Page 76 - Speedhorse June 2019

THE MANY ROLES OF VITAMIN A
“For healthy cartilage, a young, growing horse needs adequate vitamin A.”
Heather Smith Thomas
by Heather Smith Thomas
Vitamins are organic compounds required in trace amounts in the diet to help regulate body functions. Various vitamins play crucial roles in body maintenance and function. Dr. Kathleen Crandell, Equine Nutritionist, Kentucky Equine Research, says vitamin A is probably one of the most important
because it has so many different
roles in the body. “Vitamins are
either fat-soluble or water-soluble, and this just refers to the way each vitamin
is handled in the body,” she says.
The fat-soluble
vitamins are A, E, D
and K. The water-sol-
uble vitamins include
all of the B vitamins
and vitamin C. The
fat-soluble vitamins can be stored in the body, while water-soluble vitamins are not. “If there is not enough fat in the diet, the body has difficulty absorbing very much of a fat-soluble vitamin because fat helps these vitamins pass through the mucosal wall in the intes- tinal lining. The water-soluble vitamins pass through the wall readily, without the need of fat. Humans on really low-fat diets compromise the absorption of fat- soluble vitamins and are at risk of becoming deficient in vitamins A or D,” she explains.
WHAT IS VITAMIN A?
“Vitamin A is just a general term for many differ- ent forms of retinol. After it’s absorbed by the body, it goes through various forms as it goes from storage to distribution to target cells. Carotene in green plants is converted to vitamin A in the intestinal wall. Green grass is an excellent source of carotenes. The greener the color, the more carotenes in that plant,” says Crandell. Horses on green pasture or fresh alfalfa hay get plenty of vitamin A.
“The most abundant source of vitamin A is from beta-carotene,” she says. “When a horse eats grass,
a certain enzyme within the cells in the intesti-
nal tract convert carotene to vitamin A, and it is absorbed into the body as vitamin A, though some carotenes are absorbed directly.
“Sometimes in humans, if they eat too much beta-carotene, they get a yellow or orange tinge to the skin. This doesn’t happen in horses because they have poor absorption of the intact beta-carotenes.”
Horses have been eating large amounts of caro- tene in forages for a long time. “They will never get vitamin A toxicity from eating naturally-occurring carotenes, due to a limited amount of an enzyme (called Beta-carotene 15, 15’ dioxygenase) to convert the carotenes to vitamin A,” says Crandell.
“From beta-carotene, the horse can actually get two retinols, which is the form of vitamin A within the body once it is absorbed. After going through the intestinal wall cells, it goes into the lymphatic system - rather than directly into the bloodstream
- and from the lymph it goes into the liver. In the liver, it becomes bound to the retinol-binding
Equine Nutritionist
Dr. Kathleen Crandell of Kentucky Equine Research.
74 SPEEDHORSE, June 2019
EQUINE HEALTH