The effect of added dietary soybean oil on vitamin E status of the horse.
ABSTRACT Fourteen 2-yr-old Quarter Horses and Quarter Horse x Thoroughbreds were randomly assigned to either a control (CTRL) diet or a diet supplemented with 6.4% soybean oil (SBO). The amounts of both diets that were fed met current NRC nutrient requirements and were isoenergetic with similar nutrient:energy ratios. Venous blood samples (20 mL) were taken at d 0, 30, 60, and 90 of the experiment and analyzed for serum alpha-tocopherol, serum cholesterol, and serum triglyceride. The sum of serum cholesterol and serum triglyceride values was used as an estimate of serum total lipid. The ratio of serum alpha-tocopherol:serum total lipid was used as a measure of vitamin E status of horses. The SBO group had higher serum cholesterol concentrations (P < .001) at 30, 60, and 90 d. Serum triglyceride was not affected by diet. Serum total lipids tended to be greater (P < .06) for the SBO group. Serum alpha-tocopherol was greater (P < .001) at 90 d in the SBO group. There was a significant correlation (P < .001, r = .62) between serum total lipids and serum alpha-tocopherol concentrations in the SBO group. This correlation was not significant (P < .07, r = .35) in the CTRL group. The ratio of serum alpha-tocopherol:serum total lipid was not affected by diet. The results of this experiment suggest that vitamin E status in non-exercising, 2-yr-old horses fed currently recommended levels of vitamin E was not affected by the addition of 6.4% SBO to the diet.
P. D. Siciliano and C. H. Wood
The effect of added dietary soybean oil on vitamin E status of the horse
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The Effect of Added Dietary Soybean Oil on
Vitamin E Status of the Horse
P. D. Siciliano and C. H. Wood'
Department of Animal Science, University of Kentucky, Lexington 40546
ABSTRACT: Fourteen 2-yr-old Quarter Horses and
Quarter Horse x Thoroughbreds were randomly as-
signed to either a control (CTRL) diet or a diet
supplemented with 6.4% soybean oil (SBO). The
amounts of both diets that were fed met current NRC
nutrient requirements and were isoenergetic with
similar nutrient:energy ratios. Venous blood samples
(20 mL) were taken at d 0, 30, 60, and 90 of the
experiment and analyzed for serum a-tocopherol,
serum cholesterol, and serum triglyceride. The sum of
serum cholesterol and serum triglyceride values was
used as an estimate of serum total lipid. The ratio of
serum a-tocophero1:serum total lipid was used as a
measure of vitamin E status of horses. The SBO group
had higher serum cholesterol concentrations ( P <
.001) at 30, 60, and 90 d. Serum triglyceride was not
affected by diet. Serum total lipids tended to be
greater ( P < .06) for the SBO group. Serum a-
tocopherol was greater ( P < .001) at 90 d in the SBO
group. There was a significant correlation ( P < .001, r
= .62) between serum total lipids and serum a-
tocopherol concentrations in the SBO group. This
correlation was not significant ( P < .07, r = .35) in the
CTRL group. The ratio of serum a-tocophero1:serum
total lipid was not affected by diet. The results of this
experiment suggest that vitamin E status in non-
exercising, 2-yr-old horses fed currently recommended
levels of vitamin E was not affected by the addition of
6.4% SBO to the diet.
Key Words: Horses, Vitamin E, Polyenoic Fatty Acids, Cholesterol, Lipids
The use of added dietary fat as a source of energy
for horses has become increasingly popular in recent
years. Two commonly used sources of dietary fat are
corn oil and soybean oil ( SBO). Both corn oil and
SBO contain large amounts of polyunsaturated fatty
acids ( PUFA). Diets containing added fat high in
PUFA (e.g., corn oil, soybean oil, and cod liver oil)
have been shown to decrease vitamin E status, which
resulted in vitamin E deficiency symptoms in chicks
(Dam, 19621, ducks (Jager, 19721, dogs (Hayes et
al., 19681, swine (Grant, 19611, calves, and lambs
(Blaxter, 1962). There have been several studies in
horses concerning the utilization of dietary fat as a
source of energy (reviewed by Lawrence, 1990);
however, none has investigated the effect of fats,
specifically those high in PUFA, on vitamin E status
of the horse. Vitamin E has been shown to affect
immunocompetence of horses (Baalsrud and Overnes,
19861, in addition to providing protection against
'To whom correspondence should be addressed
Received December 7, 1992.
