Vitamin A repletion in thoroughbred mares with retinyl palmitate or beta-carotene.

This study examined the effects of vitamin A supplementation in the form of retinyl palmitate or water-dispersible beta-carotene on forty-five Thoroughbred mares over the course of 20 months. The findings suggest that retinyl palmitate supplementation improved the horses’ vitamin A status when compared to beta-carotene supplementation, but reaching an optimal vitamin A status required twice the recommended daily intake of vitamin A.

Research Methods and Participants

• The study involved forty-five Thoroughbred mares participating in an 8-month depletion study which was extended for an additional 20 months.
• These mares were kept on three different forage diets: orchardgrass hay and vitamin A-free concentrate on a drylot; pasture, orchardgrass/alfalfa hay, and vitamin A-free concentrate; or pasture and orchardgrass/alfalfa hay only.
• Each dietary group was further divided into three more subgroups, and each mare was provided one of three variations: retinyl palmitate at twice the recommended daily intake, an equivalent of vitamin A in water-dispersible beta-carotene form, or a control (vehicle).

Measurements and Observations

• Vitamin A status in the mares was monitored by measuring serum retinol (a form of vitamin A) levels and a relative dose response (RDR) test was conducted every 60 days.
• The results showed that retinol concentration and RDR fluctuated seasonally, regardless of vitamin A supplementation.
• Mares contained on hay and vitamin A-free concentrate (HC) had a lower vitamin A status based on serum retinol and RDR values compared to those on pasture and hay or pasture, hay, and concentrate (PH and PHC).

Findings on Vitamin A Supplementation

• Results also suggested an improvement in vitamin A status in all dietary groups supplemented with retinyl palmitate but not with beta-carotene.
• Even with vitamin A supplementation, the HC mares achieved serum retinol levels similar to that of the control groups in pasture, but not the RDR values.
• Overall, the study suggests that optimal vitamin A status in mares previously depleted might just be achieved by providing twice the currently recommended daily intake of vitamin A.