2003 Poultry Science Association, Inc.
Dietary Carvacrol Lowers Body
Weight Gain but Improves Feed
Conversion in Female Broiler
H. Everts, H. J. Kappert, K.-H. Yeom, and A. C. Beynen
Department of Nutrition, Faculty of Veterinary Medicine, Utrecht University,
P.O. Box 80.152, 3508 TD, Utrecht, The Netherlands
Primary Audience: Nutritionists, Researchers, Veterinarians, Feed Manufacturers
Dietary thymol, and its isomer, carvacrol, were evaluated as alternatives to antibiotic feed
additives in female broiler chickens. In addition, the alleged hypocholesterolemic effect of carvacrol
and thymol were tested when the chickens were fed cholesterol-free or cholesterol-containing diets.
The experiment had a 2 ×3 factorial arrangement of treatments with diets containing two levels
of cholesterol (0 or 1%) and without feed additive or with 200 ppm thymol or carvacrol.
Dietary carvacrol lowered feed intake and weight gain but also lowered the feed-to-gain ratio.
Dietary thymol, an isomer of carvacrol, did not affect growth performance. Dietary cholesterol
signiﬁcantly increased plasma and liver cholesterol concentrations. Carvacrol lowered plasma
triglyceride concentrations but did not affect plasma cholesterol. It is concluded that thymol and
its isomer, carvacrol, have different effects on growth performance and triglyceride metabolism
in broiler chickens. The two compounds did not have hypocholesterolemic activity, irrespective of
whether the diet was cholesterol free or cholesterol rich.
Key words: essential oil, carvacrol, thymol, broiler, growth performance, lipid metabolism
2003 J. Appl. Poult. Res. 12:394–399
DESCRIPTION OF PROBLEM
In studies with broilers to identify a possible
supplement alternative to antibiotics, attention is
focused on essential oils and their pure compo-
nents. Essential oils are derived mainly from
spices and herbs and their constituents, which
have antimicrobial effects in vitro [1, 2]. Thymol
(Table 1), a major component of thyme-essential
oils, has been widely studied for its antimicrobial
properties [3, 4]. It can be added to chewing gum
at levels as high as 100 ppm and to nonalcoholic
To whom correspondence should be addressed: firstname.lastname@example.org.
beverages at concentrations of 2.5 to 11 ppm .
The reported lethal dose (LD
mg/kg of BW when given orally . Carvacrol
(Table 1), an isomer of thymol, is found in essen-
tial oils isolated from oregano, thyme, marjoram,
and summer savory . Like thymol, carvacrol
also displays antimicrobial activity [3, 8, 9]. Car-
vacrol may be added to baked goods at levels as
high as 120 ppm and to nonalcoholic beverages
at 26 ppm . The LD
of carvacrol in rats is
810 mg/kg of BW when administered by gavage
. The chemical properties and structures of thy-
LEE ET AL.: ESSENTIAL OILS AND BROILERS 395
TABLE 1. Chemical properties and structures of thymol and carvacrol
Molecular weight 150 (C
O) 150 (C
Synonym 5-methyl-2-(1-methylethyl)phenol 2-methyl-5-(1-methylethyl)phenol
21CFR 172.515 21CFR 172.515
Occurrence Thyme (lamiaceae, or labiatae) Oregano (lamiaceae, or labiatae)
Appearance White crystals Colorless to pale yellow liquid
Odor Pungent, caustic taste Thymol-like odor
Boiling point, °C 233 237
Density, g/mL 0.969 0.976
The Flavor and Extract Manufacturers Association; code given refers to “generally recognized as safe” (GRAS) status for
their use as food ﬂavor .
Food and Drug Administration; code given refers to GRAS status for their use in food .
mol and carvacrol are shown in Table 1. The two
compounds have the status of generally recog-
nized as safe (GRAS), which is endorsed by the
Flavor and Extract Manufacturers’ Association
(FEMA) and the Food and Drug Administration
(FDA) of the U.S.A. . Based on in vitro antimi-
crobial studies [3, 4, 8, 9], the minimum inhibitory
concentrations of thymol and carvacrol range
from 100 to 1,000 ppm, yeasts being most sensi-
tive and gram-negative bacteria being resistant.