Accepted August 16, 1993.
J. Anim. Sci. 1993. 71:3399-3402
muscular dystrophy in foals (Wilson et al., 1976;
Schougaard et al., 1972). The following study was
designed to investigate the effect of added dietary SBO
(6.4% of the total diet) on vitamin E status of horses.
Materials and Methods
Fourteen 2-yr-old horses, of Quarter Horse and
Quarter Horse x Thoroughbred breeding, were ran-
domly assigned to one of two dietary treatments:
control ( CTRL) (n = 7 ) or SBO-supplemented (n =
7). Treatment groups were balanced for age and sex.
Mean BW were similar in both treatment groups at
the start (520 f 10 kg), and finish (CTRL, 530 f 7 kg;
SBO, 529 -t 12 kg) of the study. Both treatment
groups of horses were fed a ration composed of a grain-
based concentrate and alfalfa hay, which was formu-
lated to meet NRC (1989) requirements (Tables 1
and 2). All ingredients in both diets were analyzed for
ADF, CP, calcium, phosphorus, and selenium (Chemi-
cal Service Laboratory, Louisville, KY). Digestible
energy of ingredients was calculated according to the
equation of Fonnesbeck ( 198 1). Vitamin A concentra-
tion of ingredients was calculated using NRC (1989)
table values. All ingredients in both diets were
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SICILIAN0 AND WOOD
Table 1. Composition of control (CTRL) and added
dietary soybean oil diets (SBO)ab
Soybean meal, kg/d
Soybean oil, kgld
Trace mineral salt, g/dC
Vitamin E supplement, g/dd
Alfalfa hay, kg/d
alOO% dry matter basis.
bDuring the last 30 d of the trial, a new lot of hay with slightly
different nutrient composition (compared with hay fed during d 0 to
60) was fed. To maintain previous nutrient levels, the amount of
hay was decreased from 5.9 to 5.6 kg/d. The vitamin E supplement
was also adjusted to maintain vitamin E levels fed d 0 to 60.
CComposition of trace mineral salt: NaCl 5 98.59’~~
kg of Zn as ZnO; 34 mg/kg of Fe as Fe2O3; 20 mgkg of Mn as MnOZ;
3.3 mgkg of Cu as CuO; .7 mgkg of I as Ca(I03) 2; .5 mgkg of Co as
2 96%; 35 mg/
d176,000 IU of vitamin E activitykg supplied as all-rue-a-
tocopheryl acetate. Manufactured by Carl S. Akey, Lewisburg, OH.
analyzed for vitamin E activity (Animal Health
Diagnostic Laboratory, Lansing, MI). The SBO diet
differed from the control diet in that it contained 6.4%
SBO in the total ration (25% in the concentrate
portion of the ration). In addition, soybean meal was
added to the SBO diet so that both treatments were
isonitrogenous. Both SBO and CTRL rations were
isoenergetic and similar nutrient:energy ratios were
maintained. Vitamin E
176,000 IUkg provided by all-rac-a-tocopheryl acetate
was top-dressed on the concentrate of each individual
horse’s ration at each feeding, so that total vitamin E
intake, which included vitamin E activity from the
concentrate, hay, and vitamin E supplement, was
equal in both treatment groups. All horses were
individually fed one-half their respective rations twice
daily. Both rations were readily consumed by all
horses. Horses were individually housed in open-air
pens that were partially covered for shelter.
