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Hydrolyzed collagen (gelatin) decreases food efficiency and the bioavailability of high-quality protein in rats

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Objective Although deficient in all indispensable amino acids, gelatin is used in protein-restricted diets. Food efficiency and protein quality of casein and gelatin mixtures in low protein diets in Wistar rats were investigated. Methods The rats were treated with protein-restricted diets (10.0 and 12.5%) containing casein (control diets), casein with gelatin mixtures (4:1 of protein content), and gelatin as sources of protein. The food conversion ratio, protein efficiency ratio, relative and corrected protein efficiency ratio, true protein digestibility, and hepatic parameters were estimated. Results After 28 days of the experiment, food efficiency of 10.0% casein/gelatin diet decreased when compared to that of 10.0% casein diet, and the protein efficiency ratio of the casein/gelatin mixtures (10.0%=2.41 and 12.5%=2.03) were lower than those of the casein (10.0%=2.90 and 12.5%=2.32). After 42 days of the experiment, the weight of the liver of the animals treated with 10.0 and 12.5% casein/gelatin diets, and the liver protein retention of the 12.5% casein/gelatin diet group of animals were lower than those of the control group. Conclusion Gelatin decreases food efficiency and high-quality protein bioavailability in protein-restricted diets.
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GELATIN DECREASES CASEIN BIOAVAILABILITY | 421
Rev. Nutr., Campinas, 28(4):421-430, jul./ago., 2015 Revista de Nutrição
http://dx.doi.org/10.1590/1415-52732015000400008
ORIGINAL | ORIGINAL
Hydrolyzed collagen (gelatin) decreases
food efficiency and the bioavailability
of high-quality protein in rats
Colágeno hidrolisado (gelatina) reduz a
eficiência alimentar e a biodisponibilidade
de proteína de alta qualidade em ratos
Cláudia Cantelli Daud BORDIN
1
Maria Margareth Veloso NAVES
1
A B S T R A C T
Objective
Although deficient in all indispensable amino acids, gelatin is used in protein-restricted diets. Food efficiency
and protein quality of casein and gelatin mixtures in low protein diets in Wistar rats were investigated.
Methods
The rats were treated with protein-restricted diets (10.0 and 12.5%) containing casein (control diets), casein
with gelatin mixtures (4:1 of protein content), and gelatin as sources of protein. The food conversion ratio,
protein efficiency ratio, relative and corrected protein efficiency ratio, true protein digestibility, and hepatic
parameters were estimated.
Results
After 28 days of the experiment, food efficiency of 10.0% casein/gelatin diet decreased when compared to
that of 10.0% casein diet, and the protein efficiency ratio of the casein/gelatin mixtures (10.0%=2.41 and
12.5%=2.03) were lower than those of the casein (10.0%=2.90 and 12.5%=2.32). After 42 days of the
experiment, the weight of the liver of the animals treated with 10.0 and 12.5% casein/gelatin diets, and the
liver protein retention of the 12.5% casein/gelatin diet group of animals were lower than those of the control
group.
Conclusion
Gelatin decreases food efficiency and high-quality protein bioavailability in protein-restricted diets.
Keywords: Amino acids. Biological availability. Caseins. Gelatin. Proteins. Rats.
1
Universidade Federal de Goiás, Faculdade de Nutrição, Programa de Pós-Graduação em Nutrição e Saúde. R. 227, Qd. 68,
74605-080, Goiânia, GO, Brasil. Correspondência para/Correspondence to: MMV NAVES. E-mail: <mmvnaves@gmail.com>.
Support: Conselho Nacional de Desenvolvimento Científico e Tecnológico.
422 | CCD BORDIN & MMV NAVES
Rev. Nutr., Campinas, 28(4):421-430, jul./ago., 2015
Revista de Nutrição
http://dx.doi.org/10.1590/1415-52732015000400008
R E S U M O
Objetivo
A gelatina é deficiente em todos os aminoácidos indispensáveis, mas é usada em dietas com restrição de
proteína. A eficiência alimentar e a qualidade da proteína de misturas de caseína com gelatina em dietas com
baixo teor de proteína foram investigadas em ratos Wistar.
