Oxidation increases as the need for energy increases, like during exercise and pregnancy. As oxidation increases, so does the production of ROS, including free radicals, which can damage vital tissues in your horse.This article explores the different antioxidants that affect the horse and the role that they play to protect the horse from oxidative stress.
Carey A. Williams, Ph.D., Equine Extension Specialist; Lesleyann E. Atherly, Rutgers University, Cook College, Animal Science Research Student; Jessica D. Hirsch, Rutgers University, Cook College, Animal Science Research Student
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Oxidation and Oxidative Stress
Oxidation is defined as one of the processes by
which energy is obtained from the diet. During this
process, nutrients are broken down and converted
into energy for normal metabolic function. The
browning of an apple or rusting of metal is a
common example of oxidation in our everyday life.
The rate of oxidation depends on the amount of
activity that is occurring. At rest, the rate of
oxidation is at its lowest level. However, during
stress, exercise, growth, pregnancy, or lactation, the
rate of oxidation is elevated because the body is
rapidly breaking down nutrients (like protein,
carbohydrates, and fat) to produce energy needed
during these times. During these metabolic
processes if the regulatory systems in the body are
overwhelmed, oxidative stress can occur.
Oxidative stress causes an overabundance of
circulating molecules called reactive oxygen species
(ROS), sometimes commonly referred to as “free
radicals.” It is important to note that all free radicals
are ROS, but not all ROS are free radicals. These
free radicals are normally produced from oxygen
metabolism (see equation below).
When we inhale, we take in oxygen (O2), in our
cells the oxygen uses an electron (e-) and a
hydrogen (H+) to form water (H2O) and carbon
dioxide (CO2), then the carbon dioxide gets
exhaled. The free radicals shown in the equation
include superoxide radical (O2•), hydroxyl radical
(HO•), and hydrogen peroxide (H2O2). They
contain oxygen, but they are much more reactive
than the oxygen in the air we breathe. Free radicals
are ROS with a missing electron, therefore, making
them unstable molecules. They circulate
throughout the body searching for electrons, hoping
to achieve stability.
Are these free radicals or ROS beneficial or
detrimental to your horse? In reasonable amounts
they’re necessary. They are needed for proper
function of the immune system, as they aid the
destruction of invading foreign organisms. On the
other hand, larger amounts of circulating free
radicals are harmful. They can cause tissue
damage and cell death by destroying cell proteins,
DNA, and fatty acids. An excess of ROS leads to
fatigue and damage of vital tissues such as muscle,
nervous tissue, and skin. This can lead to illness
due to a decrease in immune function, lameness
due to destruction of muscle tissue, and other
nervous system related problems.
The Role of Antioxidants
Thankfully, there is a way to combat serious
damage from these ROS. Antioxidants such as
vitamin E, vitamin C, glutathione, and selenium, to
name a few, all have protective action against this
damage.
Antioxidants protect your horse from ROS by (see
Figure 1):
- Scavenging them or rendering them inactive (see Figure 2)
- Inhibiting excess ROS production
- Promoting repair of damaged tissues and cells
Figure 1. Illustration of a hydroxyl radical (•OH) getting scavenged or converted to water (H20) by vitamin E.
Figure 2. Illustration of how an antioxidant can scavenge a free radical or render them inactive.
The various antioxidants work together to achieve
all of this and more. So where do they come from?
Some antioxidants come from your horse’s diet,
and some are synthesized in the body. Therefore,
it is crucial that your horse is healthy, and has a
balanced diet that provides nutrients, including
essential vitamins and minerals. Let’s take a closer
look at some common antioxidants.
Vitamin E
Vitamin E is the most important antioxidant.
Vitamin E is a fat-soluble vitamin, and it protects cell
membranes from damage by free radicals. Cell
membranes are comprised of lipid molecules.
These lipid molecules are very reactive with ROS,
making cell membranes highly susceptible to ROS
damage.
Vitamin E is essential in your horse’s diet:
- Deficiency can cause uncoordination and various muscle and nervous disorders. Without vitamin E the membranes of these cells become permeable and eventually are
destroyed, thus destroying the cell.
- Vitamin E has been proven to protect against equine protozoal myeloencephalitis (EPM), equine degenerative myelonen- cephalopathy (EDM), and tying up in exercising horses.
- It is especially important for exercising horses, as exercise can:
- induce oxidative stress and ROS damage, as well as decrease circulating vitamin E levels,
- in turn cause muscle problems and an overall decrease in endurance capacity and performance.
- Supplementing vitamin E before stressing the horse (e.g. traveling, competition) could potentially be beneficial to your horse by increasing immune function and protecting muscle and nervous cells (see fact sheet FS656, “Are you ‘Stressing Out’ Your Horse?” for more on stress).
