The Impact of Fermented Gamma-Aminobutyric Acid on Poultry Growth Performance Through Insulin-like Growth Factor-1 Activation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

 

This review summarized the methods for production of Gamma-aminobutyric acid (GABA) through fermentation, mechanisms of GABA action including biochemistry and physiological functions of GABA with its molecular mechanisms. In addition, the authors described insulin-like growth factor 1 (IGF-1) pathway involved in GABA action. The content of this manuscript is too widespread in diverse species and various tissues as well as with different functions. It should focus on “Fermented GABA” and “Poultry Growth Performance”. Many studies were not carried out with “Fermented GABA” as it appears in the title.

 

Author Response

Revision Notes

These revision notes are disclosed to address comments/reviews on " fermentation-3434296", entitled: The Impact of Fermented Gamma-aminobutyric acid on Poultry Growth Performance through Insulin-like Growth Factor-1 Activation.

Introduction:

The authors would like to express their gratitude to the MDPI fermentation for permitting us to revise our manuscript, thereby improving the quality of the revised manuscript before the high standard of this publication channel.

The authors attempted as best as possible to strike a balance between conciseness and results reporting. The aim is to create an informative paper for the readers that clearly articulates the key components. The remarkable improvements of the revised manuscript are:

Major and comprehensive clarifications have been made based on the comments from the Reviewers, as well as being implemented in the corresponding section of the revised manuscript.

The modified/revised parts of the manuscript are

Reviewer 1 Comments:

Comment 1: This review summarized the methods for production of Gamma-aminobutyric acid (GABA) through fermentation, mechanisms of GABA action including biochemistry and physiological functions of GABA with its molecular mechanisms. In addition, the authors described insulin-like growth factor 1 (IGF-1) pathway involved in GABA action. The content of this manuscript is too widespread in diverse species and various tissues as well as with different functions. It should focus on “Fermented GABA” and “Poultry Growth Performance”. Many studies were not carried out with “Fermented GABA” as it appears in the title.

Response to Comment 1:

The authors would like to thank the reviewer for the careful and thorough reading of this manuscript and for the thoughtful comments and constructive suggestions, which helped to improve the quality of this manuscript. We have thoroughly checked and improved the technicality of the manuscripts and incorporated the possible suggested comments by Reviewer 1. According to the reviewer's suggestion, the following changes have been made to the revised version of the manuscript.

We would like to clarify that this review paper aims to present a comprehensive overview of Gamma-aminobutyric acid (GABA), with a primary emphasis on its production through fermentation, its biochemical and physiological mechanisms of action, and its potential to enhance poultry growth performance via the Insulin-like Growth Factor-1 (IGF-1) pathway. While we understand that focusing exclusively on “GABA fermentation” and “Poultry growth performance” would narrow the discussion, we intentionally included findings from diverse species and tissues to provide readers with a broader context of GABA's mechanisms and functions. This broader perspective of this comprehensive review paper serves two fundamental purposes:

Providing a contextual framework: By examining GABA’s role in various species and tissues, we offer insights into its universal biochemical mechanisms and physiological effects, many of which are likely relevant to poultry but may not yet have been directly studied in this context.

Bridging Knowledge Gaps: With limited studies specifically addressing "fermented GABA" in poultry, research from other species provides valuable insights into mechanisms and pathways that may influence poultry growth performance.

We would also like to emphasize that the manuscript maintains a clear focus on "fermented GABA" and its role in improving "poultry growth performance" throughout. However, as a comprehensive review, we aimed to include additional details on GABA production and its broader mechanisms to make the paper more informative and useful to a wider audience in this field.

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

This review manuscript is generally written well.

Comments:

1. The title of this review manuscript states, "fermented gamma-aminobutyric acid". Does it mean there is unfermented GABA? if yes, what is the difference between fermented vs unfermented GABA?

2. What would be the effect of glutamate, the precursor of GABA if the animals receive glutamate instead?

3. Please list all abbreviations.

4. Figure 1: Any different type of GAD? Is GAD on the figure GAD65?

5. Figure 1: the schematic pathway in Figure 1 is misleading. It looks like acetyl-CoA goes to citrate. And oxaloacetate goes to citrate too in the different way. However, it should be acetyl-CoA (2 carbons) + oxaloacetate (4 carbons) --> citrate (6 carbons). 

6. Please provide a schematic illustration for GABA-IGF1 pathway as a Figure.

Author Response

Revision Notes

These revision notes are disclosed to address comments/reviews on " fermentation-3434296", entitled: The Impact of Fermented Gamma-aminobutyric acid on Poultry Growth Performance through Insulin-like Growth Factor-1 Activation.

Introduction:

The authors would like to express their gratitude to the MDPI fermentation for permitting us to revise our manuscript, thereby improving the quality of the revised manuscript before the high standard of this publication channel.

The authors attempted as best as possible to strike a balance between conciseness and results reporting. The aim is to create an informative paper for the readers that clearly articulates the key components. The remarkable improvements of the revised manuscript are:

Major and comprehensive clarifications have been made based on the comments from the Reviewers, as well as being implemented in the corresponding section of the revised manuscript.

The modified/revised parts of the manuscript are

Reviewer 2 Comments:

Comment 1: The title of this review manuscript states, "fermented gamma-aminobutyric acid". Does it mean there is unfermented GABA? if yes, what is the difference between fermented vs unfermented GABA?

Response to Comment 1:

The authors would like to thank the reviewer for the careful and thorough reading of this manuscript and for the thoughtful comments and constructive suggestions, which helped to improve the quality of this manuscript. We have thoroughly checked and improved the technicality of the manuscripts and incorporated the possible suggested comments by Reviewer 2. According to the reviewer's suggestion, the following changes have been made to the revised version of the manuscript.

