Confusion About Carbs
Lori K. Warren, PhD, PAS
Dept. ofAnimal Sciences, University ofFlorida
Tips for feeding
1) Slowly increase the amount of
grain you offer (allow 3 days of
adjustment for every 1 pound
2) Feed no more than 5 pounds of
grain (for a 1000-lb horse) at a
single feeding. Smaller, more
frequent meals are always best.
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 2 weeks to
adjust to a new brand offered
(gradually blend the new feed in
with the old feed).
5) Cut back on the grain ifyour
horse's level of activity changes,
either due to a lay-up or a
reduction in training.
UF UNIVERSITY of
Whether it's our diet, or that of our horses, we seem to be obsessed
with carbohydrates lately. Low carb feeds, reduced starch feeds,
and "controlled energy" products have flooded feed stores, making
the choice of what to feed your horse more complicated than ever.
Are carbohydrates really that bad? It depends on which carbo-
hydrate you're talking about. It also depends on how much is fed
and to what type of horse.
Some carbohydrates have been linked with digestive disturbances
leading to colic and laminitis. Certain carbohydrates can also
exacerbate diseases, such as insulin resistance, Cushing's disease,
and polysaccharide storage myopathy (PSSM). Most horses,
however, do not experience problems with reasonable levels of
starch and sugar in their ration (see Tips at left). 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 carbo-
hydrates in feeds and how they are processed in the horse's
digestive system. In addition, key carbohydrates that can impact
horses with digestive or metabolic conditions will be discussed.
Sources of Carbs
As an herbivore, the horse is built to process high fiber roughages.
Although fiber is often discounted as "fill," it 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 gut health and motility. Therefore, fiber is one carbo-
hydrate 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.
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 (see Figure 1). 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.
When starch enters the large intestine-regardless
if it originates from hay, oats or corn-it is
rapidly fermented by microorganisms to lactic
acid (Figure 1). 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,
Figure 1: Types of High in molasses, grail
Some in forages
carbohydrates and their
digestion and absorption Starch
in the horse. High in grains
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.
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.
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 pro-
duced 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 fluctua-
tions in the glycemic response should be avoided
in horses with carbohydrate-sensitive metabolic
conditions, such as insulin resistance, Cushing's
disease and PSSM.
Fermented to Lactic Acid
*Excess lactic acid can
lead to digestive upset
S Fermented to
Volatile Fatty Acids
*Important energy source
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 (Figure 1).
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 insoluble and more slowly
fermented. Other fibers are soluble and are more
rapidly broken down by microbes (Figure 1).
Of the fibers that are more rapidly broken down,
some are good while others put the horse at risk
for digestive disturbances. More 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) (Figure 1).
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 (Figure 1). 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.
Measuring Carbohydrates in Feeds
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 carbo-
hydrates" (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 in a feed and adding this
value to the water soluble carbohydrate content of
the feed. Water soluble carbohydrates (WSC) are
determined by soaking the feed and measuring
the carbohydrates that become suspended in
water. Water soluble carbohydrates include
sugars, but they also include fructans. These car-
bohydrates are digested in different parts of the
digestive tract (sugars in the small intestine,
fructans in the cecum and colon); therefore, they
can affect a horse's blood sugar and gastro-
intestinal health differently.
For example, say you are comparing two different
hays. One has 10% sugar/starch and 5% fructan.
The other has 5% sugar/starch and 10% fructan.
Although they both have the same NSC (15%),
they are not the same hay. The hay with 10%
fructan could be more of a problem with a lami-
nitic 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.
Table 1: Carbohydrate Fractions in Common Feeds Fed to Horses
Feed % S Sac
Corn, steam flaked
8 6 26 33
Timothy, orchard hays 11 8 2.5 13
Bermudagrass hay 7.5 8 6 13
Pasture bahiagrass 5 5 1 6
Beet pulp (molassed) 10 11 1.5 12
Soybean hulls 4 1.5 2 6
Because of the limitations of NSC, most
nutritionists are moving away from this value and
are instead evaluating feeds based on the starch,
WSC and ethanol soluble carbohydrate contents.
Ethanol soluble carbohydrates (ESC) are a subset
of WSC that do not include fructans. Fructan
content can be estimated by subtracting ESC from
WSC. Ultimately, these three fractions can be
used to compare feeds for horses with different
carbohydrate-related needs (Table 1).
* For horses with insulin resistance, Cushing's
disease, or PSSM-all of which can't tolerate
large swings in blood sugar-you should select
feeds that are low in starch and ESC. This
implies there will be less carbohydrate
digested and absorbed from the small intestine,
which produces a lower glycemic response.
* For horses with a history of diet-related
laminitis, you should select feeds that are low
in starch and WSC. The less starch and
fructans (a component of WSC) 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 types of carbo-
hydrates and where they are digested along the
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 laminitis, insulin
resistance, PSSM, 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.
Also, it is likely that sensitivity to different
carbohydrates varies with the individual horse.
Starch should be limited in the diet of
horses with a history of laminitis or those
diagnosed with insulin resistance, Cushing's
disease and PSSM. 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 that 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 carbo-
hydrate 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 other sports 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.
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. Refer to the "Tips
for Feeding Starch Safely" at the beginning
of this article.