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Revista de Nutrição

Print version ISSN 1415-5273On-line version ISSN 1678-9865

Rev. Nutr. vol.28 no.4 Campinas July/Aug. 2015

http://dx.doi.org/10.1590/1415-52732015000400008 

Original Articles

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  

1Universidade Federal de Goiás, Faculdade de Nutrição, Programa de Pós-Graduação em Nutrição e Saúde. R. 227, Qd. 68, 74605-080, Goiânia, GO, Brasil

ABSTRACT

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.

Key words: Amino acids; Biological availability; Caseins; Gelatin; Proteins; Rats

RESUMO

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

INTRODUCTION

Gelatin is a soluble mixture of polypeptides produced by the partial hydrolysis of collagen1. Collagen is an insoluble protein of animal origin that holds organs and tissues together and gives strength to tendons, among other biological functions2 , 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 Sciences4. It is known that gelatin contains no tryptophan5 , 6.

Gelatin and collagen hydrolysate have been shown to improve skin hydration, transepidermal water loss, elasticity, and skin barrier dysfunction7 , 8. Furthermore, they have been used to promote weight loss because studies have shown that gelatin can inhibit appetite and promote satiety9 , 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 loss11 , 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 repair12. 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.

METHODS

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 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: C10 - 10.0% protein (casein); C8G2 - 10.0% protein (8.0% casein and 2.0% gelatin); C12.5 - 12.5% protein (casein); C10G2.5 - 12.5% protein (10.0% casein and 2.5% gelatin); G10 - 10.0% protein (gelatin), and protein-free diet. In the C8G2 and C10G2.5diets, 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 (C8G2) and supplementation (C10G2.5) when compared to the C10 diet (control). The protein content in the diets ranged from approximately 0.0% (protein free) to 10.0% (C10, C8G2 and G10) and 12.5% (C12.5 and C10G2.5), 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, respectively15 (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 G10 groups and 4 weeks for the C10, C8G2, C12.5 and C10G2.5groups. The treatment of the protein-free and G10 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 C10, C8G2, and C10G2.5 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 G10 groups) or the fourth week (for other groups) of the experiment. The liver of C10, C8G2 and C10G2.5 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 animals13, and it was approved by the Ethics Committee in Research of the Universidade Federal de Goiás (Protocol no 381/10).

Table 1. Ingredients and proximate composition of experimental diets with different contents and sources of protein. 

Component Diet
Protein-Free G10 C10 C8G2 C12 C10G2.5
Ingredient (g/100 g)
Casein
Gelatin
L-cystine
Soybean oil
Cellulose
Mineral mix
Vitamin mix
Choline bitartrate
Corn starch
-
-
-
7.0
5.0
3.5
1.0
0.3
83.2
-
11.4
-
7.0
5.0
3.5
1.0
0.3
71.8
11.0
-
0.2
6.7
5.0
3.5
1.0
0.3
72.4
8.8
2.3
0.1
6.8
5.0
3.5
1.0
0.3
72.2
13.7
-
0.2
6.6
5.0
3.5
1.0
0.3
69.6
11.0
2.8
0.2
6.7
5.0
3.5
1.0
0.3
69.7
Proximate composition (g/100 g)
Moisture
Protein
Lipids
Total carbohydrates
Ash
Energy value (kcal)
10.4
0.2
6.2
81.1
2.1
308.8
10.4
10.8
6.3
70.3
2.1
381.4
9.8
10.3
7.0
70.6
2.3
386.7
8.8
10.7
7.0
70.2
2.3
386.4
9.9
12.8
7.0
68.0
2.3
386.4
9.9
12.8
7.1
68.1
2.2
387.6

Note: PF: Protein-Free, G10: 10.0% protein (gelatin), C10: 10.0% protein (casein), C8G2: 10.0% protein (8.0% casein and 2% gelatin), C12.5: 12.5% protein (casein), and C10G2.5: 12.5% protein (10.0% casein and 2.5% gelatin). Formulation according to AIN-93G14. 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.

