Energy Expenditure and Menstruation
As a researcher at Calorify, I often think about how energy expenditure varies across different contexts, such as environmental conditions, like extreme heat, or specific disease states, like obesity. Recently, I began to wonder about energy expenditure throughout the menstrual cycle and if it fluctuates at all. I couldn’t recall any papers off the top of my head, so I hit the literature to find out.
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Before exploring energy expenditure during menstruation, I was curious to research why humans menstruate in the first place. Many other mammals don’t, so understanding the reason might provide additional context for how we use our energy.
There are 5,149 or so recognized eutherian species (fancy way of saying mammals that have a placenta, as opposed to marsupials like kangaroos–who have pouches–or monotremes like platypuses–who lay eggs). Only about 1.6% of those, or 84 species total, menstruate [1]. Menstruation appears to have evolved at least four times independently in mammalian history, but the exact reasons are still unclear. One theory is that menstruation is a consequence of spontaneous decidualization, the process by which the uterine lining prepares for pregnancy.
“In most animals, decidualization occurs in the estrogen- and progesterone-primed uterus in response to the presence of the embryo. This is induced decidualization. However, in humans, decidualization occurs even in the absence of an embryo and is therefore called spontaneous decidualization” [1].
Two proposed models explain why spontaneous decidualization occurs: one suggests it protects the mother against an aggressive fetus, while the other proposes it allows the female to test the viability of the conceptus before a definite pregnancy ensues [1].
Both models seem to be energy-saving mechanisms, as menstruation is less energy-intensive than pregnancy. Evolutionarily speaking, a small expenditure now can save a lot of unnecessary expenditure later.
The Menstrual Cycle
Menstruation is regulated by the hypothalamic-pituitary-ovarian (HPO) axis, with the brain interacting with the ovaries to coordinate the cycle. The average cycle is 28 days long, but can range from 25 to 35 days depending on the individual. Day 1 of the natural menstrual cycle is the first day of bleeding and signifies the start of the follicular phase (FP), while ovulation around day 14 signifies the transition to the luteal phase (LP), as shown in Figure 1.
Figure 1. Normal Menstrual Cycle Hormones.
The FP is named for the growth of ovarian follicles (each follicle is a fluid filled sac with one egg; normally there are about 20 follicles for each ovulatory cycle), stimulated by follicular stimulating hormone (FSH) produced by the pituitary gland. Typically, one dominant follicle releases its egg during ovulation. Estrogen, produced by the growing follicle, thickens the uterine lining and triggers a spike in luteinizing hormone (LH), leading to ovulation (release from the dominant follicle).
If pregnancy occurs, progesterone levels stay elevated and help support the uterine lining. If pregnancy does not occur, progesterone levels drop, triggering the shedding of the uterine lining and the start of a new cycle.
Energy Intake, Expenditure, and Menstruation
The NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development considers the menstrual cycle a “fifth vital sign,” indicating that any changes in menstruation can be indicative of larger problems, and should not be ignored. Unfortunately, research on energy expenditure and menstruation is limited as, historically, women have been neglected in medical research. There are a handful of studies out there that have investigated energy expenditure and intake in relation to menstruation, but most have evaluated differences in resting metabolic rate (RMR), and I didn’t come across any that used doubly labeled water (DLW).
During my research, I found one paper on PCOS and energy expenditure, but not explicitly menstrual cycle [7]. In this paper, Broskey and colleagues call for equations in order to move people with PCOS into the right calorie deficit, as weight loss is the first line of defense against PCOS. However, with the way Calorify is democratizing DLW, hopefully people with PCOS can get their expenditure measured rather than using equations to guess.
Studies of energy expenditure and menstruation have proven to be tricky to pin down. Early studies showed that total daily energy expenditure (TDEE) was 8-16% higher in the LP than in the FP [2]. In 2007, these findings were confirmed, but to a lesser degree, showing that although there were differences in sleeping metabolic rate depending on the phase of the cycle, changes in TDEE were not found to be statistically significant [3]. In 2015, investigations of RMR through the different phases of the menstrual cycle also found differences, albeit not statistically significant. The study found there was a trend towards a higher RMR in the LP and was particularly evident in females with a high intra-individual variation in their RMR [5]. However, a review paper published in 2020 showed the opposite again: that there is a significant increase in RMR in the LP, but the amount of change is small.
“Limitations include risk of bias regarding measurement of both menstrual cycle and RMR. Sample sizes were small and studies did not report control of potential confounders. Sub-group analysis demonstrated that in more recent studies published since 2000, the effect of menstrual phase was reduced and not statistically significant” [5].
The cycle has also been studied from the lens of energy intake. Women often cite differences in appetite throughout the cycle, but researchers are unsure if this is due to a higher TDEE or more complex physiological or hormonal factors. “Estrogen mediates the release of the satiating CCK from the small intestine, which acts to decrease meal sizes (rather than the frequency of eating) and attenuates the release of the appetite-stimulating hormone, ghrelin. In humans, hunger has been reported to be, on average, lower during the ‘fertile window’ (ovulatory and early luteal phase) than on other days during the same cycle, corresponding with lower energy intake” [6]. A 2023 review paper found that energy intake does vary throughout the menstrual cycle, concluding that intake is greater in the LP compared to the FP, with the lowest intake occurring around ovulation. Depending on the study protocol, the caloric increase ranges from 159 to 529 calories per day [6]. However, phase-related differences in energy intake most likely vary both between individuals, and from cycle-to-cycle” [6].
