A study of millions of blood tests has shown that there are several human hormones that fall into clear seasonal patterns, although these changes are small in magnitude.
Hormones are released from the pituitary gland, which helps control fertility, metabolism, weight and lactation, usually in late summer.
Peripheral organs that are under the control of the pituitary, such as those that secrete our sex hormones or thyroid hormones, also showed seasonality. Instead of peaking in the summer, however, these hormones go on in the winter.
For example, testosterone, estradiol, and progesterone reached their peak in late winter or spring.
The findings provide the strongest evidence to date that humans have an internal seasonal clock, which in some way affects our hormones in a seasonally consistent manner. .
“Along with a long history of studies of winter peaks – spring in action and human growth, the season of the hormone indicates that, like other animals, humans may have a physical peak season for basic biological activities,” the the authors write.
It is not yet known what the basic method of driving this circular clock is, but the authors suggest that a natural, year-long feedback cycle plays between the pituitary gland. and peripheral glands in the body.
The pituitary hormones, which are adapted to sunlight, could nourish these other organs over a year, allowing them to grow in functional beauty in a way that -side to the seasons.
“Thus, people may exhibit seasonal fixed points coordinated by winter peaks – spring in growth, weight, metabolism, and reproductive axes,” the authors write.
As the paper points out, it is not too different from what we find in other mammals, where changes in certain hormones lead to seasonal changes in animal reproduction, activity, growth, pigmentation, or migration.
Mammals such as arctic reindeer, for example, show a decrease in a hormone called leptin when winter days become shorter, and this helps to reduce their energy consumption, lowering their temperature. bodies and inhibits their reproductive capacity.
Even primroses closer to the equator show sensitivity to subtle seasonal changes. For example, Rhesus macaques ovulate much more during the post-monsoon season so that their offspring are born just before the monsoons hit in the summer.
It is not clear whether human hormones change with the seasons.
Most of the data analyzed so far are not very large and do not cover all human hormones, which makes decisions very challenging. Studies have either examined only human sex hormones, or have focused on weight and metabolic hormones. Results have also varied widely and inconsistently.
While some studies on human sex hormones suggest that seasonal changes should be considered, other studies conclude that seasons are an insignificant source of variability.
At the same time, a study of salivary cortisol levels – aka the stress hormone – found that there is some seasonal variability, and a large data study of the thyroid-stimulating hormone found higher levels of this hormone. in summer and winter.
The new research is the largest of the lottery and includes a large set of Israeli health records covering 46 million people. It also analyzes all human hormones.
Controlling changes through one day, the authors found that humans show seasonal patterns in their hormone levels, although not as strongly as other mammals.
The physiological effects of these hormonal shifts are not yet clear, but some of the changes in the thyroid hormone, T3, and the stress hormone, cortisol, are consistent with previous findings.
For example, the thyroid hormone, which was found at an altitude in winter, has been linked to thermogeneration. The seasonal period of cortisol, found at its peak in February, also agrees with previous studies spanning the northern and southern hemispheres.
The seasonal changes are small in magnitude, but as the authors point out, from a clinical perspective, “even a small systematic effect can lead to misdiagnosis if the normal ranges are not changed to the seasons, with additional testing and treatment costs. “
More studies are needed on such a large scale and in different parts of the world to further confirm the results. But the results show that we are not so different from other mammals after all.
If our hormones really ebb and flow with the seasons, even just a little, it could be important for our health that we know.
The study was published in PNAS.