Antioxidant vitamins improve the immune
response of dairy cattle.
Generally, it has been assumed that endogenously produced ascorbic acid was sufficient to meet
the metabolic demands of ruminants. However, recent research on swine and poultry (17, 23)
suggests that under specific environmental and physiological conditions the amount of ascorbic
acid produced by the animal may in fact be insufficient to meet its requirements. Lactating dairy
cattle may be predisposed to subclinical ascorbic acid deficiency because of the huge drain on the
precursor molecules for ascorbic acid production, glucose and galactose into milk. Non-lactating
ruminants may also have their ascorbic acid status compromised during periods of stress (11).
The physiological ramifications of subclinical ascorbic acid deficiency are multifactorial. Ascorbic
acid is critical if the effectiveness of the immune system is to be optimized (4, 5). Pathogenic
organisms such as bacteria and viruses cause increased free radical production in the infected host
and a concomitant decrease in ascorbic acid level and immune function (3).
Neutrophils are the primary leukocyte responsible for the killing of invading pathogens. A
deficiency of ascorbic acid can reduce the ability of the cell to migrate to the site of the
inflammation, allowing for increased oxidative damage to the neutrophils and reduced production
of the major antimicrobial agent, hypochlorous acid. Hypochlorous acid can also exert a negative
effect on lymphocyte proliferation. In the presence of adequate levels of ascorbic acid lymphocyte
proliferation occurs normally (1). Ascorbic acid and other antioxidant vitamins such as vitamin E
have been shown to enhance neutrophil function and minimize free radical damage (18).
Ascorbic acid may also modulate the immune system via its role in the regulation of hormones
associated with stress. Acute stressors activate the hypothalamic-pituitary-adrenal axis resulting in
the elaboration of corticotrophin releasing hormone (CRH). Corticotrophin releasing hormone
stimulates the anterior pituitary to release adrenocorticotropic hormone which in turn causes the
adrenal cortex to increase circulating plasma levels of glucocorticoids (20). Glucocorticoids,
specifically cortisol, compromise many facets of the cellular and humoral immune response.
Vitamin C is thought to have a role in the production and regulation of corticosteroid production
in the bovine adrenal gland (9). In addition, stress and the associated rise in corticosteroids may
reduce the circulating plasma levels of ascorbic acid (17).
Vitamin C is a known antioxidant and in this capacity can quench free radicals and thereby protect
the structural integrity of the cells of the immune system (6). The antioxidant properties of
ascorbic acid may also impart improved quality to beef (25). Cattle fed high levels of another
antioxidant vitamin, alpha-tocopherol, produced steaks that had superior shelf life in terms of
color and lipid oxidation (2). There is a close synergism between ascorbic acid and vitamin E in
that vitamin C can reduce the radical of vitamin E back to its active reduced state (15). Vitamin C,
therefore, may play a two-fold role in preventing the oxidation of the meat color pigment,
oxymyoglobin, to the less desirable metmyoglobin. Metmyoglobin is the pigment which causes
fresh beef to become brown in color thus reducing its consumer appeal. Ascorbic acid may
enhance the quality of beef directly by acting as antioxidant and indirectly by preserving the
antioxidant potential of vitamin E.
Ascorbic acid has been shown to have interactions with trace minerals in species other than
ruminants (10). In particular, there has been considerable research on the interaction between iron
and ascorbic acid in swine (23). Intestinal absorption of nonheme iron is known to increase in the
presence of ascorbic acid (23). Iron is transferred into mucosal cells in the ferrous form and
because vitamin C can reduce ferric iron to ferrous iron it may facilitate absorption (24).
In contrast to iron, copper absorption may actually be reduced by ascorbic acid. Vitamin C can
reduce the divalent copper ion to the less absorbable monovalent ion (24).
There has been far less research conducted on any interactions that may occur between zinc
absorption and ascorbic acid. The studies that have been completed in this area are often
contradictory in nature (21).