Intake and expenditure can also vary depending on certain diagnoses related to the menstrual cycle. For example, those with premenstrual dysphoric disorder, a severe form of premenstrual syndrome, had 16% greater energy intake during the LP while a control group experienced no change between phases [6].
Statistically significant or not, an increase in RMR but not necessarily TDEE makes sense. Herman Pontzer’s 2018 paper describes how the constrained energy model works to keep daily caloric burn as consistent as possible [8]. So while RMR might turn up as a result of various hormonal changes (as Webb 1986 has suggested, thinking that progesterone produced by the corpus luteum in the LP causes increased expenditure), the body is going to try to keep overall burn consistent by turning down the burn elsewhere. Remember though, this is all highly individual.
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In the beginning of this article, I mentioned that menstruation is less energetically expensive than pregnancy. Nancy Butte’s work demonstrated that a pregnancy requires on the order of 70,000 additional calories: 90 calories per day in the first trimester, 287 calories per day in the second, and 466 calories per day in the third. If menstruation expenditure increased as much as Webb 1986 saw, over the course of 9 months (equivalent length to a pregnancy), it would be less than ⅓ of the energy required for a full pregnancy.
Essentially, it’s beneficial to catch an unviable pregnancy/aggressive fetus early on so that energy isn’t wasted, and menstruation helps give our bodies the tools to do so.
Hormonal Contraceptives
In 2023, Weidlinger et al. investigated the impact of estrogens on REE in the context of hormonal contraceptives and found that they increase REE by over 200 calories [9]. This wasn’t associated with any specific phase of the cycle because hormonal contraceptives mute the hormonal spikes of a normal cycle, causing an egg to not be released (thus preventing pregnancy). Weidlinger proposed the reason for increased expenditure is that estradiol (a synthetic estrogen found in many hormonal contraceptives) causes white adipose tissue to transform into brown adipose tissue which is more metabolically healthy, causing the increased burn (via activated brown adipose tissue thermogenesis) [9].
Menstruation and DLW: Erica’s Experience
To my knowledge, no study has evaluated TDEE via DLW throughout the menstrual cycle (either natural or altered by hormonal contraceptives). So I decided to conduct a mini-experiment myself to study it.
I use a daily birth control pill (BCP), which comes in one of two forms: combination oral hormonal contraceptives, like the one I’m on, or progestin-only pills (progestin being synthetic progesterone). Typically, a pack comes with 28 pills, 21 of which contain the same amount of synthetic hormones, and the remaining 7 are placebo pills with no hormones. This means that I don’t experience a typical cycle with a full FP and LP. Rather, my hormones look more steady, similar to those shown in Figure 2. The BCP I'm using includes constant doses of ethinyl estradiol (synthetic estrogen) and norethindrone acetate (synthetic progesterone). At the end of the 21 days, when the hormonal pills run out, the drop in hormones triggers a withdrawal bleed. It can look and feel like a regular period, but it isn’t a true period (and some women don’t experience bleeding at all while on BCPs). The LH and FSH that normally trigger ovulation are suppressed by the constant flow of synthetic hormones. The suppression of FSH prevents the maturation of ovarian follicles, reducing the likelihood of ovulation, while LH suppression is designed to prevent ovulation period.
Figure 2. Menstrual Hormones While on Hormonal Contraceptives.
Although my cycle is altered by BCPs, it is still worth evaluating. In the US, 14% of women aged 15 to 49 are on BCPs, with another 10.4% using hormonal contraceptives like an IUD or the implant. So, in order to see what effect my BCPs had on my TDEE, I followed my cycle for 20 days. Just a quick disclaimer: this data is representative only of my individual experience (n=1), and shouldn’t be applied to everyone.
I tracked my cycle for 20 days using DLW, starting on the first day of taking placebo pills (cycle day 22). On the fourth day of taking placebo pills, my withdrawal bleed started and it lasted until my last placebo pill day (4 days total), as shown in Figures 3. Then I started the regular BCPs again (cycle day 1). I experienced mild “regular” period symptoms during my placebo week (bloating, cramping, etc.) but it all resolved by the time I was taking the regular BCPs again.
Figure 3. My DLW testing schedule and corresponding cycle days.
My DLW results showed that my TDEE was fairly stable. I burned the most calories during days 1-8 of my cycle, coinciding with when I started taking the hormonal pills again (Figure 4), but my activity was also slightly higher during this period.
Figure 4. My caloric burn throughout the testing period with the cycle day.
Overall, my weight remained stable, with a slight drop when I started the regular pills again, likely due to reduced fluid retention. My average caloric burn was pretty consistent, suggesting that being on the pill didn’t significantly impact my expenditure. Changes in activity, stress, and sleep likely influenced the variations in my caloric burn more than hormonal changes.
Conclusion?
The big takeaway is that from month to month, your body wants to be as consistent as possible, as dictated by the constrained energy model [8]. So, even if RMR increases for a period of time throughout the menstrual cycle, burn in other pools will decrease in order to compensate and keep your burn as stable as possible. I hope to see larger and more comprehensive studies (with better activity, sleep, and hormone tracking) use DLW alongside RMR resting to better understand energy expenditure throughout the cycle and empower women to better understand their bodies.
Finally, in terms of taking a Calorify test, the menstrual cycle doesn’t impact the gold standard measurement of TDEE. However, the cycle could impact body composition and energy intake metrics if effects of fluid retention are particularly noticeable (aka bloating; see White et al. 2011 for more information on how fluid retention is related to the natural menstrual cycle) [10].
References
[8] Pontzer et al. 2018. Energy Constraint as a Novel Mechanism Linking Exercise and Health. Physiology.
