The Relationships between Water Intake and Hydration Biomarkers and the Applications for Assessing Adequate Total Water Intake among Young Adults in Hebei, China

Round 1

Reviewer 1 Report

This study examines the association between water intake and hydration biomarkers. It also explores of the tools used for assessing water intake among young adults in Hebei Province, PRC. The findings are important additions to the existing body of literature. However, several issues should be addressed before this manuscript meets the standard for publication.

Intro:

1. Line 75-77, the authors argued that more research was needed to examine the association in China. Why? What's unique about the Chinese population that warrant more research? Dietary? Urinary functions? Body fat/muscle distribution? A brief rationale would help the readers better understand why this study is needed, and important.
2. What are the significance of the urine and plasma biomarkers? Have they been linked to health outcomes? Info on the significance of the biomarkers would be helpful for the readers to understand the importance of examining the biomarkers.

Methods

1. Figure 1 was nicely made and contains a lot of important info. But it's very hard to read in it's present form. Maybe consider making it a vertical flow to accommodate a bigger font size?
2. Participants: more details are needed on how participants were recruited. Potential self-selection issue and other selection bias should be addressed in the discussion section as well.

Results

1. In table 1, the p values should be "< .001" instead of ".000"
2. Did the author explore any sex differences? Since they seemed to infer in the intro part and the discussion part that sex plays a role in the association between TWI and various biomarkers, it seems to be a legitimate question that warrants some examination.
3. A table showing the descriptive statistics might also be helpful. Important demographic factors such as sex and age, BMI, optimal hydration rate, and dehydration rates should be presented.
4. Figure 2: the x and y axis were distorted, i.e., on different scales. For example, a 1000ml unit appears much shorter on the y axis than it does on the x axis in the upper figure.
5.  

Discussion:

1. Line 259 - 261: why would a male-only study sample differ from the sample of this study in terms of the association between volume of urine and water from food? The authors should not make a speculation without any theoretical or empirical evidence. 

Minor:

1. "TWI" should be spelled out the first time it appeared in the abstract.
2. Line 43: "While" should be "However" or "Meanwhile"?
3. Line 51: should be "of vital importance."
4. Line 56: do you mean "longitudinal studies" or "long-term studies"?
5. Line 58: "the void of 24 h urine" -- not sure what the authors meant by it. Not producing urine for 24 hours?
6. Line 61: "research" not "researches"
7. Overall this paper could significantly benefit from English language editing.

Author Response

Reviewer 1

(x) I would not like to sign my review report
( ) I would like to sign my review report

English language and style

(x) Extensive editing of English language and style required
( ) Moderate English changes required
( ) English language and style are fine/minor spell check required
( ) I don't feel qualified to judge about the English language and style

Comments and Suggestions for Authors

This study examines the association between water intake and hydration biomarkers. It also explores of the tools used for assessing water intake among young adults in Hebei Province, PRC. The findings are important additions to the existing body of literature. However, several issues should be addressed before this manuscript meets the standard for publication.

Intro:

1. Line 75-77, the authors argued that more research was needed to examine the association in China. Why? What's unique about the Chinese population that warrant more research? Dietary? Urinary functions? Body fat/muscle distribution? A brief rationale would help the readers better understand why this study is needed, and important.

Response: Thanks for your comments.

It has been revised accordingly. The relevant references have been added into the manuscript (Lines 85-92, Page 2).

Regarding to the fluid intake among Chinese, there are only limited studies. Only two large surveys about fluid intake were conducted. In 2009, Ma et al. conducted a fluid intake survey among 1483 adults from four cities in China, it was found that approximately 32% of the subjects drank less water than the 1200 mL/day recommended by the Chinese Nutrition Society in 2012. In another survey among 5868 primary and middle school students from the same four cities, it was reported that nearly two-thirds of the subjects drank less than the recommended amount in 2012. The results of above-mentioned surveys imply that large proportion of Chinese residents may be in a state of dehydration. However, the relationship between the fluids intake and hydration biomarkers have not explored among the two surveys.

In addition, in 2015, among the young male adults, the results of our previous study showed that about 75% of them did not meet the recommendation of China. The relationship between the fluids intake and hydration biomarkers was firstly investigated, but only among males.

Studies demonstrated that the fluids intake were influenced by many factors including the age, gender, and temperature. This requires additional study in China to investigate the relationship between the fluid intake and hydration biomarkers due to the following reasons. Firstly, the landscapes vary significantly across its vast area, and the climates vary in different regions due to the highly complex topography in China. Heights are also different among the people of China and other countries due to race differences, which contributes to difference in body surface area and has corresponding effects on water requirements. Secondly, the drinking patterns including the amounts and the types of the fluid intake differ among Chinese, Europeans, and Americans were different. The Chinese prefer tea, while Europeans and Americans tend to drink more coffee. Studies also showed that the types of the fluids intake may affected the hydration status of people, therefore, the differences in drinking patterns may lead to the differences in the hydration biomarkers.

