Brazilian Journal of Poultry Science
Revista Brasileira de Ciência Avícola

  ISSN 1516-635X   Oct - Dec 2006 / v.8 / n.4 / 251 - 256

251

Denadai JC1

Ducatti C2

Pezzato AC3

Carrijo AS4

Caldara FR1

Oliveira RP1

1 Animal Science PhD student, Veterinary

Medicine and Animal Science School (FMVZ),

São Paulo State University (UNESP).
2 Professor Doctor, Supervisor of the Stable

Isotopes Center, Biosciences Institute, São

Paulo State University (UNESP).
3 Professor Doctor, Veterinary Medicine and

Animal Science School (FMVZ), São Paulo

State University (UNESP).
4 Professor, Animal Production Department,

Mato Grosso do Sul Federal University (UFMS).

# Experiment sponsored by FUNDIBIO –

Fundação do Instituto de Biociências – São

Paulo State University (UNESP), Botucatu,

SP, Brazil

Juliana Célia Denadai

Centro de Isótopos Estáveis

Instituto de Biociências UNESP

Universidade Estadual Paulista

Caixa Postal 510

18.618-000. Botucatu, SP, Brazil

Phone: (+55) 14–3811 6359

Fax: (+55) 14–3815 1171

E-mail: denadaijc@gmail.com

Albumen; carbon-13; yolk; blood; egg;

comercial laying hens.

The authors are grateful to the staff of the

Stable Isotopes Center of the Biosciences In-

stitute of UNESP, Botucatu Campus, for car-

rying out the isotopic analyses.

Studies on carbon-13 turnover in eggs and
blood of commercial layers#

Mail Address

Keywords

Author(s)

Arrived: July / 2006

Approved: November / 2006

ABSTRACT

This paper aimed at evaluating the influence of diets containing
different isotopic values of carbon-13 turnover on the half-life of egg
(yolk + albumen), yolk and albumen individually, and blood of poultry
using δ‰13C isotopic variation. Commercial layers fed four experimental
isocaloric and isonitrogenous diets (RC

4
, RC

3
, RMC

4
 and RMC

3
) containing

different isotopic values, during an experimental period of 56 days.
Turnover of the studied tissues was influenced by the experimental diets.
Blood and albumen were more influenced by dietary treatments as
compared to egg and yolk. The RMC

3 
diet induced better performance

(better feed intake and higher egg production) due faster rate of carbon
substitution than the RC

4
 diet, and lower half-life for egg (yolk +

albumen), yolk, and albumen.

INTRODUCTION

Most of turnover studies using the natural variations of the delta per
thousand of carbon-13 (δ‰13C)* of foods are related to ecological issues,
focusing on the substitution velocity of tissue carbon from food with
different isotopic ratios due migrations and changes in trophic levels
(Mizutani et al., 1991, Hobson 1995). However, stable isotopes
technique can be useful in physiology and animal nutrition studies as
the substitution rates of tissue carbon can be influenced by
environmental, nutritional, and health factors (Carrijo et al., 2000).

The period during which the isotopic concentration of a tissue reflects
the isotopic signature of a diet is partially dependent on the isotopic
turnover rate of that tissue (Hobson and Clark, 1992a). Isotopic turnover
rate depends on the absorption, synthesis, and catabolism rates of each
tissue component (Gannes et al., 1998). Tissues with fast isotopic
turnover rates reflect more recent diets, while those with slower turnover
rates reflect diets fed during previous periods (Hobson and Clark, 1992a,
b). Most metabolically active tissues (liver, pancreas, and adipose tissue)
present faster turnover rates than the less metabolically active tissues,
such as bone collagen (Hobson & Clark, 1992a).

Eggs, as other animal products, such as like milk and feces, are not
considered tissues, and therefore it is not possible to calculate its tissue
renewal rate (turnover). However, turnover studies in these products
indicate that the renewal rate of nutrients for their synthesis reflect the
metabolism of the tissues that generated these products.