Métodos
Ratos foram tratados com dietas restritas em proteína (10,0 e 12,5%), contendo, como fonte de proteína:
caseína (dieta controle), misturas de caseína com gelatina (4:1 do teor de proteína) e gelatina. Foram estimados
os seguintes índices: taxa de conversão alimentar, quociente de eficiência proteica, quociente de eficiência
proteica relativo e corrigido, digestibilidade verdadeira da proteína e parâmetros hepáticos.
Resultados
Após 28 dias de experimento, a eficiência alimentar da dieta caseína:gelatina a 10,0% diminuiu em comparação
com a dieta de caseína a 10,0%, e o quociente de eficiência proteica das misturas caseína: gelatina (10,0%=2,41
e 12,5%=2,03) foi menor do que aqueles da caseína (10,0%=2,90 e 12,5%=2,32). Após 42 dias de experimento,
o peso do fígado dos animais tratados com mistura caseína:gelatina a 10,0 e 12,5% e a retenção proteica no
fígado dos animais do grupo caseína: gelatina a 12,5% diminuíram em comparação ao grupo-controle.
Conclusão
Gelatina reduz a eficiência alimentar e a biodisponibilidade de proteína de alta qualidade em dietas restritas
em proteínas.
Palavras-chave: Aminoácidos. Disponibilidade biológica. Caseínas. Gelatina. Proteínas. Ratos.
I N T R O D U C T I O N
Gelatin is a soluble mixture of polypeptides
produced by the partial hydrolysis of collagen
1
.
Collagen is an insoluble protein of animal origin
that holds organs and tissues together and gives
strength to tendons, among other biological
functions
2,3
. In nutritional terms, gelatin is an
incomplete protein because its amino acid profile
is quite atypical and deficient in all of the
indispensable (essential) amino acids
recommended by the Food and Nutrition Board
of the Institute of Medicine (IOM) of the National
Academy of Sciences
4
. It is known that gelatin
contains no tryptophan
5,6
.
Gelatin and collagen hydrolysate have
been shown to improve skin hydration,
transepidermal water loss, elasticity, and skin
barrier dysfunction
7,8
. Furthermore, they have
been used to promote weight loss because studies
have shown that gelatin can inhibit appetite and
promote satiety
9,10
. However, the use of gelatin
in a protein-restricted diets (like those before and
after gastrointestinal tract surgery, chronic renal
insufficiency treatment diets, and those aimed to
promote weight loss
11,12
) could be a potential
nutritional risk. Insufficient protein intake (in terms
of both quantity and quality) inhibits endogenous
metabolism of proteins, prolongs the inflammatory
phase of the healing process, decreases fibroblast
proliferation, angiogenesis and collagen synthesis,
and impairs tissue repair
12
. Therefore, the use of
gelatin in a protein-deficient diet could slow
patient recovery.
Considering the uses of gelatin in protein-
restricted diets, this study investigated food
efficiency and protein quality of casein (a complete
protein) and gelatin mixtures in low protein diets
administered to Wistar rats.
M E T H O D S
The biological assay was carried out with
36 weanling male Wistar rats (21-23 days old),
which were randomically distributed in six groups
with six animals each, and average body weight
ranging from 51.8 to 54.6 g per group. The rats
were kept in individual cages and under standard
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environmental conditions (12-h light/dark cycle,
relative humidity between 40-70%, temperature
between 20-23°C)
13
. The rats were fed diets
formulated according to the American Institute
of Nutrition (AIN-93G)
14
(Table 1), containing
different sources of protein and reduced protein
contents, as follows: C
10
- 10.0% protein (casein);
C
8
G
2
- 10.0% protein (8.0% casein and 2.0%
gelatin); C
12.5
- 12.5% protein (casein); C
10
G
2.5
- 12.5%
protein (10.0% casein and 2.5% gelatin); G
10
- 10.0%
protein (gelatin), and protein-free diet. In the C
8
G
2
and C
10
G
2.5
diets, 20.0% of casein protein was
replaced with gelatin protein (4:1 of protein
content) to simulate the proportion of gelatin in
a protein-restricted diet, besides the protein
complementation (C
8
G
2
) and supplementation
(C
10
G
2.5
) when compared to the C
10
diet (control).
