Created by: Izzy Grandic
Each month for several decades women go through a monthly hormonal cycle called the menstrual cycle. This cycle is the fluctuation of 4 key hormones: luteinizing hormone, follicle-stimulating hormone, estrogen and progesterone. The average cycle length is 21-35 days (Mayo Clinic Staff, 2019). Throughout a cycle, women will go through a predictive pattern of hormonal fluctuations (figure 1) which impacts the body's biochemistry, metabolism and homeostasis mechanisms.
Figure 1. Monthly hormone fluctuations for Progesterone, Luteinizing Hormone, Estrogen (Estradiol) and Follicular Stimulating Hormone. (Draper et al., 2018)
Most people have heard of their circadian rhythm, the daily biological clock that powers their lives. For women, their menstrual cycle is equally as — if not more powerful than their circadian rhythm. Every day of the month, women's hormones are different. This influences practically every system in their body from the immune system to the nitrogen utilization cycles. Menstruation is more than just preparation for sexual reproduction. Understanding how this cycle works has implications for diets, exercise, productivity, disease detection, disease treatments, and that's only just the tip of the iceberg.
The hormonal cycle is split into different phases. It starts with the menstrual phase, which is when the lining of the uterus is shed. During menstruation, hormone levels are relatively low and stable.
Next, the body enters the follicular phase. This is when the estrogen levels are low and start rising. Eventually, estrogen reaches its peak point. Shortly after follicle-stimulating hormone and luteinizing hormone reach their peaks too.
At this point, the body is at or near its ovulatory phase. This is when the ovum (egg) exits the ovaries. The exact timing is hard to perfectly define.
After ovulation, progesterone levels will begin to rise. That's when the body has entered the luteal phase. Estrogen rises somewhat too, but not as much as it rose during the follicular phase. Sometimes, the luteal phase is split into the pre-menstrual phase, as shown in figure 1, but for this report, everything after ovulation and before menstruation will be referred to as the luteal phase.
Why each of these phases happen is important to understand why physiological adaptations occur as a result.
In general, the menstrual cycle is a series of hormonal changes that help prepare the uterus (womb) for a potential pregnancy. As seen in figure 2, the four phases generally map into 4 steps.
Figure 2, diagrams to illustrate each phase. All of this action is geared to prepare a woman for reproduction. (Self-created)
First, is the menstruation, or shedding of the endometrium. The endometrium is the muscle layer, rich in blood vessels, that lines the uterus in case of fertilization. The follicular phase is when the ovaries grow "follicles" which contain immature oocytes (future eggs). One follicle becomes the dominant follicle. This follicle is released during ovulation, the third stage.
Once released, the body works to prepare for the onset of growing another human being. This includes thickening that endometrial layer.
If fertilization does not occur, the cycle repeats.
In this report, we will be discussing the effects of three main changes: amino acid levels, hormonal impact on metabolism and the thermoregulation responses throughout the cycle. These will connect the biochemistry, metabolic processes and homeostasis concepts together. Specifically, we will be looking at the differences between the menstrual (low estrogen and progesterone)/ follicular (rising estrogen) and luteal (rising progesterone) ****phases of the cycle.
During the luteal phase of the menstrual cycle, lipid and amino acid levels are present at lower levels in the body (Draper et al., 2018). Figure 3 shows a heat map comparing 39 amino acids during different stages of the menstrual cycle. Most notable is the contrast between amino acids during the menstrual cycle (high) and the luteal phase (low).