Comment on “The association between carotenoids and subjects with overweight or obesity: a systematic review and meta-analysis” by N. Yao, S. Yan, Y. Guo, H. Wang, X. Li, L. Wang, W. Hu, B. Li and W. Cui, Food Funct., 2021, 12, 4768.

In a recent issue of the journal, Yao et al.¹ published the results of an interesting meta-analysis on the influence of carotenoid supplementation on overweight or obese subjects. They came to the conclusion that the insufficiency of serum carotenoids is associated with an increase in the risk of obesity. Moreover, carotenoid supplementation could significantly change the body weight, waist circumference, and body mass index (BMI). Although these results are of great interest, several points regarding the identification, screening process, data extraction, and analysis of selected studies should be considered when interpreting these findings.

First, as it is well recognized, the literature search process for the qualified and related published data is the milestone of a systematic review and meta-analysis that can entirely affect the results. Based on inclusion criteria, it seems that some eligible studies are missed through evaluation and extraction processes.^2–14 Since anthropometric measurements are the second purpose of most studies, data regarding BMI or weight changes may not be mentioned in the abstract of articles. Consequently, in this case, literature investigation based on title, abstract, and keywords alone may not be sufficient for systematic review studies.¹⁵ Another reason is related to the missing Medical Subject Heading (MeSH) term description for the concept of carotenoids, where carotenoids are defined as a group of fat-soluble pigments, such as abscisic acid, beta-carotene, lycopene, norisoprenoids, retinoids, xanthophylls, canthaxanthin, cryptoxanthins, lutein, zeaxanthins, and zeta-carotene. As there are articles available regarding the effect of supplementation with lycopene,^4,6,8,11,13 astaxanthin,^2,14 fucoxanthin,³ β-cryptoxanthin,⁵ tocotrienol,^9,10 and lutein^7,12 on overweight subjects or obese patients with or without dyslipidemia and diabetes, further analysis on the recognized types of carotenoids is suggested. Because the number of individual studies allows for such a meaningful re-analysis.

Second, the authors declared that the search strategy is limited to published human trials in which they studied the role of carotenoids in overweight or obese populations. However, an additional pooled analysis regarding lipid profiles was conducted without providing their definitions and keywords through investigation at the baseline. In this case, other eligible studies that examined the impact of carotenoids on lipid profiles could be included in the present study.^6,7,11–13Consequently, further analysis taking into account MESH terms for circulating body lipids is suggested.

In addition, there are three published RCTs which assessed the effect of lycopene on the body weight, fat mass, body mass index (BMI), and lipid profiles^4,6,8 which could be reviewed and analyzed individually or in combination. Therefore, we suggest lycopene-related research to be included in future analysis.

Besides, it would be interesting to look at the effect of each carotenoid individually in addition to the combined approach chosen by the authors to gain more insights into the effects of individual carotenoids, which may differ, in particular astaxanthin^2,14,16,17 lycopene,^{4,6,8,11–13} fucoxanthin,^3,18 β-cryptoxanthin^5,19–23 and lutein^7,12. Despite sufficient data about the reported dosage of supplementation in the trials, they were not considered for analysis; this could affect the results of the mentioned study.

Furthermore, the authors combined the data from some observational studies on children (age < 18 years) with those on adults (age ≥ 18 years) in the meta-analysis. As we know, some risk factors of obesity in childhood and adults are the same; meta-analyses of observational studies revealed that some environmental factors could be relatively different. The abovementioned issues are so important that they should be considered. Therefore, separate analyses of the reported odds ratio for overweight and obesity in adults and children are suggested.

Finally, the authors carried out a stratified analysis in the case of weight, BMI, and WC to explore the potential sources of between-study heterogeneity; however, the degree of heterogeneity remained high and statistically significant, for overweight (I² = 97.0; P < 0.001) and obese (I² = 98.0; P < 0.001) population subgroups, respectively. To find the source of between-study heterogeneity, further subgroup analysis on healthy subjects^{4,5,9,11,12,14,19,24} and patients with dyslipidemia^{7,10,13,18,20,22,25} could be performed, as well as on individual carotenoids, as previously suggested. In addition, a meta-regression based on the baseline values of the variables and the dosage of the carotenoid supplementation could provide an explanation for the observed between-study heterogeneity.

In conclusion, although conducting a meta-analysis is a good approach to summarize the findings of previous studies together, the findings of Yao et al.¹ may need to be interpreted with caution due to the points raised above.

Author contributions

Saadat S. collected the data. Shokri.mashhadi N. collected the data and wrote the paper.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This paper was supported by Isfahan University of Medical Sciences.

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