The Basics of the menstrual cycle
Reproductive hormones play a key component in the growth and development of reproductive system in embryo and puberty. In males, testosterone is a main sex hormone responsible for the spermatogenesis and the development of secondary sex characteristics. In females, the reproductive system and its control is more complex, where two main hormones oestrogen and progesterone fluctuates through the month, preparing the female body for potential pregnancy.
Fluctuation of reproductive hormones through the approximately 28 days. Roughly it can be divided on the follicular, ovulatory and luteal phase. The beginning of the menstrual cycle is counted as the first day of the menstrual bleeding. At this point oestrogen levels are very low. As we progress through the follicular phase, the oestrogen levels gradually increase until the ovulation, which occurs on approximately 12th-15th day of the cycle. Ovulation presents a transition of the cycle into the luteal phase. Beginning of the luteal phase is defined by the drop of oestrogen after the ovulation and a rise of both progesterone and oestrogen in the mid-luteal phase. At the end of the luteal phase, both hormones drop again, and menstrual bleeding occurs.
In the background of a very simplified description of what happens in the menstrual cycle, lies a highly regulated hypothalamic-pituitary-gonadal axis (HPG), essentially controlling growth, development, metabolism, and response to stress. HPG axis represents a controlling cross-talk which results in secretion of the hormones in the correct concentration. Hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the pituitary to release the LH (luteinizing hormone) and FSH (follicle stimulating hormone). LH and FSH are released in a pulsatile pattern into the bloodstream and travel to the ovaries. Upon the LH and FSH, ovaries produce and secrete oestrogen and progesterone.
Oestrogen is responsible for the achievement of fertility and a development of secondary sex characteristics. Among others it dictates the progression of breast development, fat deposition, hair growth and bone development. Importantly, it protects women from cardiovascular disease (CVD), which is significantly noted upon the menopause, where the risk for CVD rises. Oestrogen influences substrate metabolism- it increases lipolysis and inhibits glycogen utilization during rest and acute exercise. Enhances activity of lipoprotein lipase (LPL) in muscle cells enables higher storage of intramuscular triglycerides, which are important source of energy during endurance exercise. Oestrogen dictates insulin sensitivity of the tissue, causing higher glycogen storing capacity. High levels of oestrogen also influence levels of dopamine and serotonin effecting the mood and behaviour.
Progesterone rises after the ovulation, which can be observed by the rise of basal body temperature by 0.5-1.0 °C. Its effect on metabolism, is shown as an increased metabolic rate and expenditure that is elevated by 2.5-10%, corresponding to 100-300 kcal per day. Which equals one Snickers bar. Increased energy expenditure leads to an increased appetite. As the progesterone inhibits the action of oestrogen in the luteal phase, causing a low-grade insulin resistance, unstable blood sugar levels and thus stimulating hunger or cravings for certain food. Drop in progesterone days before the period and low-grade inflammation causes significant water retention, which can be noticed as a weight gain.