In addition, the contributions of water from food were different among the countries. In Europe and the United States, the proportion of fluid intake from food accounts for about 20% of total fluid intake, whereas in China, the proportion of fluid intake from food accounts for nearly 44% or 50%.

It would be more crucial for China to have more related studies to supply the scientific data, in order to improve the attention of Chinese to take in adequate water. Thus, although the studies related about the relationships between the fluids intake and hydration biomarkers among people had been conducted in some countries, it is still necessary for China to take corresponding studies to supply more information for the relationships between fluids intake and hydration status.

Followed were the added references:

1. Bougatsas D, Arnaoutis G, Panagiotakos DB, Seal AD, Johnson EC, Bottin JH, Tsipouridi S, Kavouras SA: Fluid consumption pattern and hydration among 8-14 years-old children. EUR J CLIN NUTR 2018, 72(3):420.
2. Kenney EL, Long MW, Cradock AL, Gortmaker SL: Prevalence of inadequate hydration among US children and disparities by gender and race/ethnicity: National Health and Nutrition Examination Survey, 2009-12. AM J PUB HEALTH 2015, 105(8):e113–118.
3. Malisova O, Athanasatou A, Pepa A, Husemann M, Domnik K, Braun H, Mora-Rodriguez R, Ortega JF, Fernandez-Elias VE, Kapsokefalou M: Water Intake and Hydration Indices in Healthy European Adults: The European Hydration Research Study (EHRS). NUTRIENTS 2016, 8(204).

1. What are the significance of the urine and plasma biomarkers? Have they been linked to health outcomes? Info on the significance of the biomarkers would be helpful for the readers to understand the importance of examining the biomarkers.

Response: Thanks for your comments.

It has been revised accordingly in the Introduction Sections (Lines 63-69, Page 2).

Studies demonstrated that the hydration biomarkers including the urinary and the plasma biomarkers, were in relation to the health outcomes. For instance, the osmolality of urine indicated the ability of the kidney to concentrate the urine and reflected the antidiuretic action of vasopressin. Besides, it also could be an environmental biomonitoring index among people for measuring the exposure. There was existed an association between the osmolality of plasma and the mortality among patients undergoing hemodialysis. Therefore, it is crucial to explore the hydration biomarkers among people.

Followed were the added references:

1. Torres VE, Grantham JJ, Chapman AB, Mrug M, Bae KT, King BF Jr, Wetzel LH, Martin D, Lockhart ME, Bennett WM, Moxey-Mims M, Abebe KZ, Lin Y, Bost JE; Consortium for Radiologic Imaging Studies of Polycystic Kidney Disease (CRISP). Potentially modifiable factors affecting the progression of autosomal dominant polycystic kidney disease. CLIN J AM SOC NEPHROL 2011, 6(3):640–7.
2. Lee MJ, Chang TI, Lee J, Kim YH, Han SH: Urine Osmolality and Renal Outcome in Patients with Chronic Kidney Disease: Results from the KNOW-CKD. KIDNEY BLOOD PRESS R 2019, 44(5):1-12.
3. Yeh HC, Lin YS, Kuo CC, Weidemann D, Weaver V, Fadrowski J, Neu A, Navas-Acien A: Urine osmolality in the US pop-ulation: Implications for environmental biomonitoring. ENVIRON RES 2015, 136:482-490.
4. Tsujimoto Y, Tsutsumi Y, Ohnishi T, Kimachi M, Yamamoto Y, Fukuhara S: Low Predialysis Plasma Calculated Osmolality Is Associated with Higher All-Cause Mortality: The Japanese Dialysis Outcomes and Practice Patterns Study (J-DOPPS). NEPHRON 2020, 144(3):138-146.

Methods

1. Figure 1 was nicely made and contains a lot of important info. But it's very hard to read in it's present form. Maybe consider making it a vertical flow to accommodate a bigger font size?

Response: Thanks for your comments.

It has been revised accordingly (Line 133, Page 4).

1. Participants: more details are needed on how participants were recruited. Potential self-selection issue and other selection bias should be addressed in the discussion section as well.

Response: Thanks for your comments.

It has been revised accordingly. The recruitment notices were sent through WeChat (Tencent Holdings Ltd., Shenzhen, China) and email. Besides, the advertisements were also posted in the campus publicity window. Furthermore, the participant recruitment campus talks that all college students could participate in were also held (Lines 117-120, Page 3).

The selection bias was discussed in the manuscript (Lines 336-339, Pages 10-11).  

Results

1. In table 1, the p values should be "< .001" instead of ".000"

Response: Thanks for your comments.

