Confusion About Carbs
Nutrtionists
find new ways to evaluate sugars and starches in the equine
diet.
By Lori
K. Warren, PhD, PAS, Equine Nutritionist
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Everybody seems to be obsessed with carbohydrates lately.
As evidence, an estimated 1 in 5 Americans have tried either
the South Beach or Atkins diet. A trip to your local grocery
store reveals shelves full of low-carb or no-carb products
ranging from bread and cookies to salad dressing and beer.
Even fast food restaurants are removing the buns from their
burgers and offering low-carb alternatives to the French fries
in your combo meal.
We
seem to have the same obsession with carbohydrates in the
equine diet. Low starch feeds, reduced starch feeds, and “controlled”
starch products have flooded feed stores, making the choice
of what to feed your horse more complicated than ever. |
Horses
with insulin resistance, Cushing’s disease or EPSM should
be fed a diet low in starch and ESC (ethanol soluble carbohydrate).
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Why the emphasis on carbohydrates lately? Some types of carbohydrates
have been linked with digestive disturbances leading to colic
and laminitis. Certain carbohydrates can also exacerbate health
problems, such as insulin resistance, Cushing’s disease
and polysaccharide storage myopathy (EPSM). Most horses, however,
do not experience problems with reasonable levels of starch
and sugar in their ration. And, some horses, particularly
those that compete in high intensity activities, need starch
in their diet to perform well.
To
help you better understand the role of carbohydrates in the
equine diet, this article will describe the different types
of carbohydrates in feeds and how they are processed in the
horse’s digestive system. In addition, evaluation of
key carbohydrate fractions for horses with digestive or metabolic
conditions will be discussed.
Sources
of Carbs |
As
an herbivore, the digestive tract of the horse is built to process
high fiber roughages. Although it is often discounted as “fill,”
fiber is an important carbohydrate. Forages, including pasture
and hay, provide fiber that serves as a key source of energy
for the horse. Fiber is also needed to maintain the horse’s
gut health and motility. Therefore, fiber is one carbohydrate
no horse can do without.
Another
major class of carbohydrates is sugar. The term “sugar”
usually refers to monosaccharides, such as glucose, as well
as disaccharides, such as sucrose (a glucose unit hooked to
a fructose—best known to us as table sugar). Sources of
sugar in the horse’s diet include molasses and cereal
grains, such as oats and corn; but they can also be present
in forages, particularly in lush spring and early summer pasture.
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Inadequate starch in the diet can have a negative effect on
performance. |
Starches are also carbohydrates. Starches are made up of very
long chains of individual glucose units. Cereal grains are the
richest source of starch in the horse’s diet. Legumes
such as alfalfa and perennial peanut, as well as warm season
grasses, such as bahia and Coastal or Tifton-85 bermudagrass
also contain some starch.
How the Horse Processes Carbs
Sugars
and starches are digested by enzymes in the horse’s small
intestine. Because they are small, sugars are rapidly digested
and absorbed in the small intestine. By comparison, digestion
of starches is not quite as simple. The starch contained in
oats, for example, is more available for enzymatic digestion
in the small intestine than the starch found in corn and barley.
Some of the starch in corn and barley may escape digestion in
the small intestine and be transported to the large intestine.
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When
starch enters the large intestine—regardless if it originates
from hay, oats or corn—it is rapidly fermented by microorganisms
to lactic acid. If significant quantities of starch are fermented
in the hindgut, the lactic acid can wreak havoc, resulting in
colic and laminitis.
The
amount of starch bypassing the small intestine and reaching
the hindgut can also be affected by meal size. Grain meals larger
than 0.5% of the horse’s body weight (e.g., 5 pounds for
a 1000-pound horse) can exceed the small intestine’s ability
to process the starch contained in the grains. Such large meals
result in more starch flowing back into the large intestine
where it can be rapidly fermented to lactic acid.
Processing of cereal grains can also influence starch digestion.
Cracking, flaking or rolling oats, corn and barley disrupts
the tougher outer shell of the grain, exposing more of the starch
to enzymes in the small intestine. In addition, the heat generated
during the pelleting and extruding of some feeds will gelatinize
the starch, making it easier for the horse to digest in the
small intestine and allowing less to escape into the hindgut.
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Equine
nutritionists are moving away from NSC (non structural carbohydrate)
values, and instead are evaluating feeds based on the starch,
WSC (water soluble carbohydrate) and ESC (ethanol soluble
carbohydrate) contents.
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When
sugars and starches are digested in the small intestine, the
resulting glucose and fructose are absorbed into the bloodstream.
These sugars can be used as an immediate energy source, or
repackaged and stored as muscle and liver glycogen or fat
for use at a later time. The hormone insulin, which is produced
by the pancreas in response to elevated blood glucose, drives
this process.