Carrijo et al. (2000), studying carbon turnover in eggs and liver of
laying hens, verified that carbon substitution time was similar in those
tissues. This indicates that eggs could be used as an indirect measure of
liver turnover rate, thereby avoiding the sacrifice of the birds. These

* Content of carbon-13 expressed in parts per thousand (‰) relative to the Peedee Belemnite

(PDB) standart.

Aknowledgment



Denadai JC, Ducatti C, Pezzato AC,
Carrijo AS, Caldara FR, Oliveira RP

Studies on carbon-13 turnover in eggs and blood of
commercial layers

252

authors recommend the separate isotopic rate analysis
of egg fractions (albumen and yolk), because values
found in eggs could result from isotopic dilution of
fractions with different substitution velocity.

When working with diets based on grains of plants
of the C

3
 and C

4
 photosynthetic cycle, a feed

formulation problem arises in diets containing C
4 
plants.

There are few C
4 
ingredients that can replace soybean

meal (C
3
), which is the main protein source of balanced

diets for commercial layers. An alternative ingredient
to replace soybean meal is corn gluten, which is a by-
product of corn starch removal, and which contains
60% crude protein. However, corn gluten presents
some restrictions, as it is deficient in lysine and
tryptophan for chickens, and due to its bitter flavor.

Ducatti et al. (2002), based on literature results ,
proposed a theoretical model to express the results of
relative carbon enrichment (δ‰13C) in terms of feeding
time by supplying diets with different 13C isotopic values,
and comparing this with adult animals. We hypothesized
the metabolism of certain animal tissues depends on
the input rate of dietary carbon compounds and the
substitution rate of the pre-existent compounds. This
model is suitable for determining the total or partial
(half-life) carbon substitution time in tissues in studies
with poultry and small animals.

Kennedy & Krouse (1990) stated that stress related
to external environmental factors, physical conditions,
and diseases could cause variations in the isotopic
fractionation.

The present study aimed at evaluating the influence
of diets with different isotopic values on carbon-13
turnover of egg (yolk + albumen), yolk and albumen
individually, and blood half-lives of commercial laying
hens, using δ‰13C isotopic variation.

MATERIAL AND METHODS

commercial feed (based on corn, soybean meal, and
wheat). This diet had a δ‰13C isotopic signal = -16.06
± 0.23 (n = 2). Average egg production was 52%. As
these birds were at the end of their productive cycle,
their egg production was low.

This trial lasted 84 days, including a 28 days of pre-
experimental period. According to Carrijo et al. (2000),
this pre-experimental period is sufficient for isotopic
marking of eggs to happen. The objective of this period
objective was to replace carbon derived from the
commercial feed in the studied tissues by carbon
derived from the new diets. Birds were divided in two
groups of 48 each, with one group receiving a diet
based on grains of C

3
 (RC

3
) photosynthetic cycle plants,

whereas the other was fed with a diet consisting of
grains of C

4
 (RC

4
) photosynthetic cycle plants. At the

end of the adaptation period, birds started to receive
the experimental diets.

The group fed the RC
3 

feed during the pre-
experimental period was divided in two treatments
(n=24), which were fed RC

4
 and RMC

4 
diets afterwards.

The other group, which was being fed RC
4
, was also

divided in two treatments, and were fed RC
3
 and RMC

3

diets (see treatment description below).
Treatments consisted of four isocaloric and

isonitrogenous diets with different isotopic values,
which were fed during the entire experimental period
(56 days).

Treatments:

Treatment 1 (RC
4
) - Feed consisting of grains of C

4

photosynthetic cycle plants.
Treatment 2 (RC

3
) - Feed consisting of grains of C

3

photosynthetic cycle plants.
Treatment 3 (RMC

4
) - Mixed feed consisting of

67% C
4 
grains and 33% C

3 
grains.

