Israeli Study Shows That Our Hormones Seem to Fluctuate With the Seasons

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Our hormones, like those of other animals, seem to fluctuate synchronously with the seasons.

Fall Season
Fall Season

Most of our hormones, such as cortisol, testosterone, and estradiol, reach their peak in winter-spring and their lowest levels in summer, according to an important Israeli study.

Our reproductive, metabolic, and growth functions may fluctuate with the seasons, as in other animals, according to an important Israeli study published in the journal PNAS. In fact, all of these biological functions are regulated by our hormones, which themselves can be influenced by light. Consequently, people living in countries farther from the equator may have very significant hormonal fluctuations.

11 hormones checked in six million blood and urine tests

“Many animals living in temperate climates have strong cycles: for example, they all give birth in the same season. We believe that our hormonal systems have ‘control points’ that cause spikes, for example in stress or reproductive hormones,” said researcher Alon Bar, co-author of this work, in a statement. At the Weizmann Institute (Israel), scientists analyzed six million blood and urine samples from 3.5 million Israelis between the ages of 20 and 80. The researcher’s targets were 11 different hormones that regulate reproduction, growth, metabolism, and adaptation to stress. Four of these are the so-called “pituitary” hormones, which are produced by the pituitary gland to trigger the production of the other hormones.  They also included cortisol, which regulates the response to stress, and sex hormones such as testosterone and estradiol, which regulate reproductive functions.

Human hormones are affected by the seasons

The levels of these 11 hormones always fluctuate seasonally, but not always simultaneously. Hormones such as cortisol, testosterone, and estradiol showed a peak between the end of winter and the beginning of spring in men and women, respectively. In contrast, testosterone levels in women and estradiol levels in men were higher in the summer.

Seasonality may thus have a positive effect on the chances of becoming pregnant in winter and spring compared to the rest of the year. However, this remains a marginal effect, as the difference between the “high” and “low” hormonal seasons is only 5% for all hormones!

References

Hormone seasonality in medical records suggests circannual endocrine circuits

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