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The cycle of increased blood glucose, followed by an increase
in insulin production is referred to as the glycemic response.
Feeds that are more rapidly digested and absorbed as glucose
produce a greater glycemic response. Insulin is not produced
in response to the absorption of volatile fatty acids from fiber
digestion. Therefore, feeds that contain a higher level of sugar
and starch yield a higher glycemic response than those that
contain larger quantities of fiber. Large fluctuations in the
glycemic response should be avoided in horses with carbohydrate-sensitive
metabolic conditions, such as insulin resistance, Cushing’s
disease and EPSM.
In contrast to sugars and starches, fiber cannot be digested
directly by the horse. Instead, billions of bacteria and fungi
that reside in the horse’s large intestine digest the
fiber for the horse. In return, the microbes produce volatile
fatty acids that can be absorbed by the horse and used as a
source of energy.
However, just like starches, not all fiber is the same. Some
fibers, including hemicellulose and cellulose, are more slowly
fermented, whereas other fibers are more rapidly broken down
by microbes. Of the fibers that are more rapidly broken down,
some are good while others put the horse at risk for digestive
disturbances. Specifically, pectins contained in beet pulp,
soy hulls and alfalfa are good fibers that are converted by
microbes into volatile fatty acids (similarly to slowly fermented
fibers, just at a more rapid pace). In
contrast, fructans present in cool season grasses, such as
timothy, orchardgrass and fescue, are fibers that are rapidly
fermented to lactic acid by microbes in the large intestine.
The amount of lactic acid produced is proportional to the
fructan content. In most cases, fructan levels are not significant
enough to cause problems in healthy horses. However, when
these grasses are rapidly growing or stressed by drought or
frost, fructan levels can accumulate and may cause laminitis
in susceptible horses. It is worth noting that warm-season
grasses, including bahia and Coastal and Tifton-85 bermudagrasses,
store carbohydrates as starch, not fructans.
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Some
Horses NEED Starch |
Starch should be limited in the diet of horses with
a history of laminitis or those diagnosed with insulin
resistance, Cushing’s disease and polysaccharide
storage myopathy (EPSM). But for the vast majority of
the horse population, starch is not an enemy. In fact,
some horses will truly benefit from having starch in
their diet.
Glycogen
stored in the liver and muscle is an important source
of energy for muscles during exercise. Research has
shown that horses who begin a bout of exercise with
low to moderate levels of glycogen will fatigue easier
than those who start with a full measure of glycogen.
A horse that tires more quickly can lose a race, nick
a rail going over a jump, or take a misstep and strain
a tendon.
Glycogen
itself is a carbohydrate and can only be made from carbohydrates
provided in the diet. The best source of dietary carbohydrate
to make glycogen comes from the starch in grains like
oats, corn and barley.
Because
glycogen is an important energy reserve, horses that
compete in activities that rely heavily on glycogen
need starch to replenish their glycogen stores after
exercise. Such horses include racehorses, horses used
for timed events like barrel racing and roping, three-day
event horses and every other sport where bursts of high
speed or power are required. In addition, horses that
are ridden fairly heavily and frequently, such as a
heavily campaigned jumper or a reliable lesson horse
may also benefit from being fed a traditional grain
mix higher in starch.
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Measuring
the Different Types of Carbohydrates
Carbohydrates are a very complex class of chemicals, making
it difficult to separate and measure each specific carbohydrate
fraction in the laboratory. Instead, we have to rely on values
found by subtracting one measurable fraction of carbohydrate
from another, as well as our knowledge of feeds and the digestive
process.
Historically, the term “non-structural carbohydrates”
(or NSC) has been used to describe the starch and sugar content
of feeds. Although it varies by laboratory, NSC is determined
by measuring the starch content of feeds and adding this value
to the water soluble carbohydrate content of feeds. Water
soluble carbohydrates are determined by soaking the feed and
measuring the carbohydrates that become suspended in cool
water. Water soluble carbohydrates include sugars, but they
also include fructans. Because sugars and fructans are digested
in different parts of the digestive tract (sugars in the small
intestine, fructans in the large intestine), they can affect
a horse’s blood sugar and gastrointestinal health differently.
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For example, say you are comparing two different timothy hays.
One hay has 10% sugar/starch and 5% fructan, with an NSC value
of 15%. The second hay has 5% sugar/starch and 10% fructan,
also yielding an NSC value of 15%. They are not the same hay,
even though they have the same NSC value. The hay with 10%
fructan could be more of a problem with a laminitic horse.
In contrast, the hay with 10% sugar/starch would be more of
a concern for an insulin-resistant horse, because sugar and
starch cause a greater glycemic response than fibers like
fructan.