Treatment 4 (RMC
3
) - Mixed feed consisting of

67% C
3 
grains and 33% C

4 
grains.

The mixed diets were used to obtain intermediate
δ‰13C values as those of the pure diets with the
objective of evaluating the influence of the mixed diets
and the possible improvement of the quality of the C

4

diet.
Nutritional composition of ingredients used in feed

formulation, as well as nutritional requirements
followed recommendations of Rostagno et al. (2000).
The percentage composition, calculated nutritional
levels, and δ‰13C values of the experimental diets are
shown in Table 1.

On the 0th, 1st, 2nd, 4th, 7th, 11th, 16th, 21st, 28th, 35th,
42nd, 49th, and 56th day of the experimental period,

The study was carried out at Lageado Experimental
Farm, in the School of Veterinary Medicine and Animal
Science (FMVZ) of São Paulo State University (UNESP),
Botucatu campus.

Ninety-six Hy Line W36 120-week-old layers were
housed in metallic cages placed in four rows. Cages
were1.00m long x 0.45m deep x 0.40m high. Cages
had two internal compartments, housing four birds
each. Birds were designated to four treatments with
three replicates of eight birds each. Food and water
were available ad libitum in feeders and nipple drinkers.

Layers were adapted to the new facilities during a
28 day-period, and were fed 101.5 g/bird/day of a



Denadai JC, Ducatti C, Pezzato AC,
Carrijo AS, Caldara FR, Oliveira RP

Studies on carbon-13 turnover in eggs and blood of
commercial layers

253

4 hours. It is necessary to remove lipids from the sample
because the presence of fat makes the isotopic
signature lighter in 13C in relation to the diet. For
albumen and blood, the drying period was 24 hours,
and these were not defatted because they have low
fat content. All samples were ground in cryogenic mill
(Spex- freezer/mill 6750 model) for three minutes at -
190oC, in order to obtain homogeneous material with
microscopic aspect (Ducatti, 2004).

Isotopic composition of samples was determined
using a DELTA-S (Finnigan Mat) mass spectrometer
coupled to an EA 1108 CHN Elemental Analyzer, in
the Stable Isotopes Center facilities of the Biosciences
Institute of UNESP, Botucatu campus. The results were
expressed as δ‰13C relative to the Peedee Belemnite
(PDB) standard, with analysis error of 0.2%, according
to equation 1:

δδδδδ‰13C
(sample, standard)

=[(R
sample

/R
standard

)
 
– 1] x 103     (1)

Where:
δ‰13C = enrichment of the 13C/12C ratio of the

sample in relation to the PDB standard.
R = isotopic ratio (13C/12C) of the sample and

the standard.

In order to quantitatively measure the carbon
substitution speed of the diets after certain time, the
exponential function expressed by equation 2 (Ducatti
et al., 2002) was used:

δδδδδ‰13C(t) = δδδδδ‰13C(f) + [δδδδδ‰13C(i) - δδδδδ‰13C(f)]e-kt  (2)

Where:
δ‰13C(t) = isotopic enrichment of the tissue at any

time (t).
δ‰13C(f) = isotopic enrichment of the tissue at the

balanced base, or final status.
δ‰13C(i) = isotopic enrichment of the tissue, at the

initial condition.
k = change constant (turnover) in time units-1

t = time (in days) since feed was replaced.