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 ninhydrin16. 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 IOM4, as follows: AAS=[(mg of amino acids in 1 g of protein test/mg of amino acids in standard) à - 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 acids4. The casein/gelatin mixture (4:1 of protein content) meets the patterns of amino acid requirements (Table 2).

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. 

Amino Acid (mg of amino acid/g protein) Casein Gelatin (used in diets) Commercial gelatin Casein/gelatin
(4:1 of protein content)
Requirement pattern
Indispensable (Essential)
Histidine
Isoleucine
Leucine
Lysine
Methionine + Cysteine
Phenylalanine + Tyrosine
Threonine
Tryptophan
Valine
Total
AAS (%)
28.8
38.2
90.6
82.7
32.7
104.8
45.1
16.1
44.6
483.6
130.8
11.1
15.1
30.4
47.7
10.6
30.7
16.9
0.0
22.4
184.9
0.0 (Trp)
12.3
16.4
32.8
43.6
8.4
39.6
14.5
0.0
26.5
194.1
0.0 (Trp)
25.3
33.6
78.6
75.7
28.3
90.0
39.5
12.9
40.2
423.9
113.1
18
25
55
51
25
47
27
7
32
287
100
(Methionine + Cysteine) (Methionine + Cysteine)
Dispensable (Non-essential)
Aspragine
Glutamine
Alanine
Arginine
Glycine
Proline
Serine
Total
91.0
181.7
34.6
36.6
20.7
108.5
59.4
532.5
58.4
96.4
125.3
97.2
268.9
138.3
30.8
815.1
62.5
108.6
94.3
87.8
299.0
126.6
27.2
806.0
84.48
164.64
52.74
48.72
70.34
114.46
53.68
589.02

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.

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 for feces and liver for converting nitrogen to crude protein17. Lipid content of the diets and livers was analyzed according to the method described by Bligh & Dyer18. Moisture and ash were analyzed using the methods defined by the Association of Official Analytical Chemists17, 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 percentage19 , 20. The hepatic profile of the C10, C8G2 and C10G2.5 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 Tukey's test, with significance level set at p<0.05 using the Statistica Software Stat Soft Inc., 7th version (Tulsa, Oklahoma, United States, 2004).

RESULTS

The animals of the C10, C8G2, C10G2.5, and C12.5 groups had similar body weight at the fourth week of experiment (p>0.05), and the C10G2.5 group had lower body weight gain than C10 group at the sixth week (p<0.05) (Figure 1). The G10 and protein-free groups showed similar weight loss (p>0.05) and low diet acceptance (G10=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 (C10=85.6% ±5.5, C8G2=82.5% ±11.8, C10G2.5=81.9% ±11.2 and C12.5=73.4% ±9.6). Although weight gain and food intake of the protein diets were similar after four weeks of experiment (Figure 1), the FCR of the experimental groups were higher than that of the 10% protein control group in the protein complementation (C8G2 versus C10) and supplementation (C10G2.5 versus C10) diets (Table 3). After six weeks of experiment, the weight gain was similar in the protein complementation (C8G2=141.6 g ±22.0) diet, but in the protein supplementation diet was lower in the experimental group (C10G2.5=122.4 g ±15.5) than in the control group (C10=157.5 g ±19.8) (p<0.05).

Figure 1. Body weight (lines) and food intake (bars) of rats treated with different diets over 6 weeks of experiment. Note: C10 - 10.0% protein (casein), C8G2 - 10.0% protein (8.0% casein and 2.0% gelatin), C10G2.5 - 12.5% protein (10.0% casein and 2.5% gelatin), C12.5 - 12.5% protein (casein), G10 - 10.0% protein (gelatin), and Protein-Free (PF). After 2 weeks of experiment: *Significant differences in food intake between G10 and PF groups compared to the other groups; **Significant differences in body weight between G10 and PF groups compared to the other groups. After 6 weeks of the experiment: ***Significant difference in body weight between C10G2.5 and C10 groups (p<0.05). 