Ascorbic Acid, Rumen
In cattle that have been injected with synthetic glucocorticoids (dexamethasone) the negative effect
on the neutrophil function of dairy cows could be partially alleviated by the injection of ascorbic
acid (19). Unfortunately, injecting ascorbic acid can often lead to inflammation at the site of
injection (14). Therefore, there exists a need for a suitable form of vitamin C that could be used as
a feed supplement.
Ascorbic acid in its crystalline state is extremely unstable in the conditions which exist in the
rumen (12). Therefore, a new vitamin C product form, ascorbyl-2-polyphosphate was tested to
determine rumen stability. Table 1 illustrates that the ascorbyl-2-polyphosphate did indeed have
increased stability over the crystalline ascorbic acid. It can be concluded that
ascorbyl-2-polyphosphate possesses sufficient rumen stability characteristics to warrant in vivo
Ascorbyl-2-polyphosphate appears to be an ascorbic acid compound which has sufficient rumen
stability to increase ascorbic acid plasma levels of dairy heifers fed the product for 31 days (Table
2). Prior to the commencement of the trial plasma ascorbic acid levels of the control and test
groups were 3.83 and 3.72mg/L, respectively (P<0.05) (Table 2). The fact that the plasma levels
remained elevated even after 31 days suggests that the animals did not compensate for the
exogenous supply of ascorbic acid by reducing their endogenous production of the vitamin.
Biopsies of the biceps femoris of heifers were performed at the beginning and end of the trial. The
reasons biopsies were required were: 1) an additional indication as to the bioavailability of the
ascorbyl-2-polyphosphate, and 2) to determine the possible use of this product formula to raise
meat ascorbic acid levels. Increased meat ascorbic acid may improve the quality of retail beef
(25).Initially there was no difference between the tissue ascorbic acid levels of the test and control
animals, 7.80 versus 8.20 mg/kg wet weight (P>0.05) (Table 3). After the treatment there was a
definite trend for the animals receiving ascorbic acid to have increased levels of muscle ascorbic
acid. The tissue levels were 10.11 and 8.32 mg/kg ascorbic acid for the test and control animals
(P=.08) (Table 3). The results of the plasma and muscle ascorbic acid analyses together support
the view that ascorbyl-2-polyphosphate is a suitable source of ascorbic acid for ruminants.
Neutrophil function can be enhanced in humans by the supplementation of dietary ascorbic acid
(6). Neutrophil ascorbic acid levels in humans are normally 24ng/106 cells (13) which mimics
closely the levels reported in Table 4 for bovine neutrophils. The data were quite variable between
animals and there were not any differences (P>0.05) between groups at any stage of the trial.
There are several explanations for the enhanced plasma levels not translating into increased
neutrophil ascorbic acid levels. Firstly, neutrophils actively transport ascorbic acid against a large
concentration gradient, therefore, the cells can maintain a homeostatic level of the vitamin unless
the host animal is deficient (22). It appears the trial animals were producing sufficient ascorbic
acid to fulfil their metabolic requirements. Secondly, the amount of time needed to harvest and
separate the cells may have been too long. Ascorbic acid is extremely labile and therefore, the time
for processing may have masked any potential effects.
Ascorbic Acid Supplementation
Newborn calves are dependent upon external sources of ascorbic acid. The calf's inborn ascorbic
acid production does not begin until 2 to 3 weeks of age and does not reach a maximum until 8 to
16 weeks (16).
The ascorbic acid requirements of calves is dependent on the genetic predisposition of the
individual animals to produce the compound (16). Requirements are also influenced by
environment, for example extensive confinement can increase the metabolic demand for vitamin C
When calves are subjected to environmental stressors the supplementation of ascorbic acid has
been shown to enhance the immune response of the calves (7). The suggested levels of ascorbic
acid and all other pertinent vitamins in milk replacers are shown in Table 5. Vitamin
recommendations for dairy cows are shown in Table 6.
The most rumen stable form of ascorbic acid found to date is ascorbyl-2-polyphosphate. Ascorbic
acid in the polyphosphate form now makes it possible to conduct large scale production studies to
determine what effect the vitamin will have on the productivity and profitability of dairy herds.
It can be generally recommended that milk replacers should contain supplemental ascorbic
Applied Dairy Science Course - University of Alberta:
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