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All told, about 60 carotenoids play a provitamin A role in one species or another, and beta-carotene is the most important.
Beta-carotene
Is best known as a precursor of vitamin A, but more recent work shows it also plays an independent role in immunity. Research on the latter role is still in its infancy. The compound, like all carotenoids, originates only in plants, so animals must rely on plant-based sources or supplementation to meet their beta-carotene needs.
For decades, scientists assumed that high concentrations of carotenoids in some animals' blood and tissue were nothing more than a waiting line to become vitamin A. Recent studies at Washington State University (WSU), however, demonstrate that carotenoids play their own independent role in protecting mammalian cells.
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Mastitis, the most economically disastrous of all dairy cow diseases, is most likely to strike during two stressful periods: immediately postpartum and in the early dry period. Concentrations of blood beta-carotene and vitamin A plummet during these two periods, according to WSU animal scientist Boon P. Chew, who has studied these relationships in dairy cattle for some 15 years. In two studies with Holstein cows, WSU researchers provided three levels of daily supplementation:
- 53,000 IU of vitamin A (the NRC recommendation) per head daily.
- 173,000 IU of vitamin A per head daily.
- 53,000 IU of vitamin A with 300 mg beta-carotene (total equivalent to 173,000 IU "A") per head daily.
In the study with dry-period cows, the incidence of new intramammary infections fell by nearly half (27 percent versus 49 and 50 percent) when cows received supplementation of both vitamin A and beta-carotene instead of adequate or even increased supplementations of vitamin A alone (Figure 1). The second study, concerning postpartum somatic cell counts, saw the beta-carotene cows tally 38 percent lower count than those fed 53,000 IU of vitamin A per head daily (Figure 2).
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It is noteworthy that supplemental beta-carotene has been linked to improved fetal health in pigs and laboratory animals, and reduced incidence of retained placenta in cattle. Scientists believe that carotenoids' antioxidant function protects the highly active ovarian follicles, corpora lutea and uterine cells from free radical damage, thereby keeping all the systems optimal for secretion of critical uterine proteins.
The antioxidant role also figures into the enhanced immune response to mastitis as increased levels of carotenoids protect immune cells by preventing inadequacy. This may occur in both the specific and nonspecific cellular host defense systems.
Holstein cows supplemented with 300 to 600 mg/head/day of beta-carotene from four weeks before calving to four weeks after calving showed better immune response than did cows supplemented with 120,000 IU/day of vitamin A alone. This and other studies indicate the response is due to the intact carotenoid molecules, as opposed to their prior conversion to vitamin A.
Oral beta-carotene builds up a higher concentration of plasma beta-carotene, but not vitamin A, also suggesting a specific immunomodulatory action. Moreover, enhanced NK (natural killer, anti-cancer) cell cytotoxicity was observed in humans and mice.
Beta-carotene also modulates the polymorphonuclear neutrophils (PMNs) and macrophages that ingest and break down invading particles or pathogens. WSU studies showed that beta-carotene supplementation stimulated PMN function, enhancing its bactericidal activity. On the other hand, vitamin A supplementation alone had no effect on PMNs.
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Without an antioxidant like beta-carotene, reactive oxygen molecules and destructive free radicals from both within the immune system and outside it would greatly reduce immunity. There is also evidence that carotenoids regulate cell nuclear events. At least, lymphocyte nuclei take up significant amounts of beta-carotene when available, and it persists in cells longer than in plasma after supplementation ceases.
In practical terms, beta-carotene supplementation will depend on a variety of considerations. Trials on Holstein cows have featured supplementation levels ranging from 300 to 600 mg per head daily--the equivalent of 120,000 to 240,000 IU of vitamin A if all were converted. But not all the beta-carotene will be converted, since conversion rates decline as intake levels increase. And given beta-carotene's independent role in immunity, supplementation should exceed the amount required for vitamin A synthesis.
In addition, breeds differ in their ability to transport carotenoids intact from intestines through the blood. For instance, Guernseys transfer more carotenoids to milk than Holsteins. Feed quality also comes into play, since carotenoids are subject to rapid oxidation under the heat and humidity of storage; thus, only the freshest forage is a good source.
While it's clear that beta-carotene stimulates immunoresponse independently of vitamin A, efficacy studies will require large-scale field trials. If beta-carotene works on the wider scale, it may help dairy producers cut back on the use of antibiotics.
References:
- Chew, B.P. 1994. Beta-Carotene, Other Carotenoids Push Immunity Defense. Feedstuffs, Vol. 66, No. 18.
- Chew, B.P., and L.A. Johnston. 1985. Effects of Vitamin A and Beta-Carotene on Mastitis in Dairy Cows. J. Dairy Sci. 68 (Supp. 1): 191.
- Dahlquist, S.P., and B.P. Chew. 1985. Effects of Vitamin E and Beta-Carotene on Mastitis in Dairy Cows During the Early Dry Period. J. Dairy Sci. 68 (Supp. 1): 191.
- Heirman, L.R., et al., 1990. Effects of Dietary Beta-Carotene on Lymphocyte Function in Peripartum Dairy Cows. J. Dairy Sci. 73 (Supp. 1): 166.
- Michal, J.J., et al., 1994. Modulatory Effects of Dietary Beta-Carotene on Blood and Mammary Leucocyte Function in Periparturient Dairy Cows. J. Dairy Sci. 77: (in press).
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