NUTRITION
tase (SAD), catalase and glutathione peroxidase, and
the oxidant consumers which include Vitamin C, Vi-
tamin E, glutathione and beta-carotene. The function
of antioxidant systems is not to completely remove
oxidants, but rather to keep them at an optimum level
as they do have some positive functions. of administration, and route of administration may
vary considerably across species. Selection of specific
compounds may depend on bioavailability and sup-
plementation may, or may not, increase absorption
into tissues. Many substances have species or meal
variation differences in absorption.
Antioxidants are classified into two broad divisions, de-
pending on whether they are soluble in water (hydro-
philic) or in lipids (lipophilic). In general, water-soluble
antioxidants react with oxidants in the cell cytosol and
the blood plasma, while lipid-soluble antioxidants pro-
tect cell membranes from lipid peroxidation. (Table 2) Vitamin C
Vitamin C (ascorbic acid), a redox catalyst, is main-
tained in its reduced form by reaction with glutathione
and can reduce, and thereby neutralise, reactive oxy-
gen species such as hydrogen peroxide. Vitamin C is
found in high concentrations in fruits. It can be syn-
thesised in the canine liver but cannot be stored. Its
production is decreased in chronic liver disease. Most
commercial pet foods contain sufficient amounts of
vitamin C and supplementation should only be re-
quired for malabsorption syndromes (EPI) and where
extra antioxidant activity is required (cognitive dys-
function).
Some compounds contribute to antioxidant defence
by chelating metals and preventing them from cat-
alysing the production of free radicals in the cell.
The ability to sequester iron, which is the function
of iron-binding proteins such as transferrin and ferritin
is especially important. The trace elements seleni-
um and zinc are commonly referred to as antioxidant
nutrients, but these chemical elements have no anti-
oxidant action themselves and are instead required for
the activity of some antioxidant enzymes.
Although certain levels of antioxidant vitamins in the
diet are required for good health, there is considerable
debate on whether antioxidant rich foods or supple-
ments have anti-disease activity. Moreover, if they are
actually beneficial, it is unknown which antioxidant(s) /
combinations thereof are needed from the diet and in
what amounts beyond typical dietary intake.
In order to critically evaluate the therapeutic bene-
fit for antioxidants (and other nutraceuticals) for the
aging process is to consider the outcome measures
used to determine efficacy. A variety of new labora-
tory methods have been developed to try to measure
the effects of ROS in biologic systems and the results
of an intervention. Markers are specific for different
biomolecules such as DNA (8-oxodeoxyguanosine),
lipids (alkenals, MDA, TBARS), prostaglandins (isopros-
tanes) protein (nitrotyrosine, protein carbonyls), and
advanced glycation endproducts (AGE). Some asso-
ciations have been made between reduction of these
markers of oxidative damage and improved health
outcomes.
ANTIOXIDANT SUBSTANCES:
The supplementation and efficacy of antioxidants in
animals cannot be extrapolated from human studies.
The selection of compounds, dosage range, length
Table 2: Distribution of antioxidants in the cell
Antioxidant metabolite
Ascorbic acid (vitamin C)
Glutathione
Lipoic acid
Uric acid
Carotenes
α-Tocopherol (vitamin E)
Ubiquinol (coenzyme Q10)
Solubility
Water
Water
Water
Water
Lipid
Lipid
Lipid
Vitamin E
Vitamin E is a fat soluble vitamin found in high con-
centrations in nuts and oils. Vitamin E is the collective
name for a set of eight related tocopherols and tocot-
rienols, which are fat soluble vitamins.
Alpha-tocopherol is the most widespread form of vi-
tamin E in animal foods and organisms. It is the form
with the greatest bioavailability and biological activity,
with the body preferentially absorbing and metabolis-
ing this form. It protects membranes from oxidation
by reacting with lipid radicals produced in the lipid
peroxidation chain reaction, thus removing the free
radical intermediates and preventing the propagation
reaction. The oxidized α-tocopheroxyl radicals pro-
duced that can be recycled back to the active reduced
form through reduction by other antioxidants, such as
ascorbate, retinol or ubiquinol (Co-enzyme Q10).
Flavonoids
Flavonols are antioxidants found in plants and are the
most important plant pigments for flower colouration,
producing yellow or red/blue pigmentation. Animals
ingest significant quantities in their diet. Foods with a
high flavonoid content include parsley, onions, blue-
berries and other berries, black tea, green tea and oo-
long tea, bananas, all citrus fruits, Ginkgo biloba, red
wine, sea-buckthorns, buckwheat, and dark choco-
late (with a cocoa content of 70% or greater).
Studies indicate that flavonoids may affect anti-in-
flammatory mechanisms due to their ability to inhib-
it ROS or nitrogen compounds. Flavonoids have also
been proposed to inhibit the pro-inflammatory activity
of enzymes involved in free radical production, such
as cyclo-oxygenase, lipo-oxygenase or inducible ni-
tric oxide synthase.
Beta-carotenoids
Carotenoids are organic pigments which are pro-
Issue 06 | DECEMBER 2017 | 41