Differences in Vitamin A Levels and Their Association with the Atherogenic Index of Plasma and Subclinical Hypothyroidism in Adults: A Cross-Sectional Analysis in China

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This report examines associations between vitamin A status, the atherogenic index of plasma, and subclinical hypothyroidism.  Note that the acronym for the atherogenic index of plasma (AIP) is not consistently presented in the manuscript (see Table 1 where API is stated).  Based on Table 1, the AIP does not differ by vitamin A status and furthermore, subclinical hypothyroidism does not differ between these groups.  An AIP value of under 0.11 is associated with low risk of CVD and values >0.21 are associated increased risks.  The values reported in this paper are <0.03 on average – suggesting a very low risk in the sample.  Moreover, the prevalence of hypothyroidism is low (~10% of the sample) as is vitamin A insufficiency (<7%).  Thus, interpretation of the data is quite limited for the question posed by the research. 

As a cross-sectional study, it is important to control for confounding variables in the statistical analyses since it is not possible to eliminate these potential biases.  It is well documented that selenium, iodine, and vitamin D statuses impact both hypothyroidism and the AIP/dyslipidemia.  Since the prevalence rates for these nutrient deficiencies are rather high in China (as high as 45% for some samples), these nutrient statuses need to be controlled in the analyses in this report. 

Although the classification of hypothyroidism is stated in the report (TSH > 4.2 with fT4 within normal range), much of the analyses are using other parameters in a manner that is not clearly defined in methods.  Mainly, it is not clear what is meant by the results examining ‘AIP and thyroid hormones and diseases by vitamin A levels’ (see line 156).  What is meant by ‘diseases’? (thyroid diseases?)  These ‘diseases’ need to be defined and the analyses need to be outlined in methods. 

Vitamin A is not a proper noun and should not be capitalized unless at the start of a sentence.  Also, the term ‘vitamin A loss’ is not a defining term.  Is vitamin A deficiency what is meant?  <0.35 µmol/L indicates severe vitamin A deficiency (the cutoff used in this report) – perhaps the authors could consider using serum retinol concentration <0.70 μmol/L which indicates subclinical vitamin A deficiency.

Author Response

Dear Editors and Reviewers,

Thank you for reviewing our manuscript entitled “Vitamin A levels difference in the association between atherogenic index of plasma and subclinical hypothyroidism in adults: A cross-sectional analysis in China” (Submission ID:  nutrients-3117301). We appreciate your insightful comments and suggestions. We have incorporated these to the best extent possible into the revised manuscript. The amendments are highlighted in red in the revised version, and the point-by-point responses to the reviewer’s comments are listed below. We are re-submitting the revised manuscript for your consideration and hope it is acceptable for publication.

Comments 1: This report examines associations between vitamin A status, the atherogenic index of plasma, and subclinical hypothyroidism.Note that the acronym for the atherogenic index of plasma (AIP) is not consistently presented in the manuscript (see Table 1 where API is stated).

Response: Thank you for the thorough review. We have corrected the error in Table 1 and checked the manuscript to ensure that it consistently reads "AIP."

Comments 2: Based on Table 1, the AIP does not differ by vitamin A status and furthermore, subclinical hypothyroidism does not differ between these groups. An AIP value of under 0.11 is associated with low risk of CVD and values >0.21 are associated increased risks. The values reported in this paper are <0.03 on average – suggesting a very low risk in the sample.  Moreover, the prevalence of hypothyroidism is low (~10% of the sample) as is vitamin A insufficiency (<7%).  Thus, interpretation of the data is quite limited for the question posed by the research.

Response: Thank you for this feedback. We agree that this may be a limitation in assessing the associations. We have made revisions as follows:

Additionally, the study is limited by the low variability in the AIP and the prevalence rates of subclinical hypothyroidism and vitamin A deficiency within the sample. This restricts our ability to generalize findings and fully explore the associations between these parameters. Future research should consider including populations with higher or more diverse prevalence rates to enable a more comprehensive analysis of these associations.

Comments 3: As a cross-sectional study, it is important to control for confounding variables in the statistical analyses since it is not possible to eliminate these potential biases. It is well documented that selenium, iodine, and vitamin D statuses impact both hypothyroidism and the AIP/dyslipidemia. Since the prevalence rates for these nutrient deficiencies are rather high in China (as high as 45% for some samples), these nutrient statuses need to be controlled in the analyses in this report.

Response: Thank you for this comment. We agree that it is important to control for potential confounding variables. As suggested, we have included iodine, and vitamin D levels as covariates. We have updated the results in tables, figures, and the manuscript. The main revisions are as follows:

Covariates: Vitamin D levels were quantified using liquid chromatography tandem mass spectrometry. Urine iodine levels was measured using the As3+-Ce4+ catalytic spectrophotometric method.

Methods: All analyses were adjusted for age, BMI, sex, smoking status, vitamin D levels, and urine iodine levels.