The protein content in the diets ranged from
approximately 0.0% (protein free) to 10.0% (C
10,
C
8
G
2
and G
10
) and 12.5% (C
12.5
and C
10
G
2.5
), and
all protein diets had similar energy values, which
were estimated using the conversion factors of
4, 4, and 9 kcal/g for protein, carbohydrates and
lipids, respectively
15
(Table 1). Food and water
(filtered) were provided to the animals ad libitum
for 45 days, comprising 3 days of acclimatization
and the following experimental periods: 2 weeks
for the protein-free and G
10
groups and 4 weeks
for the C
10
, C
8
G
2,
C
12.5
and C
10
G
2.5
groups. The
treatment of the protein-free and G
10
groups was
maintained for two weeks only to avoid animal
suffering since they showed severe weight
loss. In order to evaluate the influence of
complementation and supplementation combining
casein and gelatin on the growth of the animals
and the liver protein retention, the treatment of
the C
10
, C
8
G
2,
and C
10
G
2.5
groups were maintained
for 2 more weeks (total: 6 weeks). Food intake
was monitored daily, and the acceptance of each
diet was assessed comparing the amount of food
consumed versus the amount of food offered. The
animals were weighed three times a week. For
the analysis of protein content, the animal feces
were collected over 7 days during the second (for
the protein-free and G
10
groups) or the fourth
week (for other groups) of the experiment. The
Table 1. Ingredients and proximate composition of experimental diets with different contents and sources of protein.
Ingredient (g/100 g)
Casein
Gelatin
L-cystine
Soybean oil
Cellulose
Mineral mix
Vitamin mix
Choline bitartrate
Corn starch
Proximate composition (g/100 g)
Moisture
Protein
Lipids
Total carbohydrates
Ash
Energy value (kcal)
-
-
-
007.0
005.0
003.5
001.0
000.3
083.2
010.4
000.2
006.2
081.1
002.1
308.8
-
011.4
-
007.0
005.0
003.5
001.0
000.3
071.8
010.4
010.8
006.3
070.3
002.1
381.4
011.0
-
000.2
006.7
005.0
003.5
001.0
000.3
072.4
009.8
010.3
007.0
070.6
002.3
386.7
Component
Protein-Free
G
10
C
10
C
8
G
2
008.8
002.3
000.1
006.8
005.0
003.5
001.0
000.3
072.2
008.8
010.7
007.0
070.2
002.3
386.4
011.0
002.8
000.2
006.7
005.0
003.5
001.0
000.3
069.7
009.9
012.8
007.1
068.1
002.2
387.6
C
10
G
2.5
013.7
-
000.2
006.6
005.0
003.5
001.0
000.3
069.6
009.9
012.8
007.0
068.0
002.3
386.4
C
12
Note: PF: Protein-Free, G
10
: 10.0% protein (gelatin), C
10
: 10.0% protein (casein), C
8
G
2
: 10.0% protein (8.0% casein and 2% gelatin),
C
12.5
: 12.5% protein (casein), and C
10
G
2.5
: 12.5% protein (10.0% casein and 2.5% gelatin). Formulation according to AIN-93G
14
.
Casein: 91.1% ±0.29 of protein and 2.9% ±0.21 of lipids, gelatin: 88.0% ±0.63 of protein and 0.1% ±0.01 of lipids. L-cystine: 0.3 g
for each 20 g of casein. Values are means of three replicates, with the exception of total carbohydrates, estimated by difference.
Diet
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liver of C
10
, C
8
G
2
and C
10
G
2.5
groups was removed
at the end of the sixth week. The experiment was
carried out in accordance with the principles and
procedures stated in the guide for the care and
use of laboratory animals
13
, and it was approved
by the Ethics Committee in Research of the
Universidade Federal de Goiás (Protocol n
o
381/10).
The gelatin and casein amino acid contents
were determined after hydrolysis of the protein
in an acid solution using an amino acid analyzer
(Nicolas V, Ribeirão Preto [SP], Brasil) and colorimetric
post-column detection with ninhydrin
16
. The
amino acid profile of a powdered, unflavored
gelatin (a common brand sold in Brazilian markets)
was also analyzed to ensure that the gelatin used
in the experimental diets was similar to gelatin
used for human consumption. The indispensable
Amino Acid Score (AAS) was estimated using the
amino acid analysis results, which were compared
to the requirement pattern for indispensable
amino acids proposed by the IOM
4
, as follows:
AAS=[(mg of amino acids in 1 g of protein test/
mg of amino acids in standard) × 100].