Given their antimicrobial activity, it would be
expected  that thymol and carvacrol could
have positive effects on growth performance in
broilers. The main objective of the present study
was to evaluate the potential of thymol and carva-
crol as growth enhancers in female broilers. It has
been reported that thymol and carvacrol, at dietary
concentrations of 150 ppm, reduce serum choles-
terol in Leghorn chicks . To verify the hypo-
cholesterolemic properties of thymol and carva-
crol, plasma lipid concentrations were also mea-
sured in broilers. To further assess the
hypocholesterolemic action  of thymol and
carvacrol, broilers were fed both cholesterol-free
diets, as well as diets with 1% added cholesterol.
MATERIALS AND METHODS
The experimental protocol was approved by
the animal experiments committee of the Utrecht
Faculty of Veterinary Medicine.
Animals, Diets, and Experimental Design
Seventy-two 1-d-old female broilers (Cobb)
were purchased from a local hatchery. They were
wing-banded, weighed on arrival, and randomly
allocated to one of six treatments. Each treatment
consisted of two cages with six birds per cage.
Feed and water were provided ad libitum. The
temperature of the facility was 34°C on arrival
of the chickens, was gradually decreased to 25°C
after 3 wk, and then was kept constant. Continu-
ous lighting was used throughout the 4-wk experi-
mental period. The experiment had a 2 ×3 facto-
rial arrangement of treatments with two levels of
dietary cholesterol (0 and 1%) and three treat-
ments with feed additives (none, thymol, and car-
vacrol). The composition of the basal diet is
shown in Table 2. The diet was calculated to
meet the requirement of NRC  for essential
amino acids. The basal diet served as the control
diet without cholesterol and additives. Choles-
terol was introduced into the diet at the expense
of an identical amount of corn starch. Thymol
and carvacrol  were dissolved in corn oil and
gently mixed with either the cholesterol-free or
cholesterol-rich diets so that the diets contained
5% corn oil and 200 ppm of one of the two
supplements. The cholesterol-free and choles-
terol-rich diets without additives were mixed with
corn oil only. The experimental diets were pre-
pared and supplied on a daily basis.
JAPR: Research Report396
TABLE 2. Composition of the basal (control) diet
Corn, yellow 300
Corn starch 222
Soybean meal (48% CP) 375
Corn oil 50
Sodium chloride 5
Calcium carbonate 15
Monocalcium phosphate 19
, kcal/kg 3,171
CP, % 20.8
Lysine, % 1.2
Methionine +cystine, % 1.0
Calcium, % 1.0
Available phosphorus, % 0.5
The 10 g of premix consisted of 24.0 mg of vitamin A
(500,000 IU/g); 6.0 mg of vitamin D
(100,000 IU/g); 60.0
mg of vitamin E (500 IU/g); 6.6 mg of vitamin K
22.7%); 100.0 mg of vitamin B
(purity, 0.1%); 2,000.0
mg of biotin (purity, 0.01%); 1,100.0 mg of choline chloride
(purity, 50%); 1.1 mg of folic acid (purity, 90%); 65.2 mg
of nicotinic acid (purity, 100%); 16.3 mg of d-pantothenate
(purity, 92%); 4.5 mg of vitamin B
(purity, 100%); 12.5
mg of riboﬂavin (purity, 80%); 2.5 mg of vitamin B1 (purity,
100%); 32.00 mg of CuSO
O; 333.20 mg of FeSO
166.80 mg of MnO; 1.0 mg of Na
O; 220.00 mg
O; 4.80 mg of CoSO
O; 0.56 mg of KI;
100.00 mg of ethoxyquin, and 5,742.94 mg of corn meal as
The values were calculated from NRC .
Body weights were measured weekly. Feed
was weighed daily for each cage, and the leftover
feed was discarded. Daily feed intake was calcu-
lated by dividing the amount of feed consumed
by the number of days and animals. At 28 days
of age, the chickens were killed by cervical dislo-
cation after blood had been collected into heparin-
ized tubes by heart puncture. Plasma was obtained
by centrifugation at 1,700 ×gfor 15 min and
stored at −70°C until analyzed for lipids. Liver
and pancreas were immediately sampled and
weighed. Livers were stored at −70°C, whereas
pancreases were discarded.
Plasma cholesterol , triglyceride , and
phospholipid concentrations  were measured
enzymatically on an autoanalyzer . Plasma,
very-low-density lipoproteins (VLDL), and low-
density lipoproteins (LDL) were precipitated with
according to Ass-
man et al. , and the supernatant (high-density
lipoproteins, HDL) was assayed for cholesterol
. Liver total  and free  cholesterol
that had been extracted  were measured enzy-
matically on the autoanalyzer . Liver esteri-
ﬁed cholesterol was calculated as the difference
between total and free cholesterol.
