Vitamin D status is seasonally stable in northern European dogs

Background
Numerous studies in veterinary species have recently linked vitamin D status with non-skeletal health disorders. Previous studies have indicated that dogs cannot produce endogenous vitamin D via cutaneous production and rely solely on dietary intake of vitamin D. The seasonal variation of vitamin D seen in humans due to changes in UV exposure therefore is unlikely to be replicated in these animals.

Objectives
The objective of this study was to investigate the natural variation in 25-hydroxyvitamin-D concentration in dogs subject to seasonal UV exposure.

Methods
This longitudinal study followed 18 healthy dogs fed a standardized diet over one year, with blood samples obtained monthly. Two key vitamin D metabolites, 25-hydroxyvitamin-D2 and 25-hydroxyvitamin-D3, were assessed by liquid chromatography tandem mass spectrometry in serum samples. Various other biochemical parameters were also measured. Seasonality was assessed using Cosinor statistical analysis.

Results
Although the dogs were subject to seasonally varying UV radiation, 25-hydroxyvitamin-D and related biomarkers (including calcium and parathyroid hormone) remained stable over time and did not follow a seasonal pattern. 25-hydroxyvitamin-D was not positively correlated to exposure to UV radiation. Nonetheless, variation in 25-hydroxyvitamin-D concentration between individual dogs was detected.

Conclusions
Given the standardization of diet, we concluded that the seasonal stability of 25-hydroxyvitamin-D concentration (vitamin D status) was likely a direct result of lack of cutaneous vitamin D production in this species and highlights the importance of dietary intake. The variation in 25-hydroxyvitamin-D concentration between animals warrants further investigatio