The amino acid profiles of the gelatin used
in the experimental diets and the commercial
gelatin (for human consumption) were similar,
which allows extrapolation of this study results
to the gelatin commonly used for human
consumption. There is no tryptophan in gelatin,
and according to the requirement pattern, gelatin
is also deficient in other indispensable amino
acids
4
. The casein/gelatin mixture (4:1 of protein
content) meets the patterns of amino acid
requirements (Table 2).
The Nitrogen (N) content of casein, gelatin,
experimental diets, and rats feces and liver were
analyzed by the micro-Kjeldahl method using the
factors 6.38 for casein, 5.55 for gelatin, and 6.25
Table 2. Amino acid composition of casein, gelatin, and Amino Acid Score (AAS) according to the Food and Nutrition Board/Institute
of Medicine requirement pattern.
Indispensable (Essential)
Histidine
Isoleucine
Leucine
Lysine
Methionine + Cysteine
Phenylalanine + Tyrosine
Threonine
Tryptophan
Valine
Total
AAS (%)
Dispensable (Non-essential)
Aspragine
Glutamine
Alanine
Arginine
Glycine
Proline
Serine
Total
028.8
038.2
090.6
082.7
032.7
104.8
045.1
016.1
044.6
483.6
130.8
(Methionine + Cysteine)
091.0
181.7
034.6
036.6
020.7
108.5
059.4
532.5
011.1
015.1
030.4
047.7
010.6
030.7
016.9
000.0
022.4
184.9
000.0 (Trp)
058.4
096.4
125.3
097.2
268.9
138.3
030.8
815.1
012.3
016.4
032.8
043.6
008.4
039.6
014.5
000.0
026.5
194.1
0 0.0 (Trp)
062.5
108.6
094.3
087.8
299.0
126.6
027.2
806.0
Amino Acid (mg of amino
acid/g protein)
Casein
Gelatin
(used in diets)
Commercial
gelatin
025.3
033.6
078.6
075.7
028.3
090.0
039.5
012.9
040.2
423.9
113.1
(Methionine + Cysteine)
084.48
164.64
052.74
048.72
070.34
114.46
053.68
589.02
Casein/gelatin
(4:1 of protein content)
18
25
55
51
25
47
27
7
32
287
100
Requirement
pattern
Note:
Casein/gelatin (4:1 of protein content): estimated value, not analyzed. Requirement pattern of indispensable amino acids for preschool
children (from one to three years old)
4
. (Methionine + Cysteine): lower percentage of the amino acid content compared to the requirement pattern
and (Tryptophan): limiting amino acid.
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for feces and liver for converting nitrogen to crude
protein
17
. Lipid content of the diets and livers was
analyzed according to the method described by
Bligh & Dyer
18
. Moisture and ash were analyzed
using the methods defined by the Association of
Official Analytical Chemists
17
, and the total
carbohydrate content of the diets (including fiber)
was calculated by subtracting the weight of total
fat, crude protein, moisture, and ash from the
total weight of the food.
Food and protein efficiency evaluation
The biological availability of the diets was
evaluated using the following parameters:
animals growth and weight gain curve; Food
Conversion Ratio (FCR), which represents the
efficiency of food intake in promoting weight
gain; Protein Efficiency Ratio (PER), which is based
on body weight gain compared to protein intake
over a period of four weeks; Relative Protein
Efficiency Ratio (RPER); Corrected PER, which is
estimated by the PER of the protein test adjusted
for the value of 2.5 (standard value for casein
PER, estimated in diets with 10% protein); and
true protein digestibility, assessed as the difference
between nitrogen intake and nitrogen absorbed
(fecal nitrogen input) and was expressed as a
percentage
19,20
. The hepatic profile of the C
10
,
C
8
G
2
and C
10
G
2.5
groups was evaluated by the
liver weight (g), relative liver weight (g per 100 g
of body weight), and protein and fat contents.
The results of the chemical and biological
assays were expressed as mean and standard
deviation, and the data were submitted to the
Analysis of Variance (Anova) and Tukeys test, with
Figure 1. Body weight (lines) and food intake (bars) of rats treated with different diets over 6 weeks of experiment.
