Humans are animals and their connection to nature is sometimes forgotten. Animal physiology is influenced by the moon (as so eloquently explained by Michael Colgan), the sun and the seasonal variations in temperature, light and humidity. These environmental factors can have a strong influence on the hormonal milieu of animals including humans. The most obvious example of a hormonal fluctuation that can follow the seasons is vitamin D. Vitamin D is synthesised in the skin and as such, at high latitudes, levels are highest at the beginning of autumn following the summer months. As winter ensues, vitamin D levels fall to their nadir in the early spring, before rising again with the warming sun. Although the seasonal variation in vitamin D is perhaps the most well known, other hormonal fluctuation have been reported in humans. Of these one of the most interesting is perhaps the plasma glucose and insulin responses, the curves of which are known to vary considerably with seasonal variation.
For example, in one study1, researchers performed an oral glucose tolerance test on a range of test subjects in the fall and spring. While the diet records of the subjects showed no variation in the seasonal variation of carbohydrate or protein foods, there was a significant seasonal variation in the plasma glucose and insulin curves. Fasting glucose levels were highest in the autumn and lowest in the spring. Glucagon levels were also lowest during the spring. The authors also gave the subjects an oral glucose tolerance test to assess their response to a glucose load. The results of this showed that summed glucose response was highest in the spring and lowest in the autumn. Based on the criteria at the time, following the oral glucose tolerance test, one subjects would have been classified as diabetic in the spring, but would only have been classified as having only impaired glucose tolerance if tested in the autumn. Another subject classed as diabetic in the spring showed no signs of blood sugar problems when tested in the autumn.
Therefore blood sugar control shows significant seasonal variation and this may relate to changes in the sensitivity of the insulin receptor. The link between insulin sensitivity and vitamin D levels, reported elsewhere, is interesting because it may explain the seasonal variation seen in this study. Factors such as changes to activity levels could also play a role as exercise has an insulin sensitising effect. Also the authors reported a seasonal variation in thyroid hormone levels. In particular total T4 was higher in the summer than other seasons, but total T3 (the more biologically active thyroid hormone) was higher in the winter than the summer. Therefore significant changes to the hormonal milieu appears to occur seasonally in humans, which would be expected based on known data from animals and how they interact with their environment. Of course diurnal variations in hormone levels in humans in well reported, and so it should be no surprise that longer cycles of hormonal activity are also present
Dr Robert Barrington’s Nutritional Recommendation: Seasonal variations in hormones are significant. Care should be taken when making diagnoses based on single measurements for hormonal activity as these values can change substantially over time both diurnally and seasonally.
RdB
