake during the luteal phase has been reported to be 90-500 calories per day.
The Role of Estrogen and Progesterone
It’s interesting to note that some research suggests that it is falling estrogen rather than increasing progesterone that drives hunger. Among its other roles, estrogen both improves leptin signaling in the brain along with sending a leptin-like signal to the brain. Falling estrogen would reduce overall signalling which would tend to facilitate hunger.
Of course, there is also some reason to think that it’s a combination effect of estrogen and progesterone that is having the overall effect. Given everything that’s changing at once, it is often difficult to determine exactly which hormone (or how they are interacting) is having a specific effect. This makes research on women incredibly difficult to do.
Tangentially, the paper mentions that estrogens might play an important role for weight and fat loss loss through inhibition of food intake. I’d also mention that a good bit of data suggests that estrogen is actually lipolytic, helping to mobilize fatty acids during aerobic activities. In fact, if you inject men with estrogen, they will mobilize fatty acids more effectively as well.
It’s actually even more complicated than this and estrogen can be seen to be having both positive and negative effects on fat loss (and regional fat loss). I discuss this in more detail in The Stubborn Fat Solution; sufficed to say that idea that ‘estrogen is bad’ in terms of fat loss is a simplistically incorrect one.
I bring this up for a couple of reasons. It’s interesting to note that with increasing fat loss, estrogen levels typically drop and this is probably part of why women’s hunger increases as they get leaner (there are tons of other adaptations occurring). Certainly falling estrogen doesn’t make women’s lower body fat any easier to get rid of.
The Role of Blood Glucose
Finally, the paper notes that some of the drive for appetite may be mediated by changes in blood glucose homeostasis. Empirically, some women seem to be more prone to hypoglycemia during certain phases of the menstrual cycle and falling blood sugar can stimulate hunger.
Ensuring that blood glucose levels stay stable might be extremely beneficial during those periods. For example, consuming moderate amounts of fruit during that part of the cycle (to ensure that there is always some liver glycogen to be released to maintain blood glucose) would be a useful strategy.
The next part of the paper examines changes in macronutrient intake, food cravings and PMS. Studies, as usual, are inconsistent showing variously increases in carb, fat and protein intake during the luteal phase. Some of this may simply be related to being hungrier in general with no clear increase in desire for a specific nutrient.
Some research has indicated that the increase in carbohydrate intake is due to a specific craving although, with chocolate (a combination of carbs, fat and other micronutrients) being the most craved item, other possibilities exist. Cravings for a sugar/fat combo or something else entirely may be functioning here.
Alternately, the issue could simply be one of a magnesium deficiency. Some research indicates that magnesium supplements help with PMS related cravings and chocolate tends to be high in magnesium. Women may simply be self-medicating an important micronutrient. In this vein, many females report that their cravings are ameliorated if not eliminated when they are supplementing magnesium (e.g. 400 mg of magnesium citrate at bedtime, which can also help with sleep).
The Menstrual Cycle and Energy Expenditure
Next, the paper looks at the impact of menstrual cycle on energy expenditure (this brings us back to the increased body temperature I noted above). The major increase in energy expenditure occurs also during the luteal phase (when hunger is increased) with increases of 2.5-11.5% having been reported.
This only amounts to an increase of perhaps 90-280 calories per day and this has to be considered against the average food intake of 90-500 calories. While energy expenditure is up during the luteal phase, so is appetite; increases in energy intake can easily overwhelm the small increase in energy output if food intake isn’t controlled.
The increase in metabolic rate is thought to be primarily related to the increasing progesterone levels. So while increasing progesterone may not be driving the increased appetite per se, it may be stimulating metabolic rate slightly during the luteal phase.
The Impact of Birth Control on Body Weight
Next, the study examined the impact of birth control pills on body weight, first pointing out that there a number of different types of birth control containing synthetic estrogen, progesterone or possibly both. I mention this because, given the differences in each of the hormones (and their interactions), it becomes fairly inaccurate to talk about the “effects of birth control” in general terms.