Comments 4: Although the classification of hypothyroidism is stated in the report (TSH > 4.2 with fT4 within normal range), much of the analyses are using other parameters in a manner that is not clearly defined in methods. Mainly, it is not clear what is meant by the results examining ‘AIP and thyroid hormones and diseases by vitamin A levels’ (see line 156). What is meant by ‘diseases’? (thyroid diseases?)  These ‘diseases’ need to be defined and the analyses need to be outlined in methods.

Response: Thank you for highlighting the need for clarity in our methods section. According to your suggestion, we have revised the methods to make the description clearer. The revisions are as follows:

Subclinical hypothyroidism was defined as TSH > 4.20 mIU/L, with fT4 within the normal range. In the study, subclinical hypothyroidism is known as thyroid disease.

Vitamin A levels: Venous blood samples were collected and treated with separation gel to extract the supernatant. Vitamin A levels were quantified using liquid chroma-tography-tandem mass spectrometry. According to the World Health Organization (WHO)， (http://www.who.int/vmnis/database/en/), subclinical vitamin A deficiency in adults is defined as a plasma or serum retinol concentration below 0.70 μmol/L, while severe vitamin A deficiency is characterized by a concentration below 0.35 μmol/L.

Comments 5: Vitamin A is not a proper noun and should not be capitalized unless at the start of a sentence. Also, the term ‘vitamin A loss’ is not a defining term. Is vitamin A deficiency what is meant?  <0.35 µmol/L indicates severe vitamin A deficiency (the cutoff used in this report) – perhaps the authors could consider using serum retinol concentration <0.70 μmol/L which indicates subclinical vitamin A deficiency.

Response: According to your suggestion, we have revised the expression of vitamin A throughout the manuscription as suggested. In addition, we have considered the cutoff value of vitamin A levels, the revisions are as follows:

Methods: According to the World Health Organization (WHO)， (http://www.who.int/vmnis/database/en/), subclinical vitamin A deficiency in adults is defined as a plasma or serum retinol concentration below 0.70 μmol/L, while severe vitamin A deficiency is characterized by a concentration below 0.35 μmol/L.

Statistical Analysis: In addition, we performed sensitivity analysis to analyze the association between AIP with thyroid hormones and thyroid diseases across vitamin A deficiency.

Results: Additionally, linear and logistic models were used to analyze the associations between AIP and thyroid hormones and diseases across vitamin A deficiency, as shown in Table S1. Vitamin A deficiency may also heighten the risk of the associated between AIP and subclinical hypothyroidism (OR = 1.69, 95%CI: 1.05, 2.73, P=0.031).

Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Table S1 Association between AIP and thyroid hormones and diseases, stratified by vitamin A levels;

 

Reviewer 2 Report

Comments and Suggestions for Authors

Guangming Mao et al. cross-sectionally evaluated vitamin A and subclinical hypothyroidism. Hypothyroidism is an important pathology related to cardiovascular disease, but is often overlooked by clinicians, so this study is very important.

 

This study, which has a large number of patients, is clinically important.

 

1. This time, the patient background is displayed according to whether or not there was sufficient vitamin A. On the other hand, there were few patients with vitamin A deficiency, so it seemed unbalanced. I thought it would be better to display the data by whether or not there was sufficient vitamin A, and at the same time, to divide the data into 3-4 groups according to the vitamin A concentration. Please consider this.

 

2. The analysis was performed using a binary variable based on whether or not there was sufficient vitamin A. Would the results be similar if vitamin A was analyzed as a continuous variable?

 

3. In the discussion, I thought there was little mention of the relationship between vitamin A and hypothyroidism in terms of the mechanism. I think that theory building is especially necessary in basic research.

Comments on the Quality of English Language

n/a

Author Response

Dear Editors and Reviewers,

Thank you for reviewing our manuscript entitled “Vitamin A levels difference in the association between atherogenic index of plasma and subclinical hypothyroidism in adults: A cross-sectional analysis in China” (Submission ID:  nutrients-3117301). We appreciate your insightful comments and suggestions. We have incorporated these to the best extent possible into the revised manuscript. The amendments are highlighted in red in the revised version, and the point-by-point responses to the reviewer’s comments are listed below. We are re-submitting the revised manuscript for your consideration and hope it is acceptable for publication.

Comments 1: This time, the patient background is displayed according to whether or not there was sufficient vitamin A. On the other hand, there were few patients with vitamin A deficiency, so it seemed unbalanced. I thought it would be better to display the data by whether or not there was sufficient vitamin A, and at the same time, to divide the data into 3-4 groups according to the vitamin A concentration. Please consider this.

Response: Thank you for your careful review of our manuscript. We appreciate your suggestion to categorize vitamin A levels into multiple groups rather than a binary categorization of “sufficient” vs. “loss.” According to your suggestion, we have reanalyzed the data by dividing it into tertiles based on vitamin A concentrations. These findings have now been included in the revised manuscript in Supplemental materials (Table S2).

Supplementary Materials: Table S2: Tertiles of vitamin A levels difference in the association between AIP and thyroid hormones and diseases.

