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Nutrición Hospitalaria

versión On-line ISSN 1699-5198versión impresa ISSN 0212-1611

Nutr. Hosp. vol.35 no.2 Madrid mar./abr. 2018

http://dx.doi.org/10.20960/nh.1492 

Revisiones

Sugar-sweetened beverage consumption and obesity in children's meta-analyses: wrong answers to right questions

Consumo de bebidas azucaradas y obesidad en metaanálisis de estudios realizados en niños: respuestas equivocadas para preguntas correctas

Mariela Nissensohn1  2  3  , Daniel Fuentes-Lugo4  , Lluis Serra-Majem1  2  3 

1International Chair for Advanced Studies on Hydration (CIEAH). Universidad de Las Palmas de Gran Canaria. Las Palmas, Spain

2Research Institute of Biomedical and Health Sciences. Universidad de Las Palmas de Gran Canaria. Las Palmas, Spain

3CIBER OBN. Biomedical Research Networking Center for Physiopathology of Obesity and Nutrition. Carlos III Health Institute. Madrid, Spain

4Faculty of Health Sciences. Universidad Autónoma del Carmen. Ciudad del Carmen, Mexico

Abstract

Background:

recent studies assert that sugar-containing drinks may play a key role in the etiology of obesity. However, scientific reviews show contradictory results. Whether there is just association or clear causation still is a matter of debate. It is also subject to discussion whether the quality/adequacy of the different studies may influence their outcome.

Objective:

the aim of this study is to explore the most recent scientific evidence focused on sugar-sweetened beverages (SSB) and child obesity and to further analyze the adequacy of the meta-analyses in terms of their results, with special emphasis in the methodology, clarity and transparence of their procedures.

Methods:

only meta-analyses of randomized control trial studies were selected. The search was performed on PubMed and Cochrane Website until January, 2016. Adherence to PRISMA statement was assessed.

Results:

six meta-analyses were included. All of them showed some degree of evidence of heterogeneity in their pool estimates. Two of them showed a positive association between SSB and obesity but the other four found no association. The adherence to the PRISMA criteria was higher in two of the meta-analyses that showed opposite conclusions regarding the association or non-association of SSB and obesity in children. Thus, there is no relation between the adequacy of the meta-analyses to the PRISMA criteria and the results obtained.

Conclusion:

the use of meta-analysis as a scientific tool still demands more polishing, agreement and spread out use by researchers. SSB are being accused of being the main cause of the existing obesity, and obviously they are part of the problem, but this subject requires a broader approach that includes a thorough analysis of diet and lifestyle and a stronger body of scientific evidence based on data from epidemiological studies conducted in different populations.

Key words: Sugar-sweetened beverage; Obesity; Children; Meta-analyses

Resumen

Antecedentes:

estudios recientes afirman que las bebidas azucaradas desempeñan un papel clave en la etiología de la obesidad. Sin embargo, las revisiones científicas muestran resultados contradictorios. Si es solo una asociación o es claramente una causa continúa siendo un tema de debate. También se discute si la calidad/adecuación de los diferentes estudios puede influir en sus resultados.

Objetivo:

el objetivo de este estudio fue explorar la evidencia científica más reciente enfocada a las bebidas azucaradas y a la obesidad infantil y analizar la adecuación de los metaanálisis en términos de sus resultados, con especial énfasis en la metodología, claridad y transparencia de sus procedimientos.

Método:

se seleccionaron metaanálisis realizados con estudios de ensayos aleatorios controlados. La búsqueda se realizó en PubMed y en Cochrane hasta enero de 2016. Se evaluó la adherencia a los criterios PRISMA.

Resultados:

se incluyeron seis metaanálisis. Todos ellos mostraron cierto grado de heterogeneidad en sus estimaciones ponderadas. Dos de ellos mostraron asociación positiva entre bebidas azucaradas y obesidad, pero los otros cuatro no encontraron asociación. La adhesión a los criterios PRISMA fue mayor en dos metaanálisis que mostraron conclusiones opuestas sobre la asociación o no asociación de las bebidas azucaradas y la obesidad en niños. Por lo tanto, no existe relación entre la adecuación de los metaanálisis a los criterios PRISMA y los resultados obtenidos.

Conclusión:

el uso del metaanálisis como herramienta científica todavía requiere un mayor refinamiento, consenso y difusión por parte de los investigadores. Las bebidas azucaradas están siendo acusadas de ser la causa principal de la obesidad existente, y es obvio que son parte del problema, pero este tema requiere un enfoque más amplio que incluya un análisis exhaustivo de la dieta y del estilo de vida y una evidencia científica más sólida fundamentada en datos de estudios epidemiológicos realizados en diferentes poblaciones.