Carbon-13 half-life for egg, yolk, albumen, and
blood, in the condition of 50% of each diet in t = T
was calculated by equation 3:

T = ln 2/k                                                            (3)

where,
T represents half-life in days

In order to perform isotopic analyses, egg and yolk
samples were thawed, dried in forced ventilation oven
(Marconi - MA 035 model) at 56oC for 48 hours, and
defatted in ethyl ether using the Soxhlet apparatus for

eight eggs were randomly collected per treatment
group on each collection day, with four eggs used for
yolk and albumen sampling, and the other four eggs
were homogenized in blender (yolk + albumen).
Additionally, on each collection day, blood samples
were taken from the wing vein of four birds per
treatment. The samples were labeled, frozen, and
stored at -18°C. The collections were concentrated in
the first days due the higher velocity of carbon
incorporation. Day zero was assumed when samples
were obtained immediately before feed change, and
its purpose was to express the isotopic composition of
feed offered at the pre-experimental period

Table 1 – Composition of the experimental diets (calculated
levels and values of δ‰13C).
Ingredients, T1 (RC

3
)3 T2 (RC

4
)3 T3 (RMC

3
)3 T4 (RMC

4
)3

%

Rice, broken 62.86 - 42.11 20.74

Soybean meal 23.50 - 15.75 7.76

Corn - 62.15 20.51 41.63

Corn gluten meal - 17.00 5.61 11.39

Kaolin - 8.50 2.81 5.70

Limestone 9.25 9.50 9.33 9.42

Dicalcium Phosphate 1.70 1.65 1.68 1.67

DL - methionine 0.14 0.05 0.11 0.08

L - lysine - 0.60 0.20 0.40

Salt 0.35 0.35 0.35 0.35

Soybean oil 2.00 - 1.34 0.66

Mineral premix1 0.10 0.10 0.10 0.10

Vitamin premix2 0.10 0.10 0.10 0.10

TOTAL 100 100 100 100

Calculated Nutritional Levels

ME, MJ/kg 11.56 11.56 11.56 11.56

CP, % 16.00 16.00 16.00 16.00

Met, % 0.44 0.40 0.43 0.41

Met+Cys, % 0.68 0.69 0.68 0.68

Lys, % 0.81 0.80 0.80 0.80

Trp, % 0.22 0.08 0.18 0.13

Ca, % 4.05 4.04 4.05 4.04

Available P, % 0.37 0.37 0.37 0.37

Na, % 0.20 0.20 0.20 0.20

Isotopic Values

δδδδδ‰13C (n=4)-26.70±0.04   -12.36±0.10   -22.40±0.02  -17.22±0.11

1 - Composition by kg of the mineral supplement: Copper: 8,000mg;

Iron: 50,000mg; Manganese: 70,000mg; Zinc: 50,000mg; Iodine:

1,200mg; Selenium: 200mg; QSP Excipient: 1,000g. 2 - Composition

by kg of the vitamin supplement: Vitamin A: 7,000,000 UI; Vitamin

D
3
: 2,000,000 UI; Vitamin E: 5,000 mg; Vitamin K

3
: 1,600mg; Vitamin

B
2
: 3,000mg; Vitamin B

12
: 8,000mcg; Niacin: 20,000mg; Panthotenic

Acid: 5,.000mg; Antioxidant: 15,000mg; QSP Excipient: 1,000g. 3 -

T1 (RC
4
) - Feed consisting of grains of C

4 
photosynthetic cycle plants;

T2 (RC
3
) - Feed consisting of grains of C

3 
photosynthetic cycle plants.;T3

(RMC
4
) - Mixed Feed consisting of 67% C

4 
grains and 33% of C

3

grains; T4 (RMC
3
) - Mixed Feed consisting 67% C

3 
grains and 33% C

4

grains.



Denadai JC, Ducatti C, Pezzato AC,
Carrijo AS, Caldara FR, Oliveira RP

Studies on carbon-13 turnover in eggs and blood of
commercial layers

254

ln is the Neperian logarithm, and constant k presents
time unit-1, providing an idea of “speed” in the
exchange process of stable isotopes in the tissues
(Tieszen et al., 1983; Hobson & Clark, 1992a, b; Ducatti
et al., 2002; Ducatti, 2004).

Isotopic results were analyzed using regression
equation method of Origin® 6.0 Professional (Microcal
Software, 1999) software, whereas performance
results (feed intake, egg production, and egg mass)
were statistically analyzed using analysis of variance
(ANOVA), with the aid of the GLM procedure of the
SAS software (1999). The test of Tukey at the
probability level of 5% was used to compare treatment
means.