It has been revised accordingly in Table 1 (Line 210, Pages 6-7).

1. Did the author explore any sex differences? Since they seemed to infer in the intro part and the discussion part that sex plays a role in the association between TWI and various biomarkers, it seems to be a legitimate question that warrants some examination.

Response: Thanks for your comments. The differences between males and females in the relationships between fluids intake and hydration biomarkers were added into Supplementary Material Table 3 and Table 4.

In this present study, we did not aim to investigate the differences between males and females. The differences between males and females in hydration biomarkers were explored in our previous study, in which showed that the fluids intake and osmolality were all higher in males than that in females (published in Chinese). Other studies also showed that males had higher urine osmolality than females. However, the relationship between fluids intake and hydration biomarkers were not investigated in the studies mentioned above.

We hypnotized that the relationships between fluids intake and hydration biomarkers among males only maybe different with that of adults including males and females. We demonstrated our previous study which conducted among the young male adults, in order to show that there may be a gender bias. Therefore, we evaluate the issue among young male and female adults. Surprisingly, we found the differences of the results between the present study and the study conducted before among males only. In the present study, the relationships between fluids intake and hydration biomarkers among females were mostly higher than those among males. Future studies are needed to investigate the differences between genders.

1. A table showing the descriptive statistics might also be helpful. Important demographic factors such as sex and age, BMI, optimal hydration rate, and dehydration rates should be presented.

Response: Thanks for your comments.

It has been revised accordingly. The information on hydration status has been added into the Supplementary Material Table 1 and had been added into the Results Section (Lines 206-207, Page 6).

1. Figure 2: the x and y axis were distorted, i.e., on different scales. For example, a 1000ml unit appears much shorter on the y axis than it does on the x axis in the upper figure.

Response: Thanks for your comments.

It has been revised accordingly in Figure 2 (Line 253, Page 8). 

Discussion:

1. Line 259 - 261: why would a male-only study sample differ from the sample of this study in terms of the association between volume of urine and water from food? The authors should not make a speculation without any theoretical or empirical evidence. 

Response: Thanks for your comments. It has been revised accordingly in the Discussion Sections (Lines 285-296, Page 10).

In the present study, there was a weak association between the volume of urine and the water from food (r=0.371). While, in the previous study, the strong relationship was found between the urine volume and the water from food (r=0.70). In order to investigate the reasons that attributed to the differences between the two studies, the relationships between the fluid intake and the hydration biomarkers among males and females were explored respectively, which was showed in Supplementary material Table 3. We found that the weak relationships between the water from food and the volume of urine were found in females (r=0.493) and males (r=0.412). We discussed carefully about the findings. Recently, a systematic study showed that the urine productions were affected by multiple dietary factors including the food composition, which may be attributed to the differences. Future research is needed to support current evidence and the mechanisms underlying these findings of the present study.

Minor:

1. "TWI" should be spelled out the first time it appeared in the abstract.
2. Line 43: "While" should be "However" or "Meanwhile"?
3. Line 51: should be "of vital importance."
4. Line 56: do you mean "longitudinal studies" or "long-term studies"?
5. Line 58: "the void of 24 h urine" -- not sure what the authors meant by it. Not producing urine for 24 hours?
6. Line 61: "research" not "researches"
7. Overall this paper could significantly benefit from English language editing.

Response: Thanks for your comments.

The manuscript has been carefully revised by MDPI (English edited 35443), in order to make sure that there were no errors in the English language and grammars throughout the manuscript.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments:

General:

Overall, a good research paper. Hydration is an important subject and exploring hydration status in young people is an area of interests. This is because many studies tend to focus on those on exercise regimes. The  materials and methods section is well laid out and relevant to the objectives of the study. However, there is need for an extensive review of English language in the paper before it can be considered for publication, the tense, especially.

Further comments on each section:

Title: brief and informative.

Abstract: The abstract is succinct and provides enough information to stimulate the reader’s appetite for more.

Introduction:

Hydration is an important subject and the need to understand hydration status cannot be over-emphasised. The introductory section provided a background and context for the research. The authors identified the research questions and gaps in knowledge which led to the aim and objectives of the research.

Materials and methods.

Extensive and detailed description of the methods adopted. The authors used validated or standard methods of measurements, with references provided. The statistical analysis was also detailed and showed great understanding. It will be easy for another researcher to duplicate what has been done.

The study procedure (Fig 1) is good as it provides an overview of what was done. However, it was difficult to view as it was too small. I will suggest that the authors provide a figure with better resolution. Instead of fitting it into a portrait arrangement, it could be made into a landscape.

Results

The result section is very informative and captures accurately what is reported in tables and figures. The use of sub-sections allows each group of results to be discussed adequately. The use of PLS model allowed the authors explore the relationship between total water intake (TWI) and urine biomarkers. (A The authors looked at a variety of indices related to hydration status, which may be a point of interest to researchers in this area.  It is interesting to see that in young adults, there is no relationship between plasma osmolality and total water intake.

