Serum profiles of vitamins A, E and D in mares and foals during different seasons

Twenty mares and 15 foals of the Finnhorse breed were used to determine serum levels of vitamins A, E and D during different seasons. Monthly serum samples were analyzed from mares for 22 mo and from foals for 13 mo after birth. The horses were kept on pasture from early June until early October and in stables during the winter months. Seasonal differences (P less than .001) were found in serum retinol, retinol to protein ratio, alpha-tocopherol and alpha-tocopherol to cholesterol ratio. Values were lowest from February through May and highest from June through August. Serum levels of retinol and alpha-tocopherol of the foals were much lower (P less than .001) than in mares during the first 4 mo of life. As in the mares, serum levels of retinol and alpha-tocopherol decreased (P less than .001) in winter. At 1 yr of age, serum levels of these vitamins increased rapidly on pasture to values that were close to those found in adult horses. Serum 25-hydroxyvitamin D levels were very low throughout the year. However, a small seasonal variation also was found in 25-hydroxyvitamin D concentrations in the mares. Serum 1,25-dihydroxyvitamin D levels in pregnant mares in winter were similar to those found in lactating mares in summer. These levels also were low compared with those in animals of other species.