Palabras clave: Bebidas azucaradas; Obesidad; Niños; Metaanálisis

INTRODUCTION

Although obesity is the result of an imbalance of energy homeostasis, the true mechanisms underlying this process and effective strategies for prevention and treatment remain unknown. In general, obesity reflects complex interactions of genetic, metabolic, cultural, environmental, socioeconomic, and behavioral factors 1. It has been suggested that the intake of sugar-sweetened beverages (SSB) may promote weight gain and obesity by increasing overall energy intake 2.

There are some studies asserting that sugar-containing drinks may play a key role in the etiology of overweight and obesity in children and adults. In Europe, epidemiological studies focusing on beverage intake and obesity are rather scarce 3. Regarding children and adolescents, there are few studies addressing this subject. However, recent scientific reviews show contradictory findings, highlighting the weaknesses of many studies, which describe the current evidence as "not conclusive" 4, "equivocal" 5, "probable" 6 and "strong" 2,7,8.

Although the relation between SSB and obesity exists, whether this is a mere association or a real causation effect still remains controversial if based on current available scientific evidence. Thinking that the raise in childhood obesity occurs due to a single dietary factor, sugary drinks in this case, is a very simplistic approach that underestimates the real problem. Dietary elements work together with a lot of others factors like computer use less physical activity, higher portions sizes, etc. that lead to obesity. Most studies fail to detect how SSD or other aspects of diet and lifestyle have contributed to excess body weight. Whether the effect of sugar and calories from SSBs is worse than the effect of some other food is unclear 9. As Hu (2013) wrote in his article 10, from a public health point of view, identifying dietary determinants of long-term weight gain is critical for reducing the prevalence of obesity in the population. However, for many other scientists dietary determinants are not enough. It is really important to clarify the overall lifestyle involved. As it is well known, obesity is a multifactorial condition. Then, the magnitude of effects depends on the amount and length of exposure. Furthermore, according to Gibson 8, most of the evidence is dominated by American studies where SSD consumption tends to be higher. That may be less applicable to the European context, where consumption is substantially lower.

On the other hand, it is well known that the use of review articles and meta-analysis has become an important method employed in epidemiological research. However, the analysis of published papers in a meta-analysis has several considerable limitations 11. One of those is that studies may differ considerably in their designs, data collection methods and the definition of the exposure and confounder variables. A special dilemma arises when separate studies adjust for different confounding factors. Sometimes, the pooled estimate from a group of studies is published even if strong statistically significant heterogeneity between studies results was found 12.

It is also subject to debate whether the quality and adequacy of the different studies may influence the outcome. It is logical that when the study is conducted within a rigorous methodological frame, publishing clear results will positively influence the generation of new knowledge and, in consequence, the reliability of reached conclusions will increase. In this sense, there is a new trend of using different tools to assess the methodological aspects of each study to legitimize the quality of the scientific information. Some of these tools are AMSTAR, A Measurement Tool to Assess Systematic Reviews; GRADE, Grading of Recommendations Assessment, Development and Evaluation; and PRISMA, Preferred Reporting Items for Systematic Reviews and Meta-Analyses 13. These are scales and verification of data quality lists, which differ among one another in terms of their items, validity and scoring system. Most of them are a valuable resource for readers, reviewers and journal editors, but they are in constant update. Then, we must be especially careful when considering their use and when interpreting the resulted information that arises from them.

For all the above reasons, the aim of this study is to explore the most recent scientific evidence focused on SSB and child obesity and to analyze the adequacy of the meta-analyses in terms of their results, with special emphasis on the applied methodology and the clarity/transparency of their procedures. Furthermore, to analyze the possible causes for which a consensus has not yet been found in the scientific evidence available on this topic and why it still remains a source of intense debate.

MATERIALS AND METHODS

SYSTEMATIC SEARCH AND SELECTION CRITERIA

We searched the literature for published meta-analyses of epidemiologic studies, whose primary aim was to describe the relation between SSB intake and obesity in children. Only meta-analyses including Random Control Trials (RCTs) were selected. The search was performed accessing PubMed database (http://www.ncbi.nlm.nih.gov/pubmed) and Cochrane Collaborative Website (http://www.cochrane.org) between January, 2001 and January, 2016, using the following search terms: "beverage(s)" or "soft drink(s)" or "soda" AND "obesity" or "weight", including MESH-terms. In addition, reference lists of the retrieved articles were searched to find other additional relevant reviews.

