▀eta-carotene (▀-carotene) is one of many carotenoids found in nature. In the human nutrition field, evidence suggests that
higher blood levels of ▀-carotene, as well as other carotenoids such as alpha-carotene, lycopene, lutein, zeaxanthin and
▀-cryptoxanthin might be associated with lower risk of several chronic diseases. ▀-carotene, alpha-carotene and ▀-cryptoxanthin
have pro-vitamin A activity, while the other carotenoids have no pro-vitamin A activity (DRI, 2000).
Earlier publications on nutrient requirements of horses (NRC, 1961) suggests that horses had both a carotene and vitamin A
requirement; however the latest edition (NRC, 1989) has published only a vitamin A requirement and not a carotene requirement
for all horse classes. Thus, nutritionists typically do not consider the intake of carotenoids when formulating rations for
Do all horse classes only need pre-formed vitamin A and not ▀-carotene, or is there also a need to supplement some classes
of horses with ▀-carotene as well as vitamin A?
What are carotenoids?
Carotenoids occur in almost all plants and animals. Carotenoids are essential to green plants for photosynthesis, acting in
light harvesting and protecting against destructive photo-oxidation. Without carotenoids, photosynthesis in an oxygen-containing
atmosphere would be impossible. In addition to those in green plants, carotenoids are also compounds easily recognized as
the orange-red colors of foods, such as oranges, tomatoes and carrots. Some animals use carotenoids for coloration, especially
birds (yellow and red feathers, e.g. flamingos), fish (e.g. goldfish and salmon) and a wide variety of invertebrate animals
(shrimp, lobster and other crustaceans), where binding with protein can modify their colors to blue, green or purple.
Straub (1987) described 563 different carotenoids, not counting their various cis- and trans-isomers. A few of the main carotenoids
and polyenes found in foodstuffs and feeds are alpha- and ▀-carotene, zeaxanthin, lutein, ▀-Apo-8;-carotenoids, ▀-cryptoxanthin,
astaxanthin, canthaxanthin, citranaxanthin, lycopene, neoxanthin, phytoene and phytofluene and violaxanthin. The naturally
occurring carotenoids are fat-soluble and are completely insoluble in water. They are often associated with lipids, to which
they impart their color, e.g. milk fat, egg yolks and animal fat. In fish and shrimp, however, carotenoids are typically protein-bound.
Of the major carotenoids, ▀-carotene has the highest pro-vitamin A activity, thus it has received most attention by scientists.
More recently, protective effects against serious disorders, such as cancer, heart disease and degenerative eye disease, have
been recognized for ▀-carotene and the carotenoids lutein, lycopene and zeaxanthin. Roles other than pro-vitamin A activity
have stimulated intensive research into various effects of carotenoids in humans and animals. Of the commercially available
carotenoids, ▀-carotene is the least expensive.
▀-carotene absorption, transport and tissue retentionLike other fat-soluble nutrients, ▀-carotene must be emulsified prior to metabolism and/or absorption via the action of bile
acids. -carotene either can be converted in the mucosa to retinal by the action of a specific enzyme—15,15'-dioxygenase,
and subsequently reduced to retinol, or it can be absorbed intact and incorporated in the chylomicrons. The ▀-carotene then
is transported via the lymph into the blood. In either case, ▀-carotene or vitamin A then must be incorporated into chylomicrons
and passed into the lymphatics. There are no data in horses, but in humans, 60 percent to 75 percent of the ▀-carotene is
absorbed as vitamin A, while 15 percent is absorbed intact (Goodman et al. 1966). ▀-carotene is transported in the blood
exclusively via lipoproteins, predominantly by low-density ones (LDL). ▀-carotene is excreted in the urine and bile.
Yang and Tume (1993) suggested that differences in the selective absorption process in the small intestine are responsible
for the various concentrations of carotenoids observed in different species of animals. It is generally thought that carotenoids
move into the enterocytes by passive diffusion (Furr & Clark, 1997). If this is the case, then species differences in intestinal
pH, gut motility, liposome and micelle formation, as well as the variation in the type and amount of dietary fat consumed,
might influence carotene absorption. In addition, little is known about how carotenoids are incorporated into lipoprotein
fractions and enter the circulation (Furr & Clark, 1997). Species variations in lipoprotein handling are also likely to be
large contributors to differences in the accumulation of carotenoids (Slifka et al. 1999).