We Know Next to Nothing About Vitamin D in Horses!

Serum 25(OH)D Analysis in Captive Pachyderms (Loxodonta africana, Elephas maximus, Diceros bicornis, Rhinoceros unicornis, Tapirus indicus) in Europe

This study aimed to detect seasonal and species differences in serum 25-hydroxy vitamin D (25(OH)D) concentrations during summer and winter months in captive pachyderms in Europe. Both elephant species had low 25(OH)D while African elephants did not show a seasonal variation. Asian elephants had significantly higher 25(OH)D compared to their African counterparts but also did not show a seasonal difference. Both rhinoceros species investigated had higher 25(OH)D compared to both elephant species; the Indian rhinoceros had high circulating levels year-round, while the black rhinoceroses showed significantly lower 25(OH)D in winter. Malayan tapirs have very low 25(OH)D, comparable to horses. The higher 25(OH)D of elephants and rhinoceroses could indicate that elephants and rhinoceroses are capable of producing vitamin D. This might indicate that the Indian rhinoceroses are capable of producing enough endogenous vitamin D year-round at latitudes around 52° N, while both elephant species and the black rhinoceros are not. This study also showed that it is likely that both elephant species and rhinoceros species are capable of absorbing cholecalciferol from the digestive tract, according to the existing literature, while tapirs may not.

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 D₂ (25OHD₂), 25-hydroxyvitamin D₃ (25OHD₃) 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 25OHD₂ than the UK horses; 25OHD₂ was undetectable in 15/79 HK sera and serum concentrations of 25OHD₂ declined with the duration in HK. The main determinants of vitamin D status were assessed using linear regression; the retained variables were the 25OHD₃ concentration and the duration in HK. The inverse relationship between the serum concentrations of 25OHD₂ and 25OHD₃, previously identified in humans, was observed for the first time in horses. In conclusion, HK racehorses have low serum 25OHD₂ and total 25OHD concentrations and rely on D₃ supplementation to maintain adequate vitamin D status. Further study is required to determine the optimal form of dietary vitamin D supplementation for Thoroughbred racehorses.

Age-related changes in vitamin D metabolism and vitamin D receptor expression in equine alveolar macrophages: a preliminary study

The vitamin D receptor (VDR)-vitamin D axis modulates pulmonary immunity in people but its role in equine immunity is unknown. Bacterial pneumonia causes high morbidity/mortality in foals and alveolar macrophages (AMφ) are important for pulmonary defenses. Age-related variations in vitamin D-mediated function of AMφ might contribute to the foal's susceptibility to pneumonia. Our aim was to assess the impact of age on equine vitamin D metabolism and VDR expression in AMφ. AMφ and plasma was collected from healthy foals (2, 4 and 8 weeks old) and adult horses (once). AMφ VDR expression was determined via RT-qPCR and plasma vitamin D metabolites quantified via immunoassays. Data were analyzed with linear mixed models. Inactive-vitamin D metabolite concentrations were lowest in foals at 2 weeks and lower at 2 and 4 weeks compared to adults (P < 0.001). Active-vitamin D metabolite concentrations were higher in foals than adults (P < 0.05). VDR expression was detected in AMφ in all animals and was highest in 2-week-old foals. Vitamin D metabolism and AMφ VDR expression are impacted by age in horses. This may have immunological consequences in foals given the key role that the VDR-vitamin D axis has in pulmonary immunity in other species.

