suggested reading

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 horses.

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 ß-carotene, zeaxanthin, lutein, ß-Apo-8;-carotenoids, ß-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, ß-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 ß-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, ß-carotene is the least expensive.

ß-carotene absorption, transport and tissue retentionLike other fat-soluble nutrients, ß-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 ß-carotene then is transported via the lymph into the blood. In either case, ß-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 ß-carotene is absorbed as vitamin A, while 15 percent is absorbed intact (Goodman et al. 1966). ß-carotene is transported in the blood exclusively via lipoproteins, predominantly by low-density ones (LDL). ß-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).