RESULTS AND DISCUSSION

Table 2 shows the regression equations of the
treatments in the different studied tissues. δ‰13C value
changes in tissues along time occurred gradually until
a new balance was reached. This change can be seen
in Figure 1, which illustrates turnover behavior in blood,
egg, yolk, and albumen.

Blood carbon half-life was 13.3 days in RC
4

treatment, and 5.3 days in RC
3
 treatment. Turnover

period was 2.5 times slower in the blood of RC
4

treatment layers as compared to layers fed the RC
3

diet. These results are consistent with those of Carrijo
et al. (2000), who evaluated liver and egg turnover
rates of layers, and found higher half-life values when
a C

3
 feed was replaced by a C

4
 diet as compared to

the inverse replacement. This slower turnover can be
partly explained by the drastic feed intake reduction
of layers fed RC

4
 (Table 3). Corn gluten may have

reduced palatability, and nutritionally unbalanced the
feed mainly due to the level of tryptophan–, which was

not considered when the experimental feeds were
formulated – was probably the reason of low feed
intake.

Gonzales (2002) states that very sweet (20% of
sucrose) or bitter water is usually rejected by poultry.
This is also true for solid feed; poultry reject very sweet
or bitter foods (Balog & Millar, 1989).

In addition, amino acids balance in the RC
3 
diet was

different from that in the RC
4 

diet (Table 1), where

Figure 1 – Experimental model of the turnover of carbon stable
isotopes* (mean + standard-deviation, n = 4) of blood, egg, yolk,
and albumen of commercial laying hens with respective half-life
values, in days, of treatments RC

3
, RMC

3
, RMC

4
, and RC

4
.

* Microcal Software Origin®6.0 Professional. Origin Data Analysis
and Technical Graphics. USA, Microcal Software Inc. 1999.

-5 0 5 10 15 20 25 30 35 40 45 50 55 60

-10

-12

-14

-16

-18

-20

-22

-24

-26

-28

RC
4   

T = 13.3 days

RMC
4   

T = 10.5 days

RMC
3   

T = 9.2 days

RC
3   

T = 5.3 daysBlood

δδ δδ 
‰

 13
C

Time (days)

-5 0 5 10 15 20 25 30 35 40 45 50 55 60

-10

-12

-14

-16

-18

-20

-22

-24

-26

-28

RC
3   

T = 3.8 days

RMC
3   

T = 3.5 days

RMC
4   

T = 3.8 days

RC
4   

T = 4.8 days

Egg

δδ δδ 
‰

 13
C

Time (days)

-5 0 5 10 15 20 25 30 35 40 45 50 55 60

-10

-12

-14

-16

-18

-20

-22

-24

-26

-28

RMC
3
    T = 4.6 days

RMC
4   

T = 4.8 days

RC
3
   T = 4.5 days

RC
4   

T = 5.5 days

Yolk

δδ δδ 
‰

 13
C

Time (days)

-5 0 5 10 15 20 25 30 35 40 45 50 55 60

-10

-12

-14

-16

-18

-20

-22

-24

-26

-28

RMC
3    

T = 1.5 days

RMC
4   

T = 3.2 days

RC
3   

T = 2.5 days

RC
4   

T = 5.2 days

Albumen

δδ δδ 
‰

 13
C

Time (days)

Table 2 - Regression and half-life equations of blood, whole egg, and egg fractions (yolk and albumen) of commercial laying hens.
Tissues Treatments Equations (δ13C) R2 CarbonHalf-life (days)