GABA can be obtained from both natural (unfermented) sources and through fermentation. However, this review specifically focuses on the production of GABA via microbial fermentation, a process that uses microorganisms such as Lactobacillus plantarum, Rhizopus oryzae, and other species to biosynthesize GABA. Fermented GABA typically differs from unfermented GABA in its production method and purity. The fermentation process not only enhances the production efficiency of GABA but may also confer additional functional benefits, such as enriching the product with other bioactive compounds that are co-produced during microbial fermentation.

While the title emphasizes "Fermented GABA," it is not meant to imply a direct comparison with unfermented GABA. Rather, it reflects the primary focus of this paper, which is on GABA synthesized via fermentation and its subsequent impact on poultry growth performance through the IGF-1 pathway. We have designed this review as a comprehensive overview that encompasses GABA production through fermentation, its mechanisms of action, and its physiological significance, with an emphasis on its application in poultry nutrition.

Aspect	Natural GABA	Fermented GABA
Source	Plants, sprouts, and raw foods.	Microbial fermentation of substrates.
Concentration	Typically low and variable.	Higher and controllable.
Production Process	Direct extraction from natural sources.	Involves microbial metabolism.
Applications	Limited to raw dietary intake.	Functional foods, supplements, and animal feed.

Comparison of Natural and Fermented GABA

 

Comment 2: What would be the effect of glutamate, the precursor of GABA if the animals receive glutamate instead?

Response to Comment 2:

Glutamate and GABA serve distinct physiological roles despite being biochemically related. Glutamate functions primarily as an excitatory neurotransmitter and a metabolic substrate, while GABA acts as an inhibitory neurotransmitter with additional modulatory effects on physiological pathways. Administering glutamate to animals may not replicate the same effects as GABA, as the conversion of glutamate to GABA is dependent on specific enzymatic activity (glutamate decarboxylase, GAD) and cellular conditions within the animal. Furthermore, excessive glutamate intake could lead to excitotoxicity, particularly in the central nervous system, due to overstimulation of glutamate receptors. The focus of this review is specifically on GABA produced via fermentation and its role in poultry growth performance, particularly through the Insulin-like Growth Factor-1 (IGF-1) pathway. Fermented GABA, in addition to its primary effects, may also provide other bioactive compounds co-produced during fermentation, which are absent in direct glutamate supplementation. These compounds may synergistically enhance the physiological benefits observed in poultry.

The effects of glutamate supplementation in poultry remain an interesting area for future exploration, it is beyond the primary scope of this review. We aim to provide a comprehensive overview of the production, mechanisms, and applications of fermented GABA in the context of poultry growth performance.

Comment 3: Please list all abbreviations.

Response to Comment 3:

                                     List of Abbreviations

Name                                                                                    Abbreviation

Gamma-aminobutyric acid                                                            GABA

Insulin-like Growth Factor-1                                                        IGF-1

Glutamate 54 decarboxylase                                                         GAD

Insulin-like Growth Factor-1 receptor                                          IGF-1R

Lactic acid bacteria                                                                        LAB

Reactive oxygen species                                                                ROS

Nitric oxide                                                                                     NO

Enteric nervous system                                                                  ENS

Blood-brain barrier                                                                        BBB

Central nervous system                                                                 CNS

IGF binding proteins                                                                      IGFBPs

Cyclic adenosine monophosphate                                                 Camp

Growth hormone-releasing hormone                                            GHRH

Phosphoinositide 3-kinase                                                             PI3K

Mitogen-activated protein kinase                                                 MAPK

 

Comment 4: Figure 1: Any different type of GAD? Is GAD on the figure GAD65?

Response to Comment 4:

In the context of this figure, the enzyme GAD refers to glutamate decarboxylase in general, which catalyzes the conversion of glutamate to Gamma-aminobutyric acid (GABA) by decarboxylation. This reaction is a central step in GABA biosynthesis, occurring both in microbial systems and in animals. The specific type of GAD (e.g., GAD65 or GAD67) is not distinguished in the figure because it represents a generalized depiction of the metabolic pathway, applicable to a variety of organisms, including bacteria used in fermentation and animals. If we were to reference GAD in mammals:

GAD65 and GAD67 are isoforms that differ in their cellular localization and function. GAD65 is predominantly associated with neurotransmitter pools of GABA, while GAD67 is involved in maintaining basal levels of GABA for general metabolic needs.

However, in microbial fermentation systems, the GAD enzyme originates from bacteria such as Lactobacillus plantarum or Lactobacillus brevis, which possess their own GAD enzymes distinct from mammalian isoforms. These bacterial GAD enzymes are specialized for acidic environments, optimizing GABA production during fermentation. As this figure aims to provide a simplified overview of the metabolic pathway, rather than focusing on isoform specificity, it does not specify GAD types.

Comment 5:  Figure 1: the schematic pathway in Figure 1 is misleading. It looks like acetyl-CoA goes to citrate. And oxaloacetate goes to citrate too in the different way. However, it should be acetyl-CoA (2 carbons) + oxaloacetate (4 carbons) --> citrate (6 carbons).

Response to Comment 5: The correct schematic pathway of Figure 1 has been mentioned in the paper as will as uploaded in the PDF attached file.

Comment 6: Please provide a schematic illustration for GABA-IGF1 pathway as a Figure.

Response to Comment 6: The Schematic illustration of GABA-IGF-1 pathway has been mentioned in the paper as will as uploaded in the PDF attached file.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

No more comments