Because
of the limitations of NSC, most nutritionists are moving away
from this value and are instead evaluating feeds based on
the starch, water soluble carbohydrate and ethanol soluble
carbohydrate contents. Ethanol soluble carbohydrates are a
subset of water soluble carbohydrates that do not include
fructans. Fructan content can be estimated by subtracting
ethanol soluble carbohydrates from water soluble carbohydrates.
Ultimately, these three fractions can be used to compare feeds
for horses with different carbohydrate-related problems.
For
horses with insulin resistance, Cushing’s disease, or
EPSM—all of which can’t tolerate large swings
in blood sugar—you should select feeds that are low
in starch and ethanol soluble carbohydrate. This implies there
will be less carbohydrate digested and absorbed from the small
intestine, which produces a lower glycemic response.
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Tips
for Feeding Starch Safely |
If
you have a performance horse or just simply prefer to
feed a traditional grain mix or sweet feed, the safety
of feeding the relatively high level of starch in these
products can be improved in several ways:
1) Slowly increase the amount of grain you offer (allow
3 days of adjustment for every pound you add);
2) Feed no more than 5 pounds of grain (for a 1000-lb
horse) at a single feeding. Smaller, more frequent meals
are always better;
3) Select products that have a higher level of oats than
corn or products that include processed grains, like cracked
corn, steam flaked corn or rolled barley;
4) Give your horse two weeks to adjust to a new type of
feed (gradually blend the new feed in with the old feed);
5) Cut back on the grain if your horse’s level of
activity changes, either due to a lay up or a reduction
in training. |
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For
horses with a history of diet-related laminitis, you should
select feeds that are low in starch and water soluble carbohydrate.
The less starch and fructans (a component of water soluble carbohydrate)
reaching the hindgut should mean less opportunity for excessive
fermentation and large intestinal disturbances that could trigger
laminitis.
Conclusions
About Carbohydrates
An understanding of the different classes of carbohydrates and
where they are digested along the horse’s digestive tract
can be extremely useful for making decisions on what feeds to
select for a particular horse. This information is even more
essential when managing a horse with a history of laminitis
or one that has been diagnosed with EPSM, insulin resistance,
or Cushing’s disease.
Unfortunately,
at the present time there is no satisfactory, commercially
available analytical method to segment carbohydrates into
categories that are physiologically meaningful to the horse.
Furthermore, although we know some horses are more sensitive
to some carbohydrates, defining what is meant by a “low”
level of starch or “low” level of water or ethanol
soluble carbohydrate is difficult. This is because the amount
of starch, sugar and fructans required (or tolerated) by horses
with carbohydrate-related digestive and metabolic problems
has not been researched. Furthermore, it is likely that sensitivity
to the different carbohydrate fractions varies with the individual
horse.
Ultimately,
if you have questions or concerns about the level of starch,
sugar or fructans in the feeds and forages you provide your
horse, particularly if the horse has been compromised by a
carbohydrate-related digestive or metabolic condition, consult
with an equine nutritionist or your local feed representative.
Carbohydrate
Fractions in Common Feeds Fed to Horses |
Feed |
Water
Soluble Carbohydrate (%) |
Ethanol
Soluble Carbohydrate (%) |
Starch
(%) |
NonStructural
Carbohydrate (%) |
| Molasses |
53.3 |
52.1 |
1.0 |
59.7 |
| Oats |
3.2 |
3.0 |
44.3 |
48.7 |
| Corn,
Steam Flaked |
1.9 |
2.5 |
72.6 |
73.8 |
| Barley |
7.1 |
2.1 |
54.6 |
59.5 |
| Rice
Bran |
10.1 |
9.5 |
19.4 |
29.5 |
| Wheat
midds |
7.9 |
6.1 |
26.5 |
32.9 |
| Legume
Hay |
9.0 |
7.3 |
2.1 |
11.0 |
| Cool-Season
Grass Hay (Timothy, Orchard, etc) |
10.7 |
7.8 |
2.4 |
12.9 |
Bermuda
Grass Hay
(Coastal, Tifton) |
7.5 |
8.4 |
5.8 |
13.2 |
| Fresh
Cool-season grass pasture |
10.4 |
4.4 |
2.6 |
13.1 |
| Fresh
Bahiagrass pasture |
5.3 |
4.8 |
0.3 |
5.6 |
| Beet
Pulp |
10.2 |
10.6 |
1.4 |
11.8 |
| Soybean
hulls |
3.6 |
1.5 |
1.7 |
5.1 |
| Soybean
meal |
13.1 |
11.8 |
2.0 |
15.1 |
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Dr.
Lori Warren is an assistant professor of
Equine Nutrition at the University of Florida,
Institute of Food and Animal Agriculture. |
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