Blood(n=4) RC
4

-11.95‰ – 13.63‰e-0.052t 0.99 13.3

RC
3

-26.15‰ + 12.14‰e-0.132t 0.98 5.3

RMC
4

-17.43‰ – 8.17‰e-0.066t 0.99 10.5

RMC
3

-21.90‰ + 7.87‰e-0.076t 0.98 9.2

Egg(n=4) RC
4

-13.10‰ – 13.82‰e-0.146t 0.99 4.8

RC
3

-26.36‰ + 14.95‰e-0.181t 0.99 3.8

RMC
4

-17.41‰ – 9.83‰e-0.185t 0.99 3.8

RMC
3

-21.89‰ + 9.42‰e-0.200t 0.98 3.5

Yolk(n=4) RC
4

-13.01‰ – 14.99‰e-0.127 0.99 5.5

RC
3

-27.13‰ + 16.70‰e-0.153t 0.99 4.5

RMC
4

-17.67‰ – 9.97‰e-0.143t 0.99 4.8

RMC
3

-22.50‰ + 9.96‰e-0.151t 0.99 4.6

Albumen(n=4) RC
4

-12.30‰ –13.48‰e-0.133t 0.95 5.2

RC
3

-26.14‰ +  14.54‰e-0.277t 0.98 2.5

RMC
4

-17.06‰ – 8.86‰e-0.220t 0.99 3.2

RMC
3

-21.20‰ + 11.26‰e-0.467t 0.94 1.5



Denadai JC, Ducatti C, Pezzato AC,
Carrijo AS, Caldara FR, Oliveira RP

Studies on carbon-13 turnover in eggs and blood of
commercial layers

255

exclusively C
4
 due the interference of the diet on

metabolism. This fact is probably more related to the
supply of nutritional requirements than to
photosynthetic cycle type of the plants included in the
diets.

The half-life value for the egg (yolk + albumen) of
layers fed the RC

3 
diet was 3.8 days, and 4.8 days birds

in RC
4 

treatment. These results agree with those of
Carrijo et al. (2000), who evaluated liver and egg
turnover rates of layers, and obtained half-life values
of 3.7 days when a C

4 
diet was replaced for a C

3 
diet.

These authors also found a value of 4.0 days when
the C

3
 diet was replaced for the C

4 
diet. In the present

study, carbon half-life values of eggs from RMC
4
 and

RMC
3
 treatments were 3.8 days and 3.5 days,

respectively.
The yolk carbon half-life of the RC

3 
treatment was

4.5 days, whereas 5.5 days were calculated for the
RC

4 
treatment (Table 2). Intermediary and values similar

to the RC
3
 treatment were found for yolks from layers

in treatments RMC
4
 and RMC

3
, which were 4.8 days

and 4.6 days, respectively.
The albumen of layers in the RC

4 
treatment had

carbon half-life of 5.2 days, a while for the birds in
treatments RC

3
, RMC

4
, and RMC

3
, values were 2.5,

3.2, and 1.5 days, respectively. The possible reason
for the slower turnover of albumen in RC

4 
treatment is

that, as amino acids for albumen formation are directly
taken from blood plasma and blood, and these birds
may still have had carbon from the previous diets in
the body (slow turnover), this carbon was used for
albumen formation in these birds.

Hobson (1995), studying turnover of quail egg
components, stated that total carbon replacement from
egg yolk took eight days, and from albumen, from three
to five days. These data are different from the findings
of this study, which may be due to the fact that Hobson
(1995) studied different species and diets. In addition,
quails were producing eggs daily, whereas in the
present study, layer hens produced one egg every other
day.

Egg carbon turnover values are intermediate to yolk
and albumen values, showing the influence of each
component (yolk and albumen) in their formation
process, where the yolk takes longer to reflect the new
diet.

During the physiological process of egg formation,
the ovary programs a sequence of ova in different
development stages (approximately eight days of
production). Therefore, in the eggs collected in the
beginning of the trial, yolk  had a large influence of

The high feed intake of birds in RC
3
 and RMC

3

treatments was probably due to thefeed offered in the
pre-experimental period , when these birds were fed
a diet formulated exclusively with C

4 
ingredients. After

feed change (29-35 days), feed intake was dramatically
increased to compensate the low feed intake of the
last phase.