Results: Additionally, linear and logistic models were used to analyze the associations between AIP and thyroid hormones and diseases across vitamin A levels, as shown in Table S1 and Table S2. Additionally, in the lowest quartile (Q1) of vitamin A levels, suggesting that lower vitamin A levels may increase the risk of association between AIP and subclinical hypothyroidism (OR = 1.91, 95%CI: 1.02, 3.60, P=0.044).

Comments 2: The analysis was performed using a binary variable based on whether or not there was sufficient vitamin A. Would the results be similar if vitamin A was analyzed as a continuous variable?

Response: Thank you for your comment. In our study, we believe it is clinically meaningful to use vitamin A levels as a stratification variable, categorizing the results into two groups.

Comments 3: In the discussion, I thought there was little mention of the relationship between vitamin A and hypothyroidism in terms of the mechanism. I think that theory building is especially necessary in basic research.

Response: According to your suggestion, we have expanded the Discussion section to include a review of available literature and proposed biological pathways that might explain the observed association between vitamin A and hypothyroidism. The revisions are as follows:

Vitamin A plays a multifaceted role in maintaining thyroid health through mechanisms affecting hormone synthesis, receptor activity, gland morphology, nutrient interactions, and immune function [29]. Research suggests that retinoic acid can influence TSH levels by modulating the hypothalamic-pituitary-thyroid axis, which controls the release and regulation of thyroid hormones essential for metabolism, growth, and development [30]. Furthermore, evidence shows that vitamin A is necessary for maintaining the normal structure and function of the thyroid gland. Deficiencies in vitamin A may lead to structural changes in the gland, potentially impairing its ability to produce thyroid hormones efficiently [31].

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors,

This is an interesting study aiming to evaluate the association between vitamin A levels, atherogenic index of plasma (AIP), and subclinical hypothyroidism. A major public health concern, hypothyroidism, affects many people worldwide with substantial endocrine and metabolic disorders. The authors performed a cross-sectional analysis involving 3,530 Chinese adults and demonstrated that loss of vitamin A may raise the possibility of a connection between AIP and subclinical hypothyroidism, especially in older people and women.

Only a few small edits are needed to the text; otherwise, the study is intriguing. 

The introduction and purpose of the study are well argued. 

The study design, methods and selection of participants are correctly described. I would just like to ask why participants under 18 were excluded; maybe it would be good to mention this in the exclusion criteria.
The results are nicely presented and explained.
Discussion: maybe it would be interesting to compare your results with other similar studies in the specialized literature.

 

 

Author Response

Dear Editors and Reviewers,

Thank you for reviewing our manuscript entitled “Vitamin A levels difference in the association between atherogenic index of plasma and subclinical hypothyroidism in adults: A cross-sectional analysis in China” (Submission ID:  nutrients-3117301). We appreciate your insightful comments and suggestions. We have incorporated these to the best extent possible into the revised manuscript. The amendments are highlighted in red in the revised version, and the point-by-point responses to the reviewer’s comments are listed below. We are re-submitting the revised manuscript for your consideration and hope it is acceptable for publication.

Comments 1: Only a few small edits are needed to the text; otherwise, the study is intriguing.

Response: Thank you for your positive feedback about the overall quality of our study. We have carefully reviewed the manuscript for minor errors and have made the necessary edits to ensure clarity and accuracy.

Comments 2: The introduction and purpose of the study are well argued.

Response: We sincerely appreciate your positive feedback.

Comments 3: The study design, methods and selection of participants are correctly described. I would just like to ask why participants under 18 were excluded; maybe it would be good to mention this in the exclusion criteria.

Response: According to your suggestion, we have updated the Methods section as follows:

Participants under the age of 18 were excluded due to our focus on adult-onset subclinical hypothyroidism, which is less common among younger individuals. Additionally, ethical considerations and the unique physiological and nutritional needs of minors influenced this decision.

Comments 4: The results are nicely presented and explained.

Response:. We appreciate the positive feedback.

Comments 5: Discussion: maybe it would be interesting to compare your results with other similar studies in the specialized literature.

Response: Thank you for the suggestion. We have added the following to the Discussion section:

Moreover, our study identified a significant association between vitamin A deficiency and an increased risk of subclinical hypothyroidism in the presence of elevated AIP. This finding adds to the growing body of evidence suggesting a significant interplay between lipid profiles and vitamin status in thyroid health. Previous studies have indicated that vitamin A supplementation could offer therapeutic benefits for patients with hypothyroidism [32], which is consistent with our results. For instance, a study involving 80 female patients aged 20 to 45 years found that atherogenic indices were significantly higher in those with hypothyroidism (P< 0.001) [33].

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for carefully considering the comments.  Please add vitamin D and iodine status to table 1.  

Author Response

Comments: Thank you for carefully considering the comments.  Please add vitamin D and iodine status to table 1.  

Response: According to your suggestion, we have added vitamin D and iodine status to table 1. 

Reviewer 2 Report

Comments and Suggestions for Authors

The authors improve manuscript appropriately. I think  thins paper is worth publishing. 

Comments on the Quality of English Language

n/a