Discussion:

The results obtained were discussed, compared, and contrasted with previous findings. The conclusions drawn from the study were based on the findings reported. For example, “24h urinary biomarkers were more sensitive than the first morning urinary biomarkers.” The authors were able to draw meaningful conclusions from different sections of their results, which makes it easy to identify their key findings.

Regarding strengths and weaknesses, the use of “7-day 24h fluid” is a great strength for the study. However, it is not very clear what the authors meant by “more ages of the participants were not investigated”. This may need a bit of clarification.

Conclusion: Brief and gives a good summary of the findings.

Author Response

Reviewer 2

(x) I would not like to sign my review report
( ) I would like to sign my review report

English language and style

(x) Extensive editing of English language and style required
( ) Moderate English changes required
( ) English language and style are fine/minor spell check required
( ) I don't feel qualified to judge about the English language and style

Comments and Suggestions for Authors

Comments:

General:

Overall, a good research paper. Hydration is an important subject and exploring hydration status in young people is an area of interests. This is because many studies tend to focus on those on exercise regimes. The materials and methods section is well laid out and relevant to the objectives of the study. However, there is need for an extensive review of English language in the paper before it can be considered for publication, the tense, especially.

Response: Thanks for your comments.

The manuscript has been carefully revised by MDPI (English edited 35443), in order to make sure that there were no errors in the English language and grammars throughout the manuscript.

Further comments on each section:

Title: brief and informative.

Abstract: The abstract is succinct and provides enough information to stimulate the reader’s appetite for more.

Response: Thanks for your comments.

Introduction:

Hydration is an important subject and the need to understand hydration status cannot be over-emphasised. The introductory section provided a background and context for the research. The authors identified the research questions and gaps in knowledge which led to the aim and objectives of the research.

 Response: Thanks for your comments.

Materials and methods.

Extensive and detailed description of the methods adopted. The authors used validated or standard methods of measurements, with references provided. The statistical analysis was also detailed and showed great understanding. It will be easy for another researcher to duplicate what has been done.

The study procedure (Fig 1) is good as it provides an overview of what was done. However, it was difficult to view as it was too small. I will suggest that the authors provide a figure with better resolution. Instead of fitting it into a portrait arrangement, it could be made into a landscape.

Response: Thanks for your comments.

It has been revised accordingly (Line 133, Page 4).

Results

The result section is very informative and captures accurately what is reported in tables and figures. The use of sub-sections allows each group of results to be discussed adequately. The use of PLS model allowed the authors explore the relationship between total water intake (TWI) and urine biomarkers. (A The authors looked at a variety of indices related to hydration status, which may be a point of interest to researchers in this area.  It is interesting to see that in young adults, there is no relationship between plasma osmolality and total water intake.

Response: Thanks for your comments.

Discussion:

The results obtained were discussed, compared, and contrasted with previous findings. The conclusions drawn from the study were based on the findings reported. For example, “24h urinary biomarkers were more sensitive than the first morning urinary biomarkers.” The authors were able to draw meaningful conclusions from different sections of their results, which makes it easy to identify their key findings.

Response: Thanks for your comments.

Regarding strengths and weaknesses, the use of “7-day 24h fluid” is a great strength for the study. However, it is not very clear what the authors meant by “more ages of the participants were not investigated”. This may need a bit of clarification.

Response: Thanks for your comments.

It has been revised accordingly in the Discussion Section from “more ages of the participants were not investigated” into “participants from a wide range of age groups were not investigated” (Lines 335-336, Page 10).

In China, only two large surveys about fluid intake were conducted. In 2009, Ma et al. conducted a fluid intake survey among 1483 adults from four cities in China, it was found that approximately 32% of the subjects drank less water than the 1200 mL/day recommended by the Chinese Nutrition Society. In another survey among 5868 primary and middle school students from the same four cities, it was reported that nearly two-thirds of the subjects drank less than the recommended amount. The results of above-mentioned surveys imply that large proportion of Chinese residents may be in a state of dehydration. Unfortunately, the hydration biomarkers such as the urine osmolality were not explored in the studies mentioned above. In addition, there was only one study conducted among young male adults investigating the hydration status. It showed that about 75% of them did not meet the recommendation of China in 2015. Therefore, in 2017, the present study was implemented among young adults including the males and females to explore the relationship between the water intake and the hydration biomarkers. However, in our study, the age of the participants was between 19 to 23 years old, did not include the children and adolescents and the adults aged over 23 years old. Then, in the future, more studies should be taken in this issue.  

Conclusion: Brief and gives a good summary of the findings.

Response: Thanks for your comments.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have provided sufficient responses to my comments.