Anyone who has periods can tell you about the day-to-day impact on moods, energy levels, sex drive and food cravings. But research increasingly suggests that menstruation may affect our entire system, creating cycles in our overall health, susceptibility to illness, and the symptoms we exhibit.
A paper published by Oxford University, in collaboration with female health and period-tracking app Clue, showed that susceptibility to infection is higher during the second (luteal) phase of the hormonal cycle. Researchers also found that scientists studying diseases observed different symptoms depending on where their female participants were in the menstrual cycle.
Dr Alexandra Alvergne is associate professor of anthropology at Oxford and co-authored the paper. The menstrual cycle, she explains, is itself a cycle of immunity. The immune system is suppressed during ovulation to allow sperm to enter. It then kicks in again in order to shed the lining of the uterus. It makes sense, therefore, that our health would be affected by these changes.
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"The whole function of the menstrual cycle is to produce cyclical patterns of immunity so actually we would be better to think of female health as cyclical," says Dr Alvergne.
Science still doesn’t see periods as a significant factor in the study of female health. In a 1939 paper entitled "The Degree of Normal Menstrual Irregularity", biologist Leslie Brainerd Arey wrote: "Scientific curiosity was quick to invade almost every aspect of human behavior, yet it long neglected this most obvious one." Almost 80 years later, not much has changed.
"Even now I write a research application on PMS and I think 'Will this be taken seriously?'" says Dr Alvergne.
Failure to acknowledge the impact of the hormonal cycle on symptoms makes it harder to identify diseases and can delay diagnosis, says the paper. So why is there so little research into it? It’s tempting to cry patriarchy and, while Dr Alvergne agrees that male domination of the sciences is part of the picture, it isn’t the only factor.
"It’s difficult to study," she acknowledges. "Previously you’d have to get women to come in and report and that’s an unreliable method."
Dr Virginia J. Vitzthum, a senior scientist at the Kinsey Institute, professor of anthropology at Indiana University and director of scientific research at Clue says that Western medicine is primarily concerned with correcting deviations, rather than investigating them.
"Western medicine is focussed on fixing the body. It’s not particularly concerned with how the body got that way," she says.
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Outside of its role in baby-making, medicine largely ignores menstruation, seeing it merely as a complication. Many prescription drugs are not tested on women for exactly that reason. While this may sound shocking, Dr Vitzthum says it is born out of a need for tightly controlled clinical trials rather than a disregard for the impact of drugs on female health.
"Pharmaceutical companies wanted to have clean results on their drugs assessments. They were fully aware of the possibility of the menstrual cycle to affect the results and they needed to control for that," she says.
Ironically, this points to an acknowledgment of the hormonal cycle and its significance for symptoms and side-effects. Dr Vitzthum describes it as the "double-edged sword" of science’s approach to periods. Just as it is detrimental to ignore the menstrual cycle, it is also not helpful to dismiss unusual symptoms as "women’s troubles".
"The danger in assuming that everything is attributable to the menstrual cycle is that you miss serious conditions. At the same time we haven’t investigated this real aspect of biology or acknowledged it as part of the framework."
Both Dr Vitzthum and Dr Alvergne are hopeful that things are changing. Technological innovations such as wearable devices and smart apps mean we can record information about ourselves and our cycles in real time. Tech also makes it easier to analyse data and take a more personalised approach to treatment.
Dr Vitzthum says: "The fact that there are more female doctors has also helped. They have been pushing the envelope and successfully gathering data and treating their patients as individuals, because they understand the experience of being individual."
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But it is the demand from the public that will really drive the research, says Dr Alvergne.
"If people become more aware of these things and request to know more, that can help drive research. It would be great to get the questions from the bottom up. What matters to individuals, what do they want to know?" she says.
This is starting to happen, with around 70% of us actively tracking our health indicators or symptoms. And a survey of people who track their periods showed that many do it in order to better understand their bodies at different cycle phases and to inform their conversations with healthcare providers.
"Women themselves are now more likely to speak up and say to their doctor, 'This hormonal contraception does not work for me'," says Dr Vitzthum.
"It’s critical for each woman to know her own body. Rather than measuring yourself against some medical ideal, you need to know your own pattern."
If we want science to take our periods seriously, we need to start doing so too.