Note: C
10
- 10.0% protein (casein), C
8
G
2
- 10.0% protein (8.0% casein and 2.0% gelatin), C
10
G
2.5
- 12.5% protein (10.0% casein and 2.5%
gelatin), C
12.5
- 12.5% protein (casein), G
10
- 10.0% protein (gelatin), and Protein-Free (PF). After 2 weeks of experiment:
*
Significant differences in
food intake between G
10
and PF groups compared to the other groups;
**
Significant differences in body weight between G
10
and PF groups
compared to the other groups. After 6 weeks of the experiment:
***
Significant difference in body weight between C
10
G
2.5
and C
10
groups (p<0.05).
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significance level set at p<0.05 using the Statistica
Software Stat Soft Inc., 7
th
version (Tulsa, Oklahoma,
United States, 2004).
R E S U L T S
The animals of the C
10
, C
8
G
2
, C
10
G
2.5,
and
C
12.5
groups had similar body weight at the fourth
week of experiment (p>0.05), and the C
10
G
2.5
group had lower body weight gain than C
10
group
at the sixth week (p<0.05) (Figure 1). The G
10
and
protein-free groups showed similar weight loss
(p>0.05) and low diet acceptance (G
10
=42.4%
±6.4 and protein-free=48.2% ±6.7) during the
feeding period when compared to the other
groups after two weeks of experiment
(C
10
=85.6% ±5.5, C
8
G
2
=82.5% ±11.8,
C
10
G
2.5
=81.9% ±11.2 and C
12.5
=73.4% ±9.6).
Although weight gain and food intake of the
protein diets were similar after four weeks of
Figure 2. Total liver weight (g) and liver protein (g) of rats treated with different diets for 6 weeks.
Note: C
10
- 10.0% protein (casein), C
8
G
2
- 10.0% protein (8.0% casein and 2.0% gelatin), and C
10
G
2.5
- 12.5% protein (10.0% casein and 2.5%
gelatin).
a,b
Same lowercase letters above (liver weight) and inside (liver protein) bars and, above, total protein intake indicate no significant difference
(p>0.05).
experiment (Figure 1), the FCR of the experimental
groups were higher than that of the 10% protein
control group in the protein complementation
(C
8
G
2
versus
C
10
) and supplementation (C
10
G
2.5
versus C
10
) diets (Table 3). After six weeks of
experiment, the weight gain was similar in the
protein complementation (C
8
G
2
=141.6 g ±22.0)
diet, but in the protein supplementation diet was
lower in the experimental group (C
10
G
2.5
=122.4 g
±15.5) than in the control group (C
10
=157.5 g
±19.8) (p<0.05).
The protein efficiency ratio of the diets
with the casein/gelatin mixtures was lower than
those of their respective controls (C
8
G
2
versus C
10
and C
10
G
2.5
versus C
12.5
), as well as in the protein
supplementation diet (C
10
G
2.5
)
compared to the
C
10
diet. The corrected PER of the C
8
G
2
diet was
approximately 2.0 when adjusted to the standard
value of casein PER. The True Protein Digestibility
of the diets was approximately 95% (Table 3).
Total protein intake (g):
C
10
= 61.7 + 5.5
a0
C
8
G
2
= 67.9 + 5.3
a0
C
10
G
2.5
= 69.0 + 5.93
a
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The liver weight (g) and the relative liver
weight of the casein/gelatin groups (C
8
G
2
=3.8%
±0.2, C
10
G
2.5
=3.6% ±0.4) were lower than those
of the control group (C
10
=4.2% ±0.4); the liver
protein (g) of the C
10
G
2.5
group was lower than
that of the C
10
group (p<0.05), and there were
no differences in the protein intake between the
three groups of animals (Figure 2). The total liver
fat was similar in all of the three groups (p>0.05),
as follows: C
10
=0.28 g ±0.07; C
8
G
2
=0.28 g ±0.06,
and C
10
G
2.5
=0.22 g ±0.02.
D I S C U S S I O N
Gelatin has lower contents of all the
indispensable amino acids than those considered
necessary for human health
4
; its AAS is zero. On
the other hand, some dispensable amino acids
(alanine, arginine, glycine, and proline) are present
in gelatin in excessive amounts when compared
to other proteins of animal origin
19-21
, such as
casein (Table 2). This atypical and imbalanced
amino acid profile in relation to human protein
requirements suggests that gelatin should not be
consumed by humans as a primary source of
protein
4,5
.