Looking first at appetite, some studies have shown that birth control can increase total calorie and fat intake while others have no. With limited research available it’s hard to tell if this is due to differences in the types of birth control studies or simply individual variance between women.
As far as energy expenditure, while one study showed a small increase in basal metabolic rate of 5% with birth control intake (especially those containing a synthetic progestin), others have found no effect. Again, type of pill and individual variance is probably at play here.
Looking at body weight, most studies have apparently reported no substantial change in body weight with birth control pills although many show a trend towards increased body weight. One exception is Depo-Provera injections which are associated with the worst weight gain. There is also an enormous variation in response as I discuss in some detail in The Women’s Book Vol 1.
The Menstrual Cycle and Dieting
Finally the paper looks at potential implications for dieting and weight loss. The paper argues that considering which phase of the menstrual cycle the female in when starting a diet may be important. They argue that starting the diet premenstrually when hunger and food cravings are most intense may be a bad idea, just from an adherence standpoint.
Instead, starting a diet immediately following menstruation (or at a greater extreme 4-5 days prior to ovulation) makes far more sense since appetite will be most controlled. The paper does suggest increasing calories somewhat in the 5-8 days prior to menstruation to avoid a suboptimal calorie intake. I’m not sure I find this terribly practical for women who are dieting.
That said, the slight increase in energy expenditure during the luteal phase would allow a small increase in calories or the inclusion of a controlled amount of a craved food. Basically this is a time to consider implementing flexible dieting strategies.
The paper explicitly mentions that small amounts of dark chocolate should be allowed to improve dietary adherence (they argue that it is irreplaceable as a food due to cravings). Interestingly, I first read that idea in an older book called Why Women Need Chocolate which was an interesting look at the idea of biologically driven food cravings. I’d mention that one of the few studies to compare a diet based around the menstrual cycle (called Menstralean) applied this specific strategy with good results.
Additionally as I discuss in The Women’s Book, the luteal phase (or late luteal phase) is probably the best place to include a full diet break. Energy expenditure is up slightly and the increased food intake can help to avoid completely falling off the diet wagon. Scheduling it this way also allows the diet to resume in the follicular phase when appetite will be best controlled.
Of course, it’s also easy to look at that from the other direction; in theory at least, keeping calories controlled during periods when energy expenditure is up (for hormonal reasons) might generate superior fat loss. Of course, that also means keeping calories controlled when hunger is at its worst. Life, she is full of compromises.
Summarizing the Effect of the Menstrual Cycle on Energy Balance
So that’s a look at some of the things that can occur (good and bad) in terms of food intake, energy expenditure during the menstrual cycle. It should be obvious that massive changes in hormones, including the interactions between estrogen and progesterone drive a great deal of processes that can impact on a woman’s food intake, energy expenditure and, of course, body weight.
I’d note again that the above processes tend to be exceedingly variable between women and while generalizations can be made from studies, women may need to track things like body temperature, weight and appetite through a couple of months to get an idea of how their bodies respond. Body temperature can be tracked as can things such as training capacity (some women find it very difficult to train effectively during certain periods of their cycle), appetite, etc.
In periods when hunger is off the map and/or blood glucose seems to keep crashing, increasing food intake slightly (and including moderate fruit) may be beneficial. For those women able to keep food intake under control during the part of the cycle when body temperature is up, increased fat loss may be possible.
Supplementing with various nutrients such as magnesium and fish oils seems to help with PMS symptoms and may help with food cravings during particularly difficult periods. Alternately, that may be a good time to include free meals or refeeds as discussed in A Guide to Flexible Dieting.
Rather than trying to fight the body’s tendencies (and losing control completely), finding ways to work with them may be better in terms of long-term results. Allowing controlled amounts of craved foods tends to help avoid problems with guilt and eating the whole box. Just keep it controlled.
I’d note, in concluding, that other more involved strategies have been suggested from time to time. Things like synchronizing the intake of carbs or fats with different parts of the cycle (e.g. eating relatively more carbs when carb metabolism is dominant and less when fat metabolism is dominant) have been suggested. I discuss this in even more detail specific to women in The Women’s Book.