Thoroughbred Racehorses in Hong Kong Require Vitamin D Supplementation to Mitigate the Risk of Low Vitamin D Status.

There is a paucity of data relating to the vitamin D status of racehorses. We hypothesised that the management of racehorses in Hong Kong (HK) predisposes to low vitamin D status unless they receive dietary supplementation. Serum concentrations of 25-hydroxyvitamin D 2 (25OHD 2 ), 25-hydroxyvitamin D 3 (25OHD 3 ) and total 25-hydroxyvitamin D (total 25OHD) for 79 non-grazing HK racehorses were compared with those for 22 racehorses training in the United Kingdom (UK) that grazed for ≥1 h/d, and for which published data exists. A nested group of 41 HK horses was sampled twice to determine the effect of the duration in HK on vitamin D status. The HK horses had significantly lower serum concentrations of total 25OHD and 25OHD 2 than the UK horses; 25OHD 2 was undetectable in 15/79 HK sera and serum concentrations of 25OHD 2 declined with the duration in HK. The main determinants of vitamin D status were assessed using linear regression; the retained variables were the 25OHD 3 concentration and the duration in HK. The inverse relationship between the serum concentrations of 25OHD 2 and 25OHD 3 , previously identified in humans, was observed for the first time in horses. In conclusion, HK racehorses have low serum 25OHD 2 and total 25OHD concentrations and rely on D 3 supplementation to maintain adequate vitamin D status. Further study is required to determine the optimal form of dietary vitamin D supplementation for Thoroughbred racehorses.