Articles were considered as eligible for inclusion if they were: a) systematic reviews and meta-analyses that include at least two RCTs in their study selection; b) studies with a primary focus oriented towards SSB intake and obesity in children. Exclusion criteria were: a) reviews of dietary behaviors or patterns that identified SSB intake as one of many of the factors, but did not study exclusively their impact on body weight; b) systematic reviews and meta-analyses which did not include any RCT study in their analysis; c) articles published prior to the past decade (not earlier) because the discussion regarding the need for a systematic approach to reviewing the literature has evolved, and also a consensus has emerged by which a systematic approach is to be preferred over other more subjective approaches 14.

DATA ABSTRACTION

A data extraction form was designed to obtain descriptive information of each meta-analysis evaluated, including information of the study population, the RCTs included in each one, specific details of database searches, inclusion and exclusion criteria, heterogeneity of the pooled analyses and conclusions (the final statement on the association between SSB consumption and obesity/weight gain or not), adherence to PRISMA 13, data regarding the journal in which the reviews were published (the impact factor of each one, the year of publication of the article) and industry funding. Those studies which did not specify industry funding of any sort were considered by default as "not supported by the industry". Authors that received funding for different activities were not taken into account.

In addition, the data obtained from the primary studies included in each review (RCTs) in terms of relation to the outcomes, population studied, type of intervention, conclusions as well as funding were gathered.

ADEQUACY ASSESSMENT

Among all the assessment tools that have been elaborated during the last few years to assess the adequacy of systematic reviews and meta-analyses, we employed PRISMA (13) adherence. This tool was not designed to become an instrument for evaluating the quality of the articles. It was conceived for enhancing clarity and transparency of published systematic reviews and meta-analyses. It allow to assess whether a step has been completed or not, but not necessarily whether it has been done in the correct way (http://www.prisma-statement.org). It comprises 27 items; each of them has an assigned value of 1 for "yes", 0 for "no" and 0.5 for cases in which the answer is "partial".

RESULTS

In total, six meta-analyses were included in our study. The main characteristics of the reviews are presented in Table I. Malik et al., Forshee et al., Vartanian et al. and Mattes et al. 7,15,16,17 documented their search strategies and the inclusion and exclusion criteria. All meta-analyses performed their searches in more than one database, with at least one of them being in either Medline or PubMed, and a second source was also searched (either a database or hand-searched reference lists). Three meta-analyses 15,17,18 included a flowchart of the "study search and selection process" as recommended by PRISMA. A test of heterogeneity was developed in all the studies. The I² index, which measures the extent of the heterogeneity following with a p-value for this statistic, was showed in most of the studies. Substantial heterogeneity was present in most of the cases (it means a high I² index and a p-value for this I2 lower than 0.05). When the I² Index was low, the p-value did not show statistical significance. This situation indicated the presence of heterogeneity, which somewhat compromises the validity of the pooled estimates 19. Only Vartanian study 7 showed heterogeneity as a Q statistics value, and in this case it was low.

Table I Sugar-sweetened beverages and obesity in children and adolescents: characteristics of meta-analyses evaluated 

Table II depicts adherence to PRISMA 11 percentages. Similarly to the results obtained by Bes Rastrollo et al. study 20, only two systematic reviews 15,18 which specifically used PRISMA guidelines to report their data were identified. Adherence to PRISMA guidelines 13 was 85% and 94%, respectively. In other studies, such as Mattes et al., Kaiser et al., Forshee et al. or Vartanian et al. 7,16,17,21, adherence to PRISMA was 83%, 74%, 72% and 50%, respectively. The last two papers were published before PRISMA 13 was routinely used, but back then, there was a similar available tool called QUORUM.

Two of the articles included found a positive association between SSB and obesity 7,15. On the other hand, four articles found no association between SSB and obesity 16,17,18,21.

Table II PRISMA checklist in %  

Table III shows the data obtained from the primary studies (RCTs) included in each review. Of all RCTs, only two authors were included in all six meta-analyses 22,23 and one author was included in four meta-analyses 24. Three meta-analyses 7,18,21 included RCTs which were not considered in any other meta-analysis 25,26,27,28,29,30,31,32. In some of these RCTs, authors studied different variables, conducted different interventions in different populations, and also some of them did not include a study-dropouts analysis, nor did assessed compliance among other sources of heterogeneity. Most of the included studies lacked long term follow-up, keeping usually less than ten weeks long, suggesting the need of longer term data to conduct further analysis.

