A review of millions of blood tests showed a number of human hormones falling into clear seasonal patterns, although these changes are small.
Hormone hormones, which help control reproduction, metabolism, stress and lactation, mostly peak in late summer.
Peripheral organs under the control of the pituitary gland, such as those that produce our sex hormones or the thyroid hormone, have also shown seasonality. However, instead of reaching a peak in the summer, these hormones hit their pace in the winter.
Testosterone, estradiol and progesterone, for example, peaked in late winter or spring.
The findings so far provide the strongest evidence that humans have an internal seasonal clock, which somehow influences our hormones in a way that matches the seasons.
“Together with a long history of studies on a winter-spring peak in human function and growth, hormone seasonality suggests that humans, like other animals, may have a physiological peak season for basic biological functions,” the authors write. .
The underlying mechanism that drives this circular clock is still unknown, but the authors suggest that there is a natural, year-long feedback path between the pituitary gland and the peripheral glands in the body.
The pituitary hormones, which are uniquely tuned to sunlight, can nourish these other organs over the course of a year, allowing them to grow in functional mass in a way that matches the seasons.
“People can therefore show coordinated seasonal settings with a winter-spring peak in the axis of growth, stress, metabolism and reproduction,” the authors write.
As mentioned in the newspaper, it does not differ too much from what we find in other mammals, where fluctuations in certain hormones lead to seasonal changes in the reproduction, activity, growth, pigmentation or migration of an animal.
Mammals such as arctic reindeer, for example, show a decrease in a hormone called leptin when the winter days become the shortest, and this helps to lower their energy consumption, lower their body temperature and impair their ability to reproduce.
Even primates closer to the equator show sensitivity to subtle seasonal changes. For example, Rhesus macaques ovulate significantly more during the post-monsoon season, so that their offspring are born just before the monsoons strike in summer.
Whether human hormones also vary with the seasons remains unclear.
Most datasets analyzed so far are not very large and do not cover all human hormones, which makes drawing conclusions very challenging. Studies have only examined human sex hormones, or they have focused on stress and metabolic hormones. The results were also quite diverse and contradictory.
Although some studies on human sex hormones suggest that seasonal changes should be considered, the conclusion is that other seasons are a major variability.
Meanwhile, research on salivary cortisol levels – or the stress hormone – finds that there is seasonal volatility, and a large data study on the thyroid-stimulating hormone found higher levels of this hormone in summer and winter.
The new research is the largest of the lot and contains a massive collection of Israeli health records spanning 46 million human years. It also analyzes all human hormones.
The authors looked at changes over a single day and found that humans do show seasonal patterns in their hormone levels, although not as strong as other mammals.
The physiological effects of these hormonal shifts are not yet clear, but some of the changes to the thyroid hormone, T3, and the stress hormone, cortisol, are consistent with previous findings.
The thyroid hormone, which peaks in winter, for example, has been linked to thermogenesis. The seasonal timing of cortisol, which peaked in February, is also in line with previous studies spanning the northern and southern hemispheres.
The seasonal changes are small, but as the authors point out, from a clinical perspective, “even a small systemic effect can cause misdiagnosis if the normal ranges are not adjusted according to the seasons, with associated costs of extra tests and treatment. . “
More studies on a similar scale and in different parts of the world will have to be done to further verify the results. But the findings suggest that we are not that different from other mammals.
If our hormones really decrease and flow with the seasons, even just a little, it can be important to our health.
The study was published in PNAS.