Blood samples (20 mL) were taken via venipunc-
ture at 0, 30, 60, and 90 d of the experiment, before
the afternoon feeding, and refrigerated (3 to 5°C)
immediately after collection. Serum was separated
within 4 h of collection, and stored at -4°C. Serum
samples were analyzed for a-tocopherol, cholesterol,
and triglycerides. Alpha-tocopherol was measured
using HPLC with a fluorescence detector according to
the method of Hidiroglou (1989). Cholesterol and
triglycerides were measured by colorimetric tech-
niques using commercial kits (Sigma Diagnostics, St.
Louis, MO). A lipid control (Sigma Diagnostics) was
used with both the cholesterol and triglyceride kits to
verify the procedure and reagent
Cholesterol and triglyceride concentrations (milli-
gramddeciliter) in the serum were added to estimate
serum total lipids according to the method of Thurn-
ham et al. (1986). Serum a-tocopherol was expressed
in a ratio with serum total lipid (milligrams of a-
tocophero1:grams of serum total lipid) to represent
vitamin E status more accurately, according to the
method of Horwitt et al. (1972).
Variation in response variables (serum a-
tocophero1:serum total lipid [milligrams/graml, serum
a-tocopherol [micrograms/milliliterl, serum total lipids
[milligrams/deciliter], serum cholesterol [milligrams/
deciliter], and serum triglycerides [milligrams/
deciliter]) were partitioned using ANOVA for repeated
measures designs (SAS, 1985). Differences between
treatments were analyzed using between-horse varia-
tion as the error term. Differences due to period and
period x treatment interaction were analyzed using
within-horses variation. Treatment difference were
considered significant at the P < .05 level. Where
period effects were significant ( P < .05), orthogonal
polynomials were used to determine the response
trend. Where period x treatment interactions were
significant ( P < .05), a LSD test was used as a means
Results and Discussion
Serum a-tocopherol concentrations in the present
study in both treatment groups increased (linear; P <
.001) above the concentrations measured at the
beginning of the study (Table 3). The increase may
have been due to feeding higher amounts of vitamin E
during the study as compared to the amount fed before
the experiment. There was a significant diet x period
interaction ( P < .03) on serum a-tocopherol. The
concentration of a-tocopherol in the SBO group tended
to be greater relative to the CTRL group at 60 d, and
at 90 d the a-tocopherol concentration in the SBO
group was greater ( P < .001) than that in the CTRL
group (Table 3). Serum total lipids also tended to be
greater ( P < .06) in the SBO group (Table 3). The
increase in a-tocopherol concentrations in the horses
fed the SBO diet was correlated ( P < .001; r = .62)
with an increase in serum total lipids. Correlations
between tocopherols and total lipids have been
reported in both rat and human studies (Horwitt et
Table 2. Nutrient intake of control (CTRL) and
added dietary soybean oil (SBO) dietsa
Vitamin A, IUld
Vitamin E, IU/d
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VITAMIN E STATUS OF THE HORSE
Table 3. Serum concentrations of cholesterol, total lipids, 0-tocopherol,
and a-tocophero1:total lipids in horses fed control diets (CTRL) and
soybean oil-supplemented diets (SBO)”
30 60 90
Total lipids, rng/dLd
~ ~ ~~ ~~~~
aBoth diets were fed in amounts to meet NRC requirements for 2-yr-old, nonexercising horses. Both
rations were isoenergetic with similar nutrient to energy ratios. The SBO diet contained 6.4% soybean oil.
bMain effect of diet ( P < ,041; diet x period interaction ( P < .01).
