A review of millions of blood tests has shown a whole host of human hormones that fall into clear seasonal patterns, even though these changes are small in magnitude.
Pituitary gland hormones, which help control reproduction, metabolism, stress and lactation, were found at the latest in late summer.
Peripheral organs under the control of the pituitary gland, such as those produced by our sex hormones or thyroid hormone, also showed seasonality. However, instead of reaching its peak in the summer, these hormones are released in the winter.
Testosterone, estradiol, and progesterone, for example, reached their peak in late winter or spring.
The findings provide the strongest evidence to date that humans possess an internal seasonal clock, which somehow affects our hormones in a way that adapts to the seasons.
“Along with a long history of studies of a winter-spring peak in human function and growth, hormonal seasonality indicates that, like other animals, humans may have a physiological peak season for basic biological functions. “, write the authors.
The underlying mechanism driving this circannual clock is still unknown, but the authors suggest that there is a year-round natural feedback circuit between the pituitary gland and the peripheral glands of the body.
Pituitary hormones, which are uniquely adapted to sunlight, could feed these other organs over the course of a year, allowing them to grow in functional mass in a way that aligns with the seasons.
“Thus, humans can show seasonal conjuncture points coordinated with a winter-spring peak on the axes of growth, stress, metabolism, and reproduction,” the authors write.
As the paper mentions, it is not too different from what we find in other mammals, where fluctuations in certain hormones lead to seasonal changes in an animal’s reproduction, activity, growth, pigmentation, or migration.
Mammals such as arctic reindeer, for example, show a decrease in the hormone called leptin when winter days are shorter and this helps reduce their energy consumption, lowering body temperature and inhibiting their ability to reproduction.
Even the primates closest to the equator show sensitivity to subtle seasonal changes. For example, Rhesus macaques ovulate significantly more during the post-monsoon season, so their offspring are born just before monsoons strike in the summer.
It is still unclear whether or not human hormones fluctuate with the seasons.
Most of the data sets analyzed so far are not very large and do not cover all human hormones, which makes drawing conclusions very difficult. Studies have examined only human sex hormones or have focused on stress and metabolic hormones. The results have also been quite varied and inconsistent.
While some studies on human sex hormones suggest that seasonal changes need to be considered, other studies conclude that seasons are a source of unimportant variability.
Meanwhile, research on salivary cortisol levels, also known as stress hormone, found that there is some seasonal variability, and a large data study on thyroid-stimulating hormone found higher levels of stress. ‘this hormone in summer and winter.
The new research is the largest of the batch and includes a massive data set of Israeli health records covering 46 million person-years. It also analyzes all human hormones.
Controlling the changes over a single day, the authors found that humans show seasonal patterns in their hormone levels, though not as strong as other mammals.
The physiological effects of these hormonal changes are not yet clear, but some of the changes to thyroid hormone, T3, and stress hormone, cortisol, do match the above results.
For example, thyroid hormone, which was found at its peak in winter, has been linked to thermogeneration. The seasonal timing of cortisol, which peaked in February, also coincides with past studies covering the northern and southern hemispheres.
Seasonal changes are small in magnitude, but as the authors point out, from a clinical perspective, “even a small systematic effect can cause a misdiagnosis if normal intervals do not fit the seasons, with the associated costs of additional tests and treatment ”.
More studies will be needed on a similar scale and in various parts of the world to verify the results. But the findings suggest that we are not so different from other mammals after all.
If our hormones really flow and ebb with the seasons, even a little, it could be important to our health that we know.
The study was published in PNAS.