Where can sources of vitamin E be found?
- Many forages and pasture grasses are high in vitamin E
- However, vitamin E content will decrease with the age of the plant, as well as with processing (heat, bailing, grinding, etc.).
- Sun-curing hay also decreases the content of vitamin E in the forage.
- Because vitamin E is fat soluble, it needs to be provided with fat in the diet so it can be absorbed and utilized. So if supplementing with extra vitamin E (i.e. 2,000 to 5,000 IU/day) you may want to make sure you are feeding a commercially available feed with added fat (i.e. 10 %) or a separate fat source (i.e. oil, or rice bran).
- Vitamin E supplements in high amounts are not toxic to horses. However, large doses (> 5,000 IU/day) should be carefully monitored as they may interact with other nutrients in the diet, like beta-carotene.
Vitamin C
Another important antioxidant is vitamin C. This is a
water-soluble vitamin, so it isn’t localized to the cell
membrane like vitamin E. Vitamin C in its active
form, ascorbic acid, is usually found inside and/or
outside of cells, and confronts any free radicals it
encounters in these places. It does this by
quenching and stabilizing the free radicals,
preventing future damage inside of the horse.
Ascorbic acid can also aid in regeneration of the
vitamin E radical, restoring its antioxidant capacity
(see Figure 1). Together, vitamin C and E work
together to protect vital tissues of your horse.
When is vitamin C needed?
- Horses in good health can synthesize vitamin C on their own; so there is no need to worry about a deficiency or providing it in the diet. The horse’s liver produces as much as the maintenance horse needs to stay healthy.
- However, when a horse is stressed (e.g. geriatric, intensely exercising, long hauls), production of vitamin C can’t keep up with its demand.
- Supplementation can help decrease the detrimental effects of the stress on the immune system. The usual recommendation is 7 to 10 grams of ascorbic acid a day during the short term around stressful situations (see fact sheet FS656, “Are you ‘Stressing Out’ Your Horse?” for more on stress).
- Remember vitamin C is water soluble so if you are supplementing too much it will be wasted, and the liver will slow down production due to the decreased need for the vitamin.
Selenium
Selenium is a trace mineral found in plants. By
itself, selenium does not have much antioxidant
capacity. When selenium pairs up with vitamin E, it
becomes a strong antioxidant.
Selenium with vitamin E will:
- Stop nerve cell damage caused by free radicals, therefore preventing nervous disorders that are caused by nerve damage and degeneration (i.e. Equine Motor Neuron Disease).
- Help in preventing muscle problems in horses (i.e. Tying-up or White Muscle Disease).
- Work by sitting on the surface of cells and scavenging ROS that pass by.
- They also counterbalance each other, so if availability of selenium is low, vitamin E picks up the slack, and vice versa.
- Selenium is also an integral component of the antioxidant enzyme glutathione peroxidase (see below).
Sources of selenium:
- Soil selenium levels vary in different regions. Many hays harvested in the alkaline soils of the rocky mountain region, for example, may be very high in selenium.
- Because of this variation, it is extremely important that you know the selenium status of your region and the region where your hay is harvested so toxicity does not occur.
- Most commercial feeds have additional selenium already in them so, adding additional is not necessary.
Selenium intake should always be monitored, and
never supplemented if your horse is receiving
adequate amounts. Horses require 0.3 mg per kg
diet (about 3 mg/day). If the horse receives too
much, it can cause selenium toxicity. The earliest
signs of chronic selenium toxicity are loss of mane
and tail hairs and cracks going around the
circumference of the hoof that can actually cause
the hoof wall to slough off. This can be caused by
intakes of only 10 mg/day. Acute toxicity, due to
sudden high level intakes, is also called “Blind
Staggers”. Signs include apparent blindness,
changes in behavior, anorexia, excessive salivation,
increased heart and respiration rates due to
necrosis of heart and liver tissues. Toxicity is
enhanced if the rations also contain high amounts
of copper, as many commercial feeds now do.
Glutathione
Glutathione has antioxidant properties which
include reactivating vitamin C and vitamin E
metabolites that have been oxidized by free
radicals. Glutathione depletion in cells suppresses
immune response of white blood cells; it prevents
lymphocytes entering their normal life cycle and
inhibits antibody activity. Glutathione is commonly
included in many ‘immune boosting’ supplements
on the market. Its supplementation may enhance
antibody activity in immune cells, but has not been
documented to be beneficial in horses specifically.
Antioxidant Enzymes
Besides vitamins and minerals, other types of
antioxidants exist in the form of enzymes.
