The authors have declared that no competing interests exist.
Conceived and designed the experiments: MBR MAMG. Performed the experiments: MBR MAMG MRC. Analyzed the data: MBR. Wrote the first draft of the manuscript: MBR. Contributed to the writing of the manuscript: MBR MBS MRC MAMG.
Maira Bes-Rastrollo and colleagues examine whether financial conflicts of interest are likely to bias conclusions from systematic reviews that investigate the relationship between sugar-sweetened beverages and weight gain or obesity.
Industry sponsors' financial interests might bias the conclusions of scientific research. We examined whether financial industry funding or the disclosure of potential conflicts of interest influenced the results of published systematic reviews (SRs) conducted in the field of sugar-sweetened beverages (SSBs) and weight gain or obesity.
We conducted a search of the PubMed, Cochrane Library, and Scopus databases to identify published SRs from the inception of the databases to August 31, 2013, on the association between SSB consumption and weight gain or obesity. SR conclusions were independently classified by two researchers into two groups: those that found a positive association and those that did not. These two reviewers were blinded with respect to the stated source of funding and the disclosure of conflicts of interest.
We identified 17 SRs (with 18 conclusions). In six of the SRs a financial conflict of interest with some food industry was disclosed. Among those reviews without any reported conflict of interest, 83.3% of the conclusions (10/12) were that SSB consumption could be a potential risk factor for weight gain. In contrast, the same percentage of conclusions, 83.3% (5/6), of those SRs disclosing some financial conflict of interest with the food industry were that the scientific evidence was insufficient to support a positive association between SSB consumption and weight gain or obesity. Those reviews with conflicts of interest were five times more likely to present a conclusion of no positive association than those without them (relative risk: 5.0, 95% CI: 1.3–19.3).
An important limitation of this study is the impossibility of ruling out the existence of publication bias among those studies not declaring any conflict of interest. However, the best large randomized trials also support a direct association between SSB consumption and weight gain or obesity.
Financial conflicts of interest may bias conclusions from SRs on SSB consumption and weight gain or obesity.
In our daily lives, we frequently rely on the results of scientific research to make decisions about our health. If we are healthy, we may seek out scientific advice about how much exercise to do to reduce our risk of a heart attack, and we may follow dietary advice issued by public health bodies to help us maintain a healthy weight. If we are ill, we expect our treatment to be based on the results of clinical trials and other studies. We assume that the scientific research that underlies our decisions about health-related issues is unbiased and accurate. However, there is increasing evidence that the conclusions of industry-sponsored scientific research are sometimes biased. So, for example, reports of drug trials sponsored by pharmaceutical companies sometimes emphasize the positive results of trials and “hide” unwanted side effects deep within the report or omit them altogether.
Although the effects of company sponsors on the conclusions of pharmaceutical research have been extensively examined, little is known about the effects of industry sponsorship on nutrition research, even though large commercial entities are increasingly involved in global food and drink production. It is important to know whether the scientific evidence about nutrition is free of bias because biased information might negatively affect the health of entire populations. Moreover, scientific evidence from nutrition research underlies the formulation of governmental dietary guidelines and food-related public health interventions. In this systematic review, the researchers investigate whether the disclosure of potential financial conflicts of interest (for example, research funding by a beverage company) has influenced the results of systematic reviews undertaken to examine the association between the consumption of highly lucrative sugar-sweetened beverages (SSBs) and weight gain or obesity. Systematic reviews identify all the research on a given topic using predefined criteria. In an ideal world, systematic reviews provide access to all the available evidence on specific exposure–disease associations, but publication bias related to authors' conflicts of interest may affect the reliability of the conclusions of such studies.
The researchers identified 18 conclusions from 17 systematic reviews that had investigated the association between SSB consumption and weight gain or obesity. In six of these reviews, a financial conflict of interest with a food industry was disclosed. Among the reviews that reported having no conflict of interest, 83.3% of the conclusions were that SSB consumption could be a potential risk factor for weight gain. By contrast, the same percentage of reviews in which a potential financial conflict of interest was disclosed concluded that the scientific evidence was insufficient to support a positive association between SSB consumption and weight gain, or reported contradictory results and did not state any definitive conclusion about the association between SSB consumption and weight gain. Reviews in which a potential conflict of interest was disclosed were five times more likely to present a conclusion of no positive association between SSB consumption and weight gain than reviews that reported having no financial conflict of interest.
