Page 139 - Small Animal Clinical Nutrition 5th Edition

140 Small Animal Clinical Nutrition

after oral ingestion. In plants, carotenoids play essential light- SOURCES
VetBooks.ir harvesting roles during photosynthetic events and protect yellow, orange and red fruits and vegetables, plant leaves, as well
Carotenoids are responsible for the striking colors of many
membranes against photo-oxidative damage. More than 600
as the colors in some species of fish, crustaceans and plumage
different compounds are classified as carotenoids, but fewer
than 10% can be metabolized into vitamin A. In contrast to of some birds.
many other mammals, cats are unable to convert β-carotene to
vitamin A; therefore,cats must rely solely on preformed vitamin Bioflavonoids
A in their diet (Schweigert et al, 2002). The carotenoids found The flavonoids are a group of red, blue, yellow and colorless
in greatest abundance in a variety of foodstuffs are β-carotene, compounds that have vitamin-like activity. This class of com-
α-carotene, lutein, lycopene, β-cryptoxanthin, zeaxanthin, can- pounds was originally mistaken for vitamin C because crude
thaxanthin and astaxanthin. A primary characteristic of the extracts of lemon juice and yellow peppers had antiscorbutic
carotenoids is their conjugated polyene structure. effects. Originally called citrin (mixture of eriodictyol and hes-
peridin), vitamin P or vitamin C , these compounds were
2
ABSORPTION AND TRANSPORT reclassified as flavonoids in 1950 (Combs, 1998; Machlin,
Because carotenoids are lipophilic compounds, concurrent 1991; Harborne, 1994). More than 5,000 flavonoids have been
ingestion of fat facilitates intestinal carotenoid absorption. identified (Harborne and Baxter, 1999). Flavonoids are classi-
Bile salts are necessary for absorption of ingested fat and fied in major and minor groups. Classes include flavonols, fla-
carotenoids. The aggregation of bile salts into micelles, and vanols, flavones, isoflavones and anthocyanins. Flavonols, (e.g.,
the formation of mixed micelles with the products of lipid kaempferol, quercetin and myricetin, are present in tea, apples
digestion and other lipid-soluble food constituents are essen- and onions. Flavanols (also called catechins) are found in tea,
tial in facilitating absorption of lipophilic compounds from apples and red wine. Isoflavones such as genistein and daidzein
the intestine. At the brush border, micelles interact with are constituents of soybeans. Anthocyanins provide the deep
enterocytes where the lipophilic contents of micelles diffuse red color to fruits such as berries.
out of the micelles and across the cell membrane. It is
believed that the uptake of carotenoids by enterocytes occurs ABSORPTION AND TRANSPORT
passively and is not carrier-mediated. Enterocytes package The availability varies widely among flavonoids depending
carotenoids into chylomicrons, which migrate to the basal- on the food source and the forms of flavonoids they contain.
lateral cell membrane where they are exocytosed into the Flavonoids are usually found naturally as glycosides linked to
intracellular space for passage to the lymphatic system. After sugars, except for catechins. The type of sugar moiety of the
transportation in chylomicrons via the lymphatic system, glycoside affects availability, (e.g., quercetin glucosides are more
carotenoids are carried by lipoproteins and transported in the efficiently absorbed than quercetin rutinosides) (Hollman et al,
bloodstream. 1999). Mammalian enzymatic systems are unable to hydrolyze
flavonoid glycosides, but the necessary glycosidases are present
FUNCTION in the gut microflora. After hydrolysis and absorption in the
Although carotenoids do not strictly fit the definition of a small intestine, flavonoids are bound in the liver as glucuronides
vitamin for mammalian species, they have biologic activity or sulfate conjugates (Machlin, 1991). Recent studies with fla-
beyond their provitamin A role. Carotenoids with nine or more vanols have shown that glycosides can be absorbed without pre-
double bonds function as antioxidants by quenching singlet vious hydrolysis by microorganisms (Hollman et al, 1995).
oxygen and other reactive oxygen species such as hydroxyl rad- Most of the flavonoids are further metabolized into phenolic
icals, superoxide anion radicals and hydrogen peroxide, which compounds and rapidly excreted, usually within 24 hours.
are produced in normal metabolism (Chew, 1995; Bendich,
1989). Carotenoids sacrifice highly reactive multiple double FUNCTION
bonds to free radicals via hydrogen donation, thereby stabiliz- Although many different flavonoids exist with many differ-
ing reactive products. Carotenoids also protect cell membranes ent physiologic effects, this class of compounds shares some
by stabilizing the oxygen radicals produced when phagocytic similar functions. The most notable is the sparing effect that
granulocytes undergo respiratory bursts that destroy intracellu- flavonoids have on vitamin C. Flavonoids have the ability to
lar pathogens (Bendich, 1989). perform similarly to vitamin C: reduce capillary fragility and
The immune-modulating properties of carotenoids have permeability and chelate the divalent metal ions copper and
been studied in dogs and cats. Supplementation with β- iron (Combs, 1998). Flavonoids can act as antioxidants because
carotene increases the CD4 T cell population in older dogs they are very effective scavengers of free radicals. In fact,
to levels found in young dogs and improves T-cell prolifera- flavonoid assays in vitro often exhibit stronger antioxidant
tion (Massimino et al, 2003). Supplementation with β- activity than vitamins E and C. Other non-antioxidant related
carotene or lutein, an oxycarotenoid found in corn and other beneficial effects include prevention of angiogenesis (Cao and
vegetables, stimulates cell-mediated and humoral immune Cao, 1999) and inhibition of cyclooxygenase and lipoxygenase
responses in dogs and cats (Chew et al, 2000; Kim et al, (Laughton et al, 1991). Catechins, found in abundance in tea,
2000; Kim et al, 2000a). have been shown to modulate signal transduction pathways,