Women’s body temperatures are no more variable than men’s, despite menstrual cycles and hormonal variability, a recent monitoring study suggests, casting doubt on a theory that has long coloured biomedical research – that ovarian cycles make females unsuited for drug trials and other clinical experiments.
That claim, along with gender discrimination and other factors, is thought to fuel women’s ongoing under-representation in such trials, which continues despite a concerted effort by the federal government and researchers to bolster sex-specific research, reports The Washington Post.
When researchers looked at six months’ worth of continuous body temperature data from a group of males and females, they found that despite sex differences in body temperature, neither group was more variable than the other.
They looked at temperature data from a pool of age-matched 20-to-79-year-olds: 300 females and 300 males. The participants were part of a broader University of California study, TemPredict, designed to track vital signs and help predict the onset of coronavirus symptoms.
Women still under-represented in trials
Over the course of six months, patients wore an Oura ring, a wearable “smart ring” that monitors heart rate, respiration, body temperature and movement. (Oura Health Oy, the Finnish technology company that produces the ring, was one of the study’s funders.)
When the scientists delved into the body temperature data, comparing it across sex and over time and looking for excessive variability or measurement errors among females, they did find sex differences.
Women with menstrual cycles showed temperature fluctuations across a roughly 28-day cycle, confirming that ovarian rhythms do affect body temperature.
But those differences did not confound or sully the data, and the researchers note that the predictability of the women’s temperature variability actually made their temperatures easier to predict than men’s.
“In this study, the difference between two men is bigger than the difference between the average man and the average woman,” said Lauryn Keeler Bruce, the paper’s first author and a PhD student in the Bioinformatics and Systems Biology programme at UC San Diego.
“In addition, the variability between men and women is not statistically significant.”
Temperatures cycled along with sleep and wake patterns and time of day for both sexes.
Because no one group consistently proved more variable, the researchers conclude that women’s temperature data is no more unreliable than men’s.
“These findings contradict the viewpoint that human females are too variable across menstrual cycles to include in biomedical research,” the researchers write. Pregnancy or sex-specific cancers might create larger effects, they note.
But that’s no reason to exclude women from medical research, they conclude: “Females still need to be more routinely included in research, and we find no statistical evidence that doing so would negatively affect study power.”
The study was published in the journal Biology of Sex Differences.
Study details
Variability of temperature measurements recorded by a wearable device by biological sex
Lauryn Keeler Bruce, Patrick Kasl, Benjamin Smarr, et al.
Published in Biology of Sex Differences on 13 November 2023
Abstract
Background
Females have been historically excluded from biomedical research due in part to the documented presumption that results with male subjects will generalise effectively to females. This has been justified in part by the assumption that ovarian rhythms will increase the overall variance of pooled random samples. But not all variance in samples is random. Human biometrics are continuously changing in response to stimuli and biological rhythms; single measurements taken sporadically do not easily support exploration of variance across time scales. Recently we reported that in mice, core body temperature measured longitudinally shows higher variance in males than cycling females, both within and across individuals at multiple time scales.
Methods
Here, we explore longitudinal human distal body temperature, measured by a wearable sensor device (Oura Ring), for 6 months in females and males ranging in age from 20 to 79 years. In this study, we did not limit the comparisons to female versus male, but instead we developed a method for categorising individuals as cyclic or acyclic depending on the presence of a roughly monthly pattern to their nightly temperature. We then compared structure and variance across time scales using multiple standard instruments.
Results
Sex differences exist as expected, but across multiple statistical comparisons and timescales, there was no one group that consistently exceeded the others in variance. When variability was assessed across time, females, whether or not their temperature contained monthly cycles, did not significantly differ from males both on daily and monthly time scales.
Conclusions
These findings contradict the viewpoint that human females are too variable across menstrual cycles to include in biomedical research. Longitudinal temperature of females does not accumulate greater measurement error over time than do males and the majority of unexplained variance is within sex category, not between them.
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