Studies demonstrated that poultry can control their
feed intake based on feed protein amount and quality.
Feed intake regulation theory by dietary protein
considers the principle that the detection of plasmatic
levels of amino acids, particularly of the nutritionally
essential amino acids, takes place by activating control
systems in the central nervous system (Gonzales,
2002).

Feed intake regulation mechanism is essentially
homeostatic, with variations in feed intake depending
on protein quality and amount in the feed. Feeds with
serious amino acids unbalance induce a marked
decrease in intake (Boorman, 1979). These factors may
have contributed for a reduction in the layer
metabolism rate, and consequently blood carbon
turnover.

Carbon half-life values in the blood of layers in the
intermediate treatments (RMC

4
 and RMC

3
) were 10.5

days and 9.2 days, respectively. These values were not
different from RC

4 
treatment. These results are

consistent with those obtained by Hobson & Clark
(1992a), who, working with substitution of predominant
C

3 
diet for C

4
 in quails, obtained blood carbon half-life

of 11.4 days. These half-life values were expected to
be similar to those found in the RC

3 
treatment. Feed

intake results of these treatments presented similar
values. Tryptophan balance was probably less harmful
to these birds as compared to the diet including

only methionine and lysine requirement levels were
corrected. However, RC

4 
diet tryptophan levels were

below (calculated tryptophan level was 0.08%)  the
nutritional requirement, which is 0.18% for layer in this
stage (Rostagno et al., 2000).

Table 3 – Feed intake, in g/bird/day, at the end of the pre-
experimental period (22-28 days), beginning (29-35 days) and
end of the experimental period (78-84 days).
Days                                   Treatments

RC
4

RC
3

RMC
4

RMC
3

22-28 97.26abA 93.81bC 100.83a 84.01cC

29-35 84.23cB 141.67aA 105.61b 142.69aA

78-84 81.63bB 112.24aB 112.24a 109.78aB

Different small letters in the same row are different by the test of

Tukey (p < 0.05). Different capital letters in the same column are

different by the test of Tukey (p < 0.05).



Denadai JC, Ducatti C, Pezzato AC,
Carrijo AS, Caldara FR, Oliveira RP

Studies on carbon-13 turnover in eggs and blood of
commercial layers

256

carbon-13 isotopic ratio from the previous feed.
Albumen deposition around the yolk takes about three
hours in the magnum, where proteins are synthesized
from amino acids directly derived from blood plasma
(Hobson, 1995). This explains the lower half-life of the
albumen.

Blood and albumen showed to be strongly
influenced by diet composition as compared to egg and
yolk. This is why RC

4
 treatment turnover was slower

than the other treatments. The possible explanation is
that, since blood transports nutrients, there was intense
mobilization of reserves in layers in this treatment to
try to maintain egg production. As albumen synthesis
is directly dependent from plasma, this reflects the
blood turnover.

Moreover, blood turnover is slower than the
turnover of the egg and its fractions. A possible
explanation is the blood consists of a cellular fraction
and plasma, and the cellular fraction takes longer to
exchange existing carbon as compared to plasma. This
reflects nutrient transit in the body (Hobson and Clark,
1993), which is similar to what occurs during the
formation of the egg and its fractions.

CONCLUSION

The RMC
3 

diet was better because it promoted
better productive performance (high feed intake and
high egg production), faster carbon substitution rate
than the unbalanced diet (exclusively C

4
), and shorter

half-life of egg (yolk + albumen), yolk, and albumen.
Turnover is influenced by the quality of the diet fed

to the birds. In addition, diet composition has less
influence on egg and yolk turnover, as compared to
blood and albumen. Carbon substitution rate is faster
for egg and its components (yolk and albumen) than
for blood.

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