Each pen was considered an experimental
unit. The data were evaluated by two-way AN-
OVA using the program Genstat 4.2 , with
feed additives and dietary cholesterol as main
factors. There was no interaction between dietary
cholesterol and feed additives; therefore, the data
are presented as overall means for each factor.
Treatment means were tested for statistically sig-
niﬁcant differences according to the RPAIR pro-
cedures in Genstat Release 4.2 for PC . A
value of P<0.05 was considered signiﬁcant.
Growth Performance and Organ Weights
Chickens fed carvacrol gained signiﬁcantly
less weight than those fed thymol, whereas the
controls showed intermediate weight gain (Table
3). Carvacrol versus thymol reduced weight gain
by 4.6%. Feed intake showed a similar pattern:
the carvacrol group ate 5.1 and 6.7% less feed
than the control and thymol group, respectively.
The feed-to-gain ratio was lowest in the carvacrol
group, the decrease being signiﬁcant when com-
pared with the controls. Dietary cholesterol had
no effect on growth performance (Table 3) while
relative liver weight (g/100 g of BW) was sig-
niﬁcantly increased by cholesterol feeding (P<
0.05). In contrast, the additives had no effect
(Table 3). The color of the livers in chickens fed
cholesterol was yellowish, whereas those of their
counterparts fed the diets without cholesterol
were pinkish. Dietary cholesterol and additives
did not affect pancreas weights (Table 3).
No interaction between cholesterol and addi-
tives was found with regard to plasma and liver
lipids. Cholesterol feeding signiﬁcantly increased
total plasma cholesterol and triglycerides, but
lowered phospholipids and HDL cholesterol ex-
pressed as a percentage of total plasma cholesterol
LEE ET AL.: ESSENTIAL OILS AND BROILERS 397
TABLE 3. Effect of dietary thymol and carvacrol on growth performance (d 0 to 28) and organ weights (d 28) of
female broiler chickens
Parameters None Carvacrol Thymol SEM
Weight gain, (g/d per bird) 42.4
0.623 42.4 42.3 0.508
Feed intake, (g/d per bird) 58.9
0.832 58.3 58.2 0.679
Feed:gain, (g:g) 1.389
0.010 1.375 1.376 0.008
Organ weights, (g/100 g of live BW)
Liver 2.39 2.32 2.39 0.081 2.14
Pancreas 0.21 0.22 0.21 0.004 0.22 0.21 0.003
Means in the same row within the same main factor not sharing a common superscript are signiﬁcantly different (P<
Values are expressed as means of four replicates per dietary group.
Values are expressed as means of six replicates per dietary group. A minus sign (−) means without added cholesterol; a
plus sign (+) means with added cholesterol.
Pooled standard error of mean.
(Table 4). HDL cholesterol was not affected by
cholesterol feeding. Dietary carvacrol signiﬁ-
cantly lowered triglycerides, when compared
with the control and thymol treatments, by an
average of 10.3%. Carvacrol also decreased
plasma phospholipids when compared with the
control treatment. Plasma total and HDL choles-
terol were not changed by the dietary additives.
Cholesterol feeding drastically increased liver
free and esteriﬁed cholesterol, but thymol and
carvacrol had no effect (Table 4).
Chickens fed either the diet containing thymol
or the control diet showed no signiﬁcant differ-
TABLE 4. Effect of dietary thymol and carvacrol on plasma and liver lipids in female broiler chickens at 28 d of
Parameter None Carvacrol Thymol SEM
Plasma lipids mmol/L mmol/L
Total cholesterol 4.54 4.58 4.62 0.196 3.35
HDL cholesterol 2.36 2.21 2.30 0.056 2.33 2.25 0.045
cholesterol, (% of total) 55.9 54.1 54.3 1.343 69.9
Liver cholesterol µmol/g of liver µmol/g of liver
Free cholesterol 23.0 23.8 22.8 0.955 16.0
Esteriﬁed cholesterol 56.5 64.0 65.0 11.804 4.0
Means in the same row within the same main factor and treatment group, not sharing a common superscript are signiﬁcantly
Pooled standard error of mean.
