Any leftover energy is then dedicated to reproduction; there is a balance between resource allocation to maintenance and reproductive efforts, and environmental stressors can lessen available resources, Clancy, who co-directs the Laboratory for Evolutionary Endocrinology at Illinois, said.
The researchers collected the urine and saliva samples from a group of healthy, premenopausal, rural Polish women who participate in traditional farming practices, during the harvest season, when physical activity levels are at their peak.
They then measured participants' salivary ovarian hormone levels daily over one menstrual cycle and also tested urine samples for levels of C-reactive protein (CRP), a commonly used marker of inflammation.
They observed a negative relationship between CRP and progesterone in the Polish women - in women with high CRP, progesterone was low, further they found that estradiol and the age of first menstruation were the strongest predictors of CRP levels.
Clancy believes that there are two possible pathways that explain these results.
"One is that there is an internal mechanism, and this local inflammation drives higher levels of CRP, and that is what's correlating with the lower progesterone."
"The other possibility is that there is an external stressor like psychosocial or immune stress driving allocation to maintenance effort, which in turn is suppressing ovarian hormones," she said.
The study has been published in the American Journal of Human Biology.