Table III Characteristics of the RCTs included in the meta-analysis 

QUALITY OF META-ANALYSIS

Some authors recently reviewed the quality of SR on SSB and some health outcomes using the validated instrument AMSTAR, which is a one-page tool with eleven questions. Weed et al. 33 concluded that "the comprehensive reporting of epidemiologic evidence and use of systematic methodologies to interpret evidence was underused in published reviews on SSB and health". They suggest a dubious quality in the currently available scientific evidence in this area with the exception of a few studies. Massougbodji et al. 34 concluded that "there is no consensus on the strength of the evidence on causality, and that available quality-assessment tools have limitations, and many contextual factors beyond the intrinsic characteristics of the reviews may influence their conclusions".

According to the analyses of Keller et al., the quality of the original papers or RCTs included in the reviews or meta-analyses could have suffered from some bias that may hence also introduce some errors 35.

DISCUSSION

Our findings indicated that, opposed to our expectations, there is no clear relation between the adequacy of the meta-analyses (using PRISMA criteria) and the results reached.

We believe that controlled trials and systematic reviews with inadequate methodology are more prone to bias 36. It is clear that, in this particular issue, the analysis of the methodology developed is not enough to shed light on this task. Otherwise it is not understood how the meta-analyses with the highest and the lowest adherence to PRISMA found a positive association between SSB and obesity in both cases. This very same situation was also detected in the Keller et al. 35 analyses using the AMSTAR analysis of quality. This situation leads us to consider several points.

First of all, small sample sizes, short follow-ups and different dietary measurement tools used are linked to problems in exposure measurements that can influence the direction of the association between SSBs and weight and change research results 35.

Despite we consider that systematic reviews and meta-analyses of RCTs are the "gold standard" of evidence, and furthermore that the PRISMA checklist used for this analysis as a good tool of items guidelines, the statistical methods for pooling data from different sources have to be better implemented in order to minimize heterogeneity between studies. Our work is based on a limited group of meta-analyses and each one of them, in turn, presents a certain degree of heterogeneity, which indicates different features in each of the included studies. This heterogeneity cannot be ignored; quite the opposite, it is mandatory to find out with further research why these differences are present since they may compromise somehow the validity of the pooled estimates. Even, there are authors who claim that a pooled estimate should not be published if heterogeneity between studies is high, and that in many publications of meta-analysis the problem of heterogeneity is not sufficiently assessed 12. As far as we understand, heterogeneity is another ingredient to account for, but in no way we consider that a study with high heterogeneity should not be published. A meta-analysis should just extract a common estimate when it may truly assume that a common parameter exists; otherwise, it would be advisable to use it just to show and highlight the differences between the results of the various studies available, in order to clarify that it is not possible to combine what is so different 37. Even studies with negative results should be taken into account with the same academic value as the ones with positive results; it is important not to forget that negative findings also contribute to the improvement of science.

Another relevant aspect to consider in our analysis is the possible existence of economic interests and their consequent bias when reaching conclusions. The debate over the influences of corporate sponsorship on research findings has been always highly thrashed out. However, the issue of "funding source" has recently taken on a special interest, particularly in the realm of obesity research. Addressing this issue is of paramount importance especially in the nutrition field, because inexact data may have negative repercussions in general public health. Bes Rastrollo et al. 20 published a survey where financial conflicts of interest and reporting bias regarding the association between SSB and weight gain were analyzed in a series of studies. Their results show that those systematic reviews with stated sponsorship or conflicts of interest with food or beverage companies were five times more likely to report conclusions of negative association between SSB intake and weight gain (or obesity) than those which reported no industry sponsorship nor conflict of interest. Nonetheless, this study did not consider the methodological quality of the meta-analyses, but focused mainly on sponsor influence. To add more confusion to the subject, other authors such as Kaiser et al. 38 states that, "in general, the quality of industry-funded studies seems better than non-industry funded research; there is evidence that shows that industry-funding was associated with higher quality reporting". Scientific research must always pursue the truth, regardless of financial or other interests. The consequences of reaching biased conclusions may affect directly from health care practitioners to the process of decision-making by health policy institutions, posing a threat for public health. Industry and science need to work together in order to achieve worthwhile goals, especially nowadays when research resources are scarce. Kaiser et al. 38 underline that efforts to increase the overall quality of the scientific literature can be achieved through activities that support high quality in reporting and create transparency. Approaching questions from different perspectives with diverse methods but with a strong ethical commitment may be on the one hand appealing to the industry and, on the other hand, able to produce solid outcomes free of sponsor bias 39. Research funding is beneficial and necessary, and scientist should remain aware of potential bias of all types 38,40. In addition, conflicts of interest are not always related to financial relationships, but can also occur because of personal relationships, academic competition, intellectual passion, or political engagement 41. It is very difficult to identify these types of influences, and we cannot exclude their presence in reviews not funded by industry.