High density lipoprotein.
ence in growth performance. However, dietary
carvacrol at the level of 200 ppm signiﬁcantly
lowered feed intake and weight gain when com-
pared to thymol. When compared with the control
group, carvacrol feeding signiﬁcantly lowered the
feed-to-gain ratio. Thus, when comparing thymol
and carvacrol in relation to growth performance,
it appears that carvacrol suppressed feed intake,
leading to lower weight gain in spite of improved
feed conversion. Possibly, carvacrol affected feed
intake by modulating appetite. Deyoe et al. 
showed that the ﬂavor of chickens’ diets can stim-
ulate or depress feed intake. The carvacrol effect
on feed-to-gain ratio could relate to increased
efﬁciency of feed utilization and/or altered car-
JAPR: Research Report398
cass composition, although the latter was not an-
Case et al.  reported that dietary carvacrol
and thymol, at 150 ppm, did not inﬂuence BW
gain of cockerels with initial weights of 126 g
that were followed during a 21-d feeding trial. In
a previous experiment with female broilers, Lee
et al.  also found a lack of effect of thymol
on growth performance and digestive enzyme ac-
tivity when fed at a level of 100 ppm for a period
of 6 wk. It was suggested that the antimicrobial
activity of thymol may be masked by diet compo-
sition and/or environment, in that no effect of
thymol on growth performance was seen when
a well-balanced diet was fed and the birds were
kept in a clean environment, as was done in
It has been reported that dietary thymol and
carvacrol lower serum cholesterol concentrations
in chickens . The hypocholesterolemic effect
of thymol and carvacrol has been ascribed to
inhibition of 3-hydroxy-3-methylglutaryl coen-
zyme A (HMG-CoA) reductase , the rate-
controlling enzyme of the cholesterol synthetic
pathway. The present experiment was designed
to determine if there was an interaction between
dietary cholesterol and either thymol or carvacrol.
It was reasoned that by inhibition of HMG-CoA
reductase activity through cholesterol feeding
, there would be no apparent hypocholestero-
lemic effects of thymol and carvacrol. However,
our results failed to reveal a hypocholesterolemic
effect of either thymol or carvacrol, irrespective
of whether the diet was cholesterol free or choles-
CONCLUSIONS AND APPLICATIONS
1. The results of the present study revealed that dietary carvacrol, but not its isomer, thymol,
lowered feed intake and weight gain but also lowered the feed-to-gain ratio.
2. Dietary carvacrol and thymol did not have hypocholesterolemic activity, irrespective of whether
the diet was cholesterol free or cholesterol rich.
3. Dietary carvacrol, but not thymol, lowered plasma triglycerides.
4. Our observation implies that dietary essential oil components can affect feed utilization or lipid
metabolism. Furthermore, it is likely that essential oils can contain compounds with different
or possibly even opposite effects on growth performance and lipid metabolism.
5. This notion may have impact on the use of essential oils as growth enhancers.
terol rich. As would be expected , dietary
cholesterol signiﬁcantly increased plasma total
cholesterol but did not modulate plasma HDL
cholesterol. The increase in plasma total choles-
terol may have been located in VLDL, as reported
earlier [27, 28], lowering the percentage of total
cholesterol in HDL. Thus, the present study cor-
roborates earlier studies [27, 28] showing that a
substantial portion of plasma total cholesterol in
chickens fed on a cholesterol-free diet is carried in
the form of HDL and that exogenous cholesterol
increases the secretion of VLDL by the liver.
The concentrations of liver free and esteriﬁed
cholesterol were not changed either by thymol
and carvacrol. Dietary carvacrol signiﬁcantly
lowered plasma triglycerides and phospholipids
by 12 and 7%, respectively. According to Yasni
et al. , α-curcumene, a component of essential
oils from Curcuma xanthorrhiza, speciﬁcally
suppressed hepatic fatty acid synthase. Feeding
of the extracts of Curcuma xanthorrhiza lowered
serum triglycerides without affecting serum cho-
lesterol. On the other hand, d-limonene, an essen-
tial oil component from Citrus sinensis, inhibited
HMG-CoA reductase but did not affect fatty acid
synthetase, which led to a signiﬁcant reduction
in serum cholesterol . In the light of earlier
studies [29, 30], dietary carvacrol, but not thymol,
may have had more impact on de novo lipogen-
esis than on cholesterol biosynthesis in this study.
An explanation in molecular terms for the obser-
vation that carvacrol lowered plasma triglycerides
should provide further insight in lipid me-
LEE ET AL.: ESSENTIAL OILS AND BROILERS 399
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The authors are indebted to A. Lankhorst for technical assistance.
K-W. Lee was supported by the National Institute for International
Education Development, South Korean Ministry of Education.