The animals fed only gelatin as protein
source lost body weight in a similar way to those
Table 3. Weight gain, food and protein intake, and protein utilization in rats after four weeks of experiment.
Weight gain (g)
Total energy intake (kcal)
Daily food intake (g)
Food intake (%)
Total protein intake (g)
FCR
PER
RPER
Corrected PER
True digestibility (%)
Note: C
10
: 10.0% protein (casein), C
8
G
2
: 10.0% protein (8.0% casein and 2.0% gelatin), C
12.5
: 12.5% protein (casein) and C
10
G
2.5
: 12.5% protein
(10% casein and 2.5% gelatin). Different lowercase letters within a row indicate significant differences (p<0.05). Food intake=[(diet offered - diet
rejected)/ diet offered) x 100].
FCR: Food Conversion Ratio, PER: Protein Efficiency Ratio, RPER: Relative PER; M: Mean; SD: Standard Deviation.
Parameter
0114.0
1467.0
0013.6
0081.5
0039.2
0003.35
00002.90
0
00
95.3
015.9
a
0
145.1
a
0
00
1.3
a
0
00
6.6
a
0
00
3.9
a
0
00
0.19
c
000.17
a
001.0
a
0
M
SD
100.0
a
00 2.50
a
C
10
Diet
0099.60
1491.60
00
13.80
00
80.50
00
41.30
000
3.91
00002.410
00
83.60
000
2.08
0094.90
00
14.7
a
0
149.8
a
0001.4
a
0
000
7.4
a
0
000
4.2
a
0
000
0.34
a
0000.22
b
0007.7
b
0
000
0.19
b
0000.5
a
0
M
SD
C
8
G
2
0106.30
1382.50
00
12.80
00
78.30
00
45.80
000
3.39
0002.32
0094.60
0
16.5
aa
0
157.7
aa
0
00
1.5
a
0
a
008.0
a
0
a
005.2
aa
0
00
0.28
b,c
000.19
ba
001.2
a
0
a
M
SD
C
12.5
100.0
a
-
0187.60
1296.80
00
12.00
00
73.10
00
42.80
000
3.86
0002.03
087.7
0094.30
0
16.1
a
0
160.4
a
0
00
1.5
a
0
00
7.5
a
0
00
5.3
a
0
000
0.32
a,b
000.16
c
006.9
b
0
00
0.8
a
0
M
SD
C
10
G
2.5
-
fed the protein-free diet, as reported in other
studies
22,23
. This result can be explained by the
low protein quality of gelatin since its deficiency
in indispensable amino acids inhibits endogenous
protein synthesis. Furthermore, there is evidence
in the literature that rats reject diets deficient in
indispensable amino acids. As a result, the food
intake of the animals is lower than their energy
requirements
24
. This was confirmed in the present
study as the G
10
group rejected more than half
(57.6%) of the diet offered. The low energy intake
along with the deficiency in indispensable amino
acids accelerated the protein malnutrition of the
animals
25
.
The similar food and energy intake resulted
in no differences in the weight gain between the
casein and casein/gelatin groups during the four
weeks of experiment. Therefore, the different
protein sources did not influence the weight gain
in the different groups of animals. In addition,
the similar energy intake between the treatments
did not interfere with the bioavailability of protein
sources, as recommended for in vivo protein
quality evaluation
19,25
.
Food conversion ratio of the experimental
groups was higher than that of the control group,
both regarding casein complementation (C
8
G
2
428 | CCD BORDIN & MMV NAVES
Rev. Nutr., Campinas, 28(4):421-430, jul./ago., 2015
Revista de Nutrição
http://dx.doi.org/10.1590/1415-52732015000400008
versus C
10
) and supplementation (C
10
G
2.5
versus
C
10
) with gelatin. Therefore, food efficiency was
lower when gelatin was present in the diet,
making it necessary a higher intake of the casein/
gelatin diet than the control diet (10% protein)
in order to reach similar weight gain.
The high quality of casein was reduced
when it was complemented with gelatin at the
two protein levels evaluated (C
8
G
2
versus C
10
and
C
10
G
2.5
versus C
12.5
), as well as when it was
supplemented with gelatin (C
10
G
2.5
versus C
10
).
PER is a more sensitive method to evaluate protein
quality since it uses growing animals, which have
a higher protein requirement than adult animals.