CLinear period effect ( P < .01).
dLinear period effect ( P < .001).
eDiet x period interaction IP < ,031.
f,gMeans within an item within a column lacking a common superscript letter differ ( P < .001).
al., 1972). The correlation between serum
tocopherol and serum total lipids was much stronger
in the SBO group ( P < .001; r = .62) than in the CTRL
group ( P < .07; r = .35). This is in agreement with the
report of Bieri (1990), who found the correlation
between serum tocopherols and serum total lipids to
be much weaker in normolipemic subjects. It is
thought that as serum total lipids increase, vitamin E
moves out of tissues and into circulating high-density
lipoproteins (Bieri et al., 1977). This migration of
vitamin E from tissues to circulating high-density
lipoproteins in association with increased serum lipids
may explain the increased serum a-tocopherol in the
SBO group in the present study. It is also possible
that the added fat in the SBO diet may have increased
vitamin E absorption. Vitamin E absorption is closely
associated with fat absorption (Bieri, 1990), and
increasing the amount of added dietary fat in the diet
of ponies has been shown to increase the digestibility
of ether extract (Kane et al., 1979); therefore, added
dietary fat may enhance vitamin E absorption.
There was a significant effect of diet ( P < .04) and
a significant diet x period interaction ( P < .01) on
cholesterol. At 30, 60, and 90 d cholesterol concentra-
tions were significantly greater ( P < .001) in the SBO
group than in the CTRL group (Table 3). The higher
concentration of serum cholesterol in the horses fed
the SBO diet resulted in the tendency toward in-
creased concentrations of serum total lipid in the
horses fed the SBO diet, as discussed previously.
Increases in serum cholesterol in horses and ponies
fed added dietary fat have been reported in other
studies (Bowman et al., 1977; Hambleton et al., 1980;
Kurcz et al., 1991). It is possible that the increase in
serum cholesterol in the horses fed the SBO diet was a
result of an increase in cholesterol-containing
lipoproteins needed for fat transport.
Serum cholesterol (Table 3 and serum triglyceride
(data not shown) concentrations increased (linear, P
< .01) throughout the experimental period in both
treatment groups. This increase in serum cholesterol
and triglyceride concentrations contributed to a sig-
nificant linear effect of period ( P . : .001) in serum
total lipids (Table 3). Increasing concentrations of
serum total lipids in both groups may reflect synthesis
of excess energy metabolites into triglycerides for
storage. Both groups of horses were assumed to be in a
positive energy balance because they gained approxi-
mately . 1 kg/d throughout the experiment.
Vitamin E status, measured by the ratio of serum
a-tocophero1:serum total lipids increased (linear, P <
.O 1) above the ratios measured at the beginning of the
study (Table 3). The increase may have been due to
differences in vitamin E content of diets fed before the
experiment as compared with experimental diets.
Serum a-tocophero1:serum total lipids was not
affected by the addition of SBO to the diet in the
present study (Table 3). Feeding diets high in PUFA
can increase the amount of PUFA in cell membrane
phospholipids (Rice and Kennedy, 1988). Membranes
containing large amounts of PUFA in their phos-
pholipids are especially susceptible to lipid peroxida-
tion (Combs et al., 1975). Vitamin E is a biological
antioxidant that protects cell membranes against
damage from lipid peroxidation (Combs et al., 1975;
Chow, 1991). Increasing PUFA content of tissues has
been shown to increase vitamin E utilization in rats
fed corn oil as a source of PUFA (Weiser and Salkeld,
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SICILIAN0 AND WOOD
1977). The absence of any effect of SBO supplementa-
tion on vitamin E status in this experiment suggests
that the level of vitamin E in the diet was adequate to
compensate for the additional PUFA content of the
diet. Hayes et al. (1968) reported no sign of decreased
vitamin E status in vitamin E-supplemented dogs fed
graded levels of PUFA; however, the vitamin E status
of dogs fed diets containing PUFA that were not
supplemented with vitamin E decreased with increas-
ing PUFA. In addition, the level of PUFA in this study
may not have been high enough to interfere with
vitamin E. Jager (1972 1 found interference between
PUFA and vitamin E in rats only at high dietary
levels of fat (approximately 15%) &at
in PUFA (approximately 50%).
were also high
The results of the present study
soybean oil fed at 6.4% of the total
interfere with vitamin E status in
2-yr-old horses fed currently recommended levels of
vitamin E. These results may not be applicable to the
exercising horse fed soybean oil.
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