Superoxide dismutase, catalase, glutathione
peroxidase and glutathione reductase all serve as
free radical quenchers by providing them with
electrons. These enzymes also work within cells,
rather than on the surface like vitamin E and
selenium. These enzymes also have a universal
nature, as they can be found in many tissues,
including liver, muscle, and brain.
- Superoxide dismutase is found in a multitude of organs; the highest activity of which is in the liver, followed by the kidney, brain, heart, and muscle.
- Glutathione peroxidase is primarily found in the red and white blood cells of mammals, which helps prevent oxidation of cell membranes by consuming free radicals in the cell. Selenium is important to the structural integrity of glutathione peroxidase, and without adequate selenium, its activity is severely handicapped.
- Glutathione reductase is essential for glutathione peroxidase to effectively stabilize free radicals and protect tissues from damage. It then reduces the oxidized glutathione to complete the cycle.
- Minerals are also a structural component of catalase, the main one being iron. An iron deficiency does slow the performance of catalase. The catalase enzyme is found mostly in liver and muscle.
What Does This Mean for Your Horse?
Exercise
Any stressful condition in horses, including exercise,
involves an adjustment of the antioxidants in the
body to take care of the ROS produced by the
increase in oxygen consumption. Horses that are
especially traveling long distances and competing in
several shows, races, or events in a short period of
time are more prone to deficiencies in antioxidant
status. This makes it even more important that the
horse is on a good balanced diet with plenty of
fresh green forage in the form of pasture grass or
good quality hay. If necessary, an antioxidant
supplement may be required; this is especially true
if the horse is on limited pasture turnout.
Pregnancy
Besides through exercise, oxidative stress can be
induced by pregnancy. A growing fetus can exert
an enormous amount of stress on the dam, as her
body is trying to produce enough energy for herself
as well as for her developing foal. Antioxidant
activity can usually keep up with the demand for
energy, however, during the final weeks of
pregnancy before foaling, fetal development peaks.
During this time, it has been shown levels of
antioxidants fluctuate, so it is important to keep
supplying the pregnant mare with adequate
amounts of vitamin E, selenium, and other essential
minerals. This should be in the form of good
quality forage, pasture preferably.
Aging
As horses age, metabolic function slows and is less
efficient. Efficiency of organ function also
decreases. This increases susceptibility to oxidative
stress and damage, thus worsening organ and
tissue function. Supplementation of antioxidants is
extremely important for an aging horse in order to
decrease their susceptibility to oxidative damage.
Older exercising horses need more antioxidants as
well, because exercise can intensify their
vulnerability to ROS damage. Vitamin E and C are
possibly needed as supplements to a geriatric
horses’ diet.
Illness
Horses in a diseased state are also vulnerable to
oxidative stress. Although free radicals to some
extent do aid in fighting sickness, the increased
levels still need to be monitored. Sickness may
also decrease food intake and absorption in the
intestines. Vitamin deficiencies can occur, which
can make an existing problem worse, so additional
antioxidant supplementation may be necessary to
fortify the normal diet in these ill horses.
Equine motor neuron disease (EMND) is a
neurodegenerative disorder in the adult horse.
There is a significant association between EMND
and vitamin E status; lower plasma levels of alphatocopherol
are found in diseased horses than in
control horses. This hypothesis of vitamin E
deficiency has been replaced with the newer theory
that vitamin E is low due to its increased utilization
of scavenging the ROS that are damaging the
affected nerves.
Chronic rheumatic disease and degenerative bone
and joint diseases are linked to excessive ROS
production. The ROS are also capable of degrading
components of the joint and this has been
implicated in the pathogenesis of equine joint
disease.
The Bottom Line
The main point to be concerned about is that
oxidation increases as the need for energy
increases, like during exercise and pregnancy. As
oxidation increases, so does the production of ROS,
including free radicals, which can damage vital
tissues in your horse. Horses do have internal
mechanisms to keep up with the increased
production of ROS, such as vitamin C synthesis and
antioxidant enzymes, but these internal
mechanisms may not be sufficient when ROS
levels rise. The best way to prevent serious
damage is to keep your horse healthy with a
balanced diet with the essential vitamins and
minerals, but avoiding oversupplementation.
Supplemental Reading
- Chew, B.P. 1996. Importance of antioxidant vitamins in immunity and health in animals. An. Feed Sci. Tech. 59:103-114.
- Clarkson, P. and Thompson, H. 2000. Antioxidants: What role do they play in physical activity and health? Am. J. of Clin. Nutr. 72 (supp.): 637S-46S.
- Thomas, Heather S. 2004. The Role of Antioxidants. The Horse Magazine.
- Williams, C.A. 2005. Are you ‘Stressing Out’ your horse? Rutgers Cooperative Extension. FS656.
