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In this study, the authors sought to evaluate if increases in body mass index occurred derived from the COVID-19 pandemic by comparing the BMI of children and adolescents attending well consults in the pre-pandemic and pandemic periods. A retrospective cohort study design with generalized linear mixed models were described by the authors to evaluate the hypothesis. A greater increase in BMI was observed during the pandemic year 2020, compared to the prior trend of a lower increase in magnitude for BMI from 2018 to 2019. The authors further report that children and adolescents were not equally affected across different socioeconomic and ethnic groups. The study objective is well-defined, although I have questions regarding the study design which need to be clarified by the authors. The statistical analysis approach is probably appropriate; however, it is conditional to the study design being correctly identified. Overall, the manuscript is scientifically sound. I would like to make a few suggestions which could allow the authors to improve reporting of their study, as well as a few recommendations which could allow them to strengthen their conclusions.

Major comments:

1.     There are several aspects which make me think that the authors mistakenly identified their study design as a retrospective cohort study, whilst it seems like they instead performed a cross-sectional study. This is due to the nature of measurements and comparisons described. There is no clear definition and characterization of a cohort of patients which would require baseline descriptions of patient characteristics, as well as description of follow-up variables, and descriptions of patients who were lost to follow up during the follow-up period which would be 3 years in this case. Since the authors instead describe comparison of rates of obesity across different years, the correct study design would be a cross-sectional study in which the investigators measure the exposure (year) and outcomes (increase in BMI, and obesity) at the same time.

2.     The statistical analysis seems correct for a cross-sectional study, since the authors report odds ratios. However, a retrospective cohort design would be suitable to determine hazard ratios to evaluate the risk of presenting the outcome during the follow-up period.

3.     In case that the authors have different measurements for the same patient in their datasets for the years 2018-2020, I would suggest re-structuring their study descriptions and analyses to be consistent with a cohort study since this is a stronger and more informative study design than a cross-sectional study to address their hypothesis. If the authors decide that they intended to do a cross-sectional study instead, it needs to be described as such.

4.     The authors need to report their study according to recommendations included in the STROBE statement (https://pubmed.ncbi.nlm.nih.gov/17941715/) since this is an observational study (independent of it being labeled as a cohort study or cross-sectional stydu). Furthermore, they need to review the RECORD extension (https://pubmed.ncbi.nlm.nih.gov/26440803/) for observational studies since their study was based on routinely collected health data. Please provide the STROBE checklist + RECORD checklist as supplementary material for peer-review only: https://docs.google.com/viewer?url=http://www.record-statement.org/Files/checklist/RECORD%20Checklist.pdf

5.     Inclusion and exclusion criteria have not been described. If a cohort of patients is intended to be described, there needs to be a full explanation of how many patients could have been eligible on 2018, while also describing how many of those patients where lost to follow-up (i.e. had no data available for comparison during a comparable month) on 2019 and 2020.  A figure could be built to describe the flow of participants by describing number of patients meeting inclusion criteria, number patients excluded according to reason for exclusion, and number of patients eliminated (if applicable).

Minor comments:

1.     The introduction is too long, and some aspects may not be appropriate for this section and could be moved to the discussion section. The following paper could be useful to provide a more concise and informative introduction: https://breathe.ersjournals.com/content/4/3/224

2.     The study design section mentions that the objective was to determine increase in BMI and rates of obesity in the “months” following the COVID-19 pandemic. However, analyses were conducted in a year-to-year basis and were conditioned to the months when the clinics were open, thus, months immediately after the onset of the pandemic were not studied. Please correct for conciseness.

3.     Why were only patients between 3 to 17 years included? If this was a cohort study, are you referring to their age during 2019, 2020, or 2021? Otherwise, I believe the age reported corresponds to the age at the time of measurement, which further suggests that this was indeed a cross-sectional study.  

4.     Please describe in your manuscript how the sample was arrived at. If sample size calculation does not apply to your study for any reason, please explain. If your sample was different from the originally calculated sample size, please comment on it briefly with its implications in the discussion section. 

5.     The data analysis section does not explain descriptive statistics.

I cannot make any further comments on the authors’ results, discussion, and conclusions since I need clarifications regarding the study design to be able to comment accordingly. Please read the full STROBE explanation and elaboration document since this will allow you to make all clarifications for the points that I have mentioned.