Another important element to consider, as mentioned above, is that consumption of SSB is usually associated with an unhealthy lifestyle characterized by a higher caloric intake, less practice of exercise, smoking habit and presence of a poorly balanced dietary pattern. According to Khan et al. 42, sugar content should not be the sole determinant of a healthy diet. There are many other factors in the diet. Rather than just focusing on one energy source, the whole diet should be considered for health benefits 42.

Lastly, unintentional or inadvertent omissions or unclear or misleading statements in research reporting are another source of bias in health research. In this sense, there is now a tendency to reject studies that produce non-significant results. Many authors do not publish their data when it shows negative results to avoid rejection by editors of scientific literature, thus creating a vicious circle. Therefore, a great number of trials have not been published yet. Including all data, regardless of their results (either positive or negative), is imperative in order to increase reliability and, in turn, produce a solid foundation which will support conclusions with more scientific strength. Lack of information may lead to questioning the validity and adequacy of an entire analysis. It is imperative to avoid this kind of publication bias.

In this sense, the lack of a statistically significant result (p-value > 0.05) does not mean that there was no underlying effect: it means that no effect was detected. A small study may not have the power to detect a real difference. As opposite, small responses are less likely to be detected. A study with many replicates might result in a statistically significant result but have a small effect size (and so, perhaps, be unimportant). The importance of an effect size is a biological, physical or social question, and not a statistical one 43.

Evidently, available adherence or quality-assessment tools have limitations, and many contextual factors beyond the intrinsic characteristics of the reviews may influence their results 34. The meta-analysis area still requires more refining, consensus and widespread use by the scientific community 12.

From another point of view, it seems that lowering SSB intake as an isolated measure will not produce a decrease in obesity prevalence 44. The suggestion that SSB consumption is the main cause of obesity cannot explain why overweight and diabetes have also increased in regions with limited intakes of SSB such as Asia and Africa. Therefore, current evidence alone is insufficient to demonstrate such a role in the global obesity epidemic. Many other causal factors should be taken into account as well. The confusing message as several nutrition researchers stand in opposite to others about the available evidence of SSB has turned a purely scientific dialogue into an intensely emotional one 42.

Summarizing, scientists need to practice good science, sponsors must commit to transparency and no influence, media needs to practice responsible scientific journalism, and we all need to base our evaluations on scientific data and not on predetermined opinions rooted in our own emotion-laden bias for or against specific funding sources 40.

Why do we remain incapable of reaching a true consensus regarding this issue? Are there so huge interests that can mask the results so treacherously that could lead us to the other side of the street? The first step is to firmly underline that funding should not lead to conclusions that are either for or against the industry. To finance must imply to generate a better service and to guarantee the continuity of that service, especially in times of economic crisis as the one that science crosses in the last times. The ethics and professionalism of scientists have nothing to do with financial aid. Ethics and professionalism must always exist with or without funding.

In conclusion, to formally accuse SSB of causing the current obesity pandemic is a narrow approach of this subject. At this point, the evidence currently available is not capable of supporting with strength this relationship. The limited data and the heterogeneity of methods employed make it impossible to reach any conclusion. They surely may contribute to a certain degree. However, better-designed, best quality and longer-term studies and a broader approach that includes a thorough analysis of diet and lifestyle with a stronger body of scientific evidence based on data from epidemiological studies conducted in different populations are needed in order to reach real science-based conclusions to establish this relationship properly.

ACKNOWLEDGEMENTS

The authors' responsibilities were as follows: Lluis Serra-Majem prepared the main outline of the manuscript. Mariela Nissensohn helped to select the data and write the manuscript. Daniel Fuentes Lugo contributed to the selection of studies and data extraction. All authors contributed to the preparation of the final manuscript.

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Received: August 09, 2017; Accepted: October 09, 2017

Correspondence: Mariela Nissensohn. Research Institute of Biomedical and Health Sciences. Universidad de Las Palmas de Gran Canaria. Paseo Blas Cabrera Felipe "Físico", s/n. 35016 Las Palmas, Spain e-mail: mnissensohn@acciones.ulpgc.es

DECLARATION OF INTEREST

This study was financially supported by a grant from the former European Hydration Institute, now International Chair for Advanced Studies on Hydration (CIEAH) to the Canarian Science Foundation and Technology Park of the University of Las Palmas de Gran Canaria. Neither Mariela Nissensohn, Daniel Fuentes Lugo nor Lluis Serra-Majem have any conflict of interest to disclose.

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