Effect of Season on 25-OH Vitamin D in Serum of Rhesus Monkeys Housed in Puerto Rico

Vitamin D blood levels have been shown to be partially dependent upon season in temperate climates, however, this same evaluation has not yet been reported in fully tropical climates. Herein, we assessed the vitamin D levels in the blood of Rhesus monkeys housed at the Puerto Rico Caribbean Primate Research Center collected in the island's "summer"(May-October) and "winter" (November-April) months.

In 2006 through 2014, repeated measurements of blood samples were collected from 5 Rhesus monkeys (IACUC-approved) during "summer" and "winter" months to assess 25-OH vitamin D, determined via HPLC. UV-B and UV-A (KJ/m2/day) were measured using a ground based radiometer for these time periods. A paired t-test and a multilevel mixed- effect model approach was performed for data analysis.

The difference of the mean serum values of 25-OH vitamin D between seasons showed lower levels during "winter" than "summer" months. About 23% of the variance in levels can be attributed to difference between the monkeys. The means of UV-B and UV-A, as a proxy for sunlight intensity, were greater (over the entire study interval) during the "summer" as opposed to "winter" months (p < 0.001).

Vitamin D levels were substantially higher in the "summer" rather than the "winter" months. This observation implies that even in fully tropical regions, such as Puerto Rico, time of year can have an influence on vitamin D status. While comparable studies have not been undertaken in humans, it would not be unreasonable to suggest that similar results would be obtained should such a study be done.