A recent study assessed the seasonality of 25(OH)D levels in cattle, a sun exposed population.
The seasonality of 25(OH)D levels are well known in many populations. However, the seasonality of 25(OH)D in a sun-exposed population is not well known. That is, what is the seasonal variation in 25(OH)D of those who live and work in the sun?
We don’t know the answer for humans but we do for cattle! Dr. Tim Reinheardt of the USDA assessed seasonal vitamin D levels of 195 steers and heifers at latitude 41 degrees North over a two-year period.
In late Spring, mean 25(OH)D levels of the cattle were 26 ng/ml. By late summer those levels were about 49 ng/ml, and by September the levels were up to 65 ng/ml.
Then, the vitamin D levels began falling to about 55 ng/ml in October, and continued to decline to about 15 ng/ml by the end of winter with 28% of the cattle having levels below 10 ng/ml.
These cattle received supplemental vitamin D in their feed but only about 2 IU/KG/day. The authors called for additional feed fortification, especially in winter.
One cannot compare the 25(OH)D levels of different species. However, it is interesting to note that the vitamin D status of cattle ranges are similar to the known ranges in humans (about 60 ng/ml for lifeguards at the end of summer and 15 ng/ml levels for many populations at the end of winter).
It appears cattle and perhaps sun-exposed humans experience drastic seasonal changes in their vitamin D status at latitude 41 degrees North.