Furthermore, the PER index takes longer to
complete than other methods, such as Net Protein
Ratio (NPR) (28 days versus 10-14 days,
respectively). For these reasons, the PER is sensitive
to small differences in protein quality
19,25,26
.
The reference protein efficiency ratio of
casein is 2.5
18,25
, based on the Association of
Official Analytical Chemists (AOAC) official
method
17
, which does not include supplementation
with sulfur-containing amino acids. However,
when the AIN-93G diet at 10% protein is used,
the PER value of casein is higher than 2.5, as found
in the present study, because the AIN diet is
supplemented with L-cystine in order to correct
the caseins deficiency in sulfur-containing amino
acids
12
. The corrected PER (adjusted to 2.5) of the
C
8
G
2
diet was lower than that of casein (p<0.05)
and close to 2.0, which is the minimum requirement
for a good quality protein
25
.
The high protein digestibility of the diets
(approximately 95%) indicates that gelatin did not
influence casein digestibility. The absorption of
the amino acids in gelatin is facilitated since it is
a partially-hydrolyzed protein derived from
collagen, and its digestibility is comparable to that
of casein
3,21
. Therefore, it can be concluded that
protein digestibility did not influence the decrease
in protein efficiency in the casein/gelatin mixture.
After six weeks of experiment, the total
protein intake of the groups was similar, but in
the protein supplementation diet (C
10
G
2.5
), the
hepatic protein was lower than that of control
group. This fact suggests that the additional
gelatin decreases the protein retention in the liver,
i.e., the protein bioavailability.
Considering that the casein/gelatin
mixture (4:1 of protein content) has sufficient
indispensable amino acid content, as well as high
digestibility, this mixture could be expected to
have protein quality close to that of casein.
However, this study showed that gelatin
negatively influenced the protein quality of the
casein/gelatin mixture. One possible explanation
for this result could be the high contents of some
dispensable amino acids in gelatin, such as
glycine. During the protein absorption by the
amino acid transport system, glycine (like other
amino acids) has affinity for more than one carrier
in the intestinal brush border. Therefore,
molecules with a similar chemical structure
compete for the same carrier
26
. As a result, the
absorption of some indispensable amino acids can
be impaired due to the excess of dispensable
amino acids, which reduces the bioavailability of
indispensable amino acid
27,28
. Thus, further
investigations are needed on the reduction in
protein bioavailability by gelatin when it is
mixed with a high-quality protein, for example,
performing a fecal amino acid profile analysis. We
also suggest further investigation on the influence
of gelatin on malnutrition recovery in different
protein mixtures since it can be used to induce
experimental malnutrition
29
.
In the present study, using casein and
gelatin mixtures of (20% of the protein content),
it was observed that gelatin decreases the
efficiency of a high-quality protein in protein-
restricted diets. This finding suggests that in some
clinical practices, mainly in situations that involve
protein catabolism, gelatin, or partially-hydrolyzed
collagen could eventually worsen the catabolic
status and thus slow patient recovery. Moreover,
in cases of high rate of catabolism, such as major
burns, cancer, and malnutrition, special attention
to the supplements prescription is suggested
because these products can have gelatin as source
of protein or amino acids.
GELATIN DECREASES CASEIN BIOAVAILABILITY | 429
Rev. Nutr., Campinas, 28(4):421-430, jul./ago., 2015 Revista de Nutrição
http://dx.doi.org/10.1590/1415-52732015000400008
C O N C L U S I O N
Gelatin interferes with protein efficiency
of high-quality protein. This influence depends
on the gelatin concentration in the mixtures and
the amount of protein in the diet. When there is
low protein intake, gelatin decreases food
efficiency and the bioavailability of high-quality
protein.
A C K N O W L E D G E M E N T S
The authors are grateful for the financial
support provided by the Conselho Nacional de
Desenvolvimento Científico e Tecnológico (research
scholarship) and to the Professors Alceu Afonso Jordão
Júnior and Daniela Canuto Fernandes for theoretical
support.
C O N T R I B U T O R S
CCD BORDIN was responsible for data
collection, literature review, and manuscript writing.
MMV NAVES was responsible for the research design
(idea and planning), participated in data analysis and
synthesis, and contributed to the drafting and critical
revision of the manuscript.
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Received: July 16, 2014
Final version: April 29, 2015
Approved: May 22, 2015
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