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Evolutionary explanation for menstruation

I was wondering if there was an evolutionary explanation for menstruation that explained how such a process could fit into "survival of the fittest." I rarely feel fit (or sometimes as if I'm even going to make it through the day) when I am experiencing this wonderful and natural phenomenon (PLEASE note irony).

So, I came across this interesting article: The Impact of Increased Menstruation Rates on Women’s Health and Reproductive Cancers

It basically says that available evolutionary theories on menstruation are not plausible. However, it points out some other interesting facts.

Particularly interesting quotes from the article:

1. "...research shows that the number of menstrual cycles modern women experience differs greatly from the number experienced by pre-agricultural women. [...] American women currently experience three times as many menstrual periods as women who have continued living in the ways of earlier ancestors. Foraging women are 16 years old at menarche, 19.5 year old at first birth, nurse for three to four years, have a completed family size of 5.9 live births, and an average age at menopause of 47 years. They experience a total of 160 ovulations in their lifetime. Contemporary American women are 12.5 years old at menarche, 24 years old at age of first birth, nurse for 3 months (if at all), have a completed family size of 1.8, and are 50.5 years old at menopause. American women experience approximately 450 ovulations within their lifetime (Eaton et al 1994). A study done with the Dogon women of Mali shows a similar relationship. The Dogon are a foraging society that practices natural fertility by not using modern contraceptive methods. The Dogon have a fertility rate of 8.6 ± 0.3 live births per woman. Median number of lifetime menses experienced by the Dogon was 109, with a U-shaped relationship between menstruation and age showing that, from menarche to menopause, women in primary child-bearing years (20 —34 years old) rarely menstruated (Strassman 1999). Overall, this data indicates that monthly menstruation for decades on end is not the historical norm. Today, women have earlier menarche, later first birth, and fewer pregnancies. There is also a decreased suppression of menstruation through lactation as [most] children are never breast-fed and the rest only breast-feed for 3 months. Early menarche is an especially recent development. In the 19th century, the age of first menarche was the same as in the hunter-gatherer women observed today. The earlier age of first menarche can be linked to an increase in caloric intake, while at the same time occupational, educational, and social forces have led to a later first birth (Eaton and Eaton III 1999). The consequences for these changes in menstrual cycling may be seen in cancer rates among women in industrialized nations."

2. "The increased number of menstrual cycles experienced by western women may increase the risks of developing cancer in the reproductive organs. Breast cancer afflicts one out of eight women (Strassman 1999). [...] One model [of breast cancer rates in ancestral hunter-gatherer societies] suggests a 100-fold increase in breast cancer rates from those in ancestral women (Eaton and Eaton III 1999). Less than 2% of current breast cancer cases are caused by heritable mutations. The other 98% of breast cancer cases are probably due to the longer time span between menses and first live birth, along with increased menstruation rates (Strassman 1999). The chances of developing breast cancer decrease with later menarche, earlier first birth, high parity, and earlier menopause. "

3. "The "incessant ovulation hypothesis" may explain increased rates of ovarian cancer (Eaton et al 1994). Ovarian cancer is the fourth leading cause of cancer in women and is fatal in 80% of cases (Coutinho and Segal 1999). The hypothesis states that each ovulation mechanically injures the ovarian epithelium, while the escaping follicle exposes the tissue to locally high hormone levels. This means that the more a woman ovulates, the greater her chances are for developing ovarian cancer. Pregnancies, lactation, and oral contraceptives all reduce the total number of ovulations, and therefore decrease the chances of developing ovarian cancer. The high number of ovulations experienced by women in western societies is a departure from the number experienced by pre-agricultural women and may explain the high rates of ovarian cancer (Eaton et al 1994). "

4. "Millions of women currently use oral contraceptive pills, which have been shown to reduce the risk of developing ovarian and endometrium cancers. However, there is no evidence that oral contraceptives decrease breast cancer rates."

5. "Delaying the onset of puberty could also reduce the rates of all reproductive cancers."

6. "Changing dietary habits could also decrease the rates of reproductive cancers. There is a strong correlation between fat intake and breast cancer development. Studies on lab animals strongly suggest that increased fat intake promotes breast tumor development. Women in the United States consume 40% of daily calories from fat and have five times the risk of developing breast cancer as women in Japan, who consume 19% of daily calories from fat. Paleolithic humans probably received 20 to 25% of their daily total caloric intake from fat (Eaton et al 1994). Studies suggest that reproductive cancer rates may be linked to dietary fat intake as a child. There is a clear-cut association between overall dietary fat intake and breast cancer rates within a country. However, no correlation has been found between individual fat intake and breast cancer rates in the adult women studied. This suggests that breast cancer rates may be influenced by childhood fat intake (Eaton and Eaton III 1999). Dietary fiber levels may also affect reproductive cancer rates because a high fiber diet increases intestinal reabsorption of estrogen. Non-human primates, which have almost non-existent rates of reproductive cancers, consume large amounts of fiber containing foods (Eaton et al 1994). Therefore, changing dietary habits to a low fat/high fiber diet could result in lower reproductive cancer rates.

7. (my personal favourite) "Viewing continuous menstrual cycling as the natural state, because it brings relief of these symptoms [pms], is an inaccurate assessment given that throughout history women probably spent most of their time pregnant or lactating, both of which suppress menstruation."

So, my new mission is to eat less fat and more fibre and, above all, to find a man (as soon as possible), have kids (as soon and as many as possible), and breastfeed (for forever and a day).

On an interesting side note about breastfeeding, check this out:
Woman breastfeeding tiger cubs in Myanmar

posted by Steph @ 1:19 PM