Table 3. Weight gain, food and protein intake, and protein utilization in rats after four weeks of experiment. 

Parameter Diet
C10 C8G2 C12.5 C10G2.5
M SD M SD M SD M SD
Weight gain (g)
Total energy intake (kcal)
Daily food intake (g)
Food intake (%)
Total protein intake (g)
FCR
PER
114.0
1467.0
13.6
81.5
39.2
3.35
2.90
15.9a
145.1a
1.3a
6.6a
3.9a
0.19c
0.17a
99.6
1491.6
13.80
80.50
41.30
3.91
2.41
14.7a
149.8a
1.4a
7.4a
4.2a
0.34a
0.22b
7.7b
0.19b
0.5a
106.3
1382.5
12.8
78.3
45.8
3.39
2.32
16.5aa
157.7aa
1.5aa
8.0aa
5.2aa
0.28b,c
0.19ba
187.6
1296.8
12.0
73.1
42.8
3.86
2.03
16.1a
160.4a
1.5a
7.5a
5.3a
0.32a,b
0.16c
RPER
Corrected PER
100.0a
00 2.50a
83.6
2.08
100.0a
-
87.7 6.9b
-
True digestibility (%) 95.3 1.0a 94.9 94.6 1.2a 94.3 0.8a

Note: C10: 10.0% protein (casein), C8G2: 10.0% protein (8.0% casein and 2.0% gelatin), C12.5: 12.5% protein (casein) and C10G2.5: 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.

The protein efficiency ratio of the diets with the casein/gelatin mixtures was lower than those of their respective controls (C8G2 versus C10 and C10G2.5 versus C12.5), as well as in the protein supplementation diet (C10G2.5) compared to the C10 diet. The corrected PER of the C8G2 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).

The liver weight (g) and the relative liver weight of the casein/gelatin groups (C8G2=3.8% ±0.2, C10G2.5=3.6% ±0.4) were lower than those of the control group (C10=4.2% ±0.4); the liver protein (g) of the C10G2.5 group was lower than that of the C10group (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: C10=0.28 g ±0.07; C8G2=0.28 g ±0.06, and C10G2.5=0.22 g ±0.02.

Figure 2. Total liver weight (g) and liver protein (g) of rats treated with different diets for 6 weeks. Note: C10 - 10.0% protein (casein), C8G2 - 10.0% protein (8.0% casein and 2.0% gelatin), and C10G2.5 - 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). 

DISCUSSION

Gelatin has lower contents of all the indispensable amino acids than those considered necessary for human health4; 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 origin19 - 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 protein4 , 5.

The animals fed only gelatin as protein source lost body weight in a similar way to those fed the protein-free diet, as reported in other studies22 , 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 requirements24. This was confirmed in the present study as the G10 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 animals25.

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 evaluation19 , 25.

Food conversion ratio of the experimental groups was higher than that of the control group, both regarding casein complementation (C8G2 versus C10) and supplementation (C10G2.5 versus C10) 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 (C8G2 versus C10 and C10G2.5 versus C12.5), as well as when it was supplemented with gelatin (C10G2.5 versus C10). 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 quality19 , 25 , 26.

The reference protein efficiency ratio of casein is 2.518 , 25, based on the Association of Official Analytical Chemists (AOAC) official method17, 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 casein's deficiency in sulfur-containing amino acids12. The corrected PER (adjusted to 2.5) of the C8G2diet was lower than that of casein (p<0.05) and close to 2.0, which is the minimum requirement for a good quality protein25.

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 casein3 , 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 (C10G2.5), 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 carrier26. 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 acid27 , 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 malnutrition29.

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.

CONCLUSION

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.

ACKNOWLEDGEMENTS

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.

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Support: Conselho Nacional de Desenvolvimento Científico e Tecnológico.

Received: July 16, 2014; Revised: April 29, 2015; Accepted: May 22, 2015

Correspondência para/Correspondence to: MMV NAVES. E-mail: <mmvnaves@gmail.com>

CONTRIBUTORS 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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