Plasma transport of ergocalciferol and cholecalciferol and their 25-hydroxylated metabolites in dairy cows

In cattle, there are 2 significant forms of vitamin D: ergocalciferol (ERG) from fungi on roughage and cholecalciferol (CHO) from vitamin supplements or endogenous synthesis in the skin. The hypothesis of the present study is that vitamin D from the 3 sources is transported in different plasma fractions in the body. This is hypothesized to explain the lower efficiency of ERG compared to CHO in securing a sufficient plasma status of 25-hydroxyvitamin D and explain the inefficient excretion of dietary CHO into milk compared to endogenous CHO. Twenty vitamin D-depleted cows were assigned to 5 treatments: D2, housed indoor and fed 625-μg/d (25.000 IU) ERG; D3, housed indoor and fed 625-μg/d CHO; D2+D3, housed indoor and fed 625-μg/d ERG and 625-μg/d CHO; SUN, let out for daily pasture to facilitate CHO synthesis from sunlight; and D2+SUN, fed 625-μg/d ERG and let out for daily pasture. Blood samples were taken twice weekly and plasma fractionated by ultracentrifugation into 3 fractions: light lipoprotein (LLP), heavy lipoprotein (HLP), and protein and analyzed for content of ERG and CHO and their liver derived metabolites 25-hydroxyergocalciferol (25ERG) and 25-hydroxycholecalciferol (25CHO), respectively. Liver biopsies were taken on the last day of the study to asses gene expression related to vitamin D metabolism. During 4 wk of study, the vitamin D status in plasma increased to 19.3 to 22.8 ng/mL 25ERG in ERG-treated cows with the highest concentration in D2 (P ≤ 0.05) and to 25.0 to 33.4 ng/mL 25CHO in pasture or CHO-treated cows with the highest concentration in SUN (P ≤ 0.01). In plasma fractions, CHO was mainly found in the HLP fraction, whereas 25CHO was almost exclusively found in the protein fraction, probably due to its reported high binding affinity to vitamin D-binding protein. About 70% to 90% of 25ERG was found in the protein fraction and the remaining 25ERG was found in HLP, whereas ERG was found in both HLP and LLP fractions. In liver tissue, the expression of vitamin D-25-hydroxylase was lower in D2+D3 (P ≤ 0.05) and SUN (P ≤ 0.05) than that in the remaining groups, and the vitamin D receptor was expressed in the liver to a larger extent in D2+SUN than that in D2+D3 (P ≤ 0.05) and SUN (P ≤ 0.05). In conclusion, different plasma transport mechanisms may explain the lower physiological efficiency of ERG compared to CHO in securing the vitamin D status in plasma but do not explain the lower efficiency of synthetic CHO compared to endogenous CHO from sunlight or UV light in securing a high CHO content in milk.

Physiological limit of the daily endogenous cholecalciferol synthesis from UV light in cattle

The link between UV light (sunlight) and endogenous cholecalciferol (vitamin D₃ ) synthesis in the skin of humans has been known for more than a 100 years, since doctors for the first time successfully used UV light to cure rickets in children. Years later, it was shown that UV light also had a significant effect on the cholecalciferol status in the body of cattle. The cholecalciferol status in the body is measured as the plasma concentration of 25-hydroxycholecalciferol, which in cattle and humans is the major circulating metabolite of cholecalciferol. Very little is, however, known about the quantitative efficiency of UV light as a source of cholecalciferol in cattle nutrition and physiology. Hence, the aim of this study was to determine the efficiency of using UV light for increasing the plasma 25-hydroxycholecalciferol concentration in cholecalciferol-deprived cattle. Twelve cows deprived of cholecalciferol for 6 months were divided into three treatment groups and exposed to UV light for 30, 90 or 120 min/day during 28 days. UV-light wavelengths ranged from 280 to 415 nm and 30-min exposure to the UV light was equivalent to 60-min average summer-sunlight exposure at 56 °N. Blood samples were collected every 3-4 days and analysed for 25-hydroxycholecalciferol and cholecalciferol. Results showed that increasing the exposure time from 90-120 min/day did not change the slope of the daily increase in plasma 25-hydroxycholecalciferol. Hence, it appears that cholecalciferol-deprived dairy cattle are able to increase their plasma 25-hydroxycholecalciferol concentration by a maximum of 1 ng/ml/day from UV-light exposure.