Proc Natl Acad Sci USA. 2021; 118 (7) e2003926118https://bit.ly/35huXba
By analysing data from Israeli health records, including millions of hormone blood tests, the authors find peaks during winter−spring in the circulating levels of all primary hormones involved in human growth, stress, metabolism and reproduction. By contrast, the pituitary-derived regulatory hormones peak in summer. This circannual clock impacts on function; the endocrine gland masses grow with a timescale of months due to trophic effects of the pituitary hormones, generating a feedback circuit with a natural frequency of about a year according to seasons.
Animals show seasonal changes in both pituitary and primary effector hormones that govern seasonality in reproduction, activity, growth, pigmentation, morphology, and migration. This adaptive physiology results in seasonal changes in body composition, organ size, and function. Hormone seasonality is thought to be a dominant regulator of physiological and behavioral traits in animals. These researchers now show that a circannual rhythm is a general principle for hormones. Endocrine seasonality indicates that, like other animals, humans may have a physiological peak season for basic biological functions.
The classical model predicts that changes in pituitary hormones should coincide with their regulated primary hormones, and thus share the same seasonal peaks and troughs. Instead, the authors suggest a functional-mass model, that the observed phase shifts arise from trophic effects of pituitary hormones that peak in winter-spring to stimulate endocrine gland secretion that peaks in summer. The model predicts that the amplitude of seasonal variations should increase with latitude, due to greater photoperiod variation with the seasons. To test this, they compared cortisol profiles in studies from Australia (30°S), the United Kingdom (51°N), and Sweden (58°N). Indeed, the amplitude of cortisol seasonality rose with latitude, in agreement with the model predictions. The functional-mass model makes another testable prediction: The masses of the glands that secrete the hormones should vary with the seasons with specific phases. They confirmed this by analyzing data on MRI brain scans and computed the volume of the pituitary.
The authors hypothesis that, because of the coordinated peaks in all hormone axes during the same winter/spring season and the widespread effects of hormones on metabolic and behavioral systems, even small changes from hormone baseline levels may have a selectable impact on organism fitness.