These findings indicate that systematic reviews that reported financial conflicts of interest or sponsorship from food or drink companies were more likely to reach a conclusion of no positive association between SSB consumption and weight gain than reviews that reported having no conflicts of interest. A major limitation of this study is that it cannot assess which interpretation of the available evidence is truly accurate. For example, the scientists involved in the systematic reviews that reported having no conflict of interest may have had preexisting prejudices that affected their interpretation of their findings. However, the interests of the food industry (increased sales of their products) are very different from those of most researchers (the honest pursuit of knowledge), and recent randomized trials support a positive association between SSB consumption and overweight/obesity. Thus, these findings draw attention to possible inaccuracies in scientific evidence from research funded by the food and drink industry. They do not imply that industry sponsorship of nutrition research should be avoided entirely. Rather, as in other research areas, clear guidelines and principles (for example, sponsors should sign contracts that state that they will not be involved in the interpretation of results) need to be established to avoid dangerous conflicts of interest.
Please access these websites via the online version of this summary at
The Research Ethics Program at the University of California, San Diego provides an overview of
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A 2012 policy brief from the Yale Rudd Center for Food Policy and Obesity discusses
The US National Institutes of Health has
Wikipedia has pages on
The use of industry funding for scientific research might influence the results of published studies, leading to conclusions that could ultimately support the industry's interests. Examples of this include biased findings by the tobacco industry
The influence of sugar-sweetened beverages (SSBs) on weight gain and obesity has been extensively researched and debated in the last few years
Systematic reviews (SRs) and meta-analyses represent an efficient and comprehensive way to access the available evidence on particular exposure–disease associations. However, publication bias related to authors' conflicts of interest in a SR may affect the reliability of its conclusions. Beverage and sugar industries tend to play leading roles in the reported conflicts of interest of some researchers actively publishing in the field of SSB and obesity. Therefore, we assessed whether the disclosure of potential financial conflicts of interest with these industries was associated with conclusions on SSB consumption and weight gain or obesity in published SRs.
Standard methods for conducting and reporting SRs were used
We conducted a search of three databases (PubMed [
Two researchers, blinded to the authors' financial conflicts of interest and stated sources of funding, independently extracted the conclusions stated in the articles. The agreement between the researchers was 93.3% (Kappa index: 0.86;
A third researcher classified the SRs according to the authors' financial conflicts of interest or stated sources of funding. Potential financial conflicts of interest were identified if an explicit statement in this regard was made in the text by any of the authors, or if a declared affiliation or financial disclosure by the author evidenced a link with a food industry.
We also evaluated whether the identified SRs used standard methods for conducting and reporting SRs.
The association between stated conflicts of interest and the articles' conclusions was assessed using Poisson regression analysis to adjust for year of publication and whether the article was published in a journal in the first quartile of impact factor within its category (according to ISI Web of Knowledge), or to adjust for the number of citations per year for each SR since its publication. We also adjusted for the existence/inexistence of summarizing point estimates (meta-analyses). Risk ratios and their 95% confidence intervals were calculated. Stata 12.1 (StataCorp) was used for all analyses.
Our final study was based on 17 out of 405 potentially eligible articles (
The search used the following combinations of terms: (soft drink or soft drinks or beverage* or soda) and (body mass index or bmi or weight or obes* or overweight); filters: Review and Human; date: up to August 31, 2013.
SR | Year of Publication | Number of Studies Included in the SRs | Population | Study Type | Conflicts of Interest with Food Companies | Grading of SR Conclusion | SR Conclusions (Meta-Analysis Estimates) |
Malik et al. |
2013 | 22 prospective studies10 RCTs | Adults and children/adolescents | SR and meta-analysis | No | Positive association | “Our systematic review and meta-analysis of prospective cohort studies and RCTs provides evidence that SSB consumption promotes weight gain in children and adults.” (Prospective studies—children: BMI change: 0.06; 95% CI: 0.02–0.10; prospective studies—adults: weight change: 0.22 kg, 95% CI: 0.09–0.34, per one daily serving increment; RCTs—children: BMI change: −0.17, 95% CI: −0.39 to 0.05, when SSBs were reduced; RCTs—adults: weight change: 0.85 kg, 95% CI: 0.50–1.20, when SSBs were added.) |
Te Morenga et al. |
2012 | 7 prospective studies5 RCTs | Adults and children/adolescents | SR and meta-analysis | No | No positive association and positive association | “Trials in children which involved the recommendations to reduce intake of sugar sweetened foods and beverages showed no overall change in body weight….In prospective studies SSB consumption was associated with the risk of being overweight or obese.” (odds ratio: 1.55, 95% CI: 1.32–1.82, highest consumption versus lowest consumption). |
Hauner et al. |
2012 | 23 prospective studies10 RCTs | Adults and children/adolescents | SR | No | Positive association | “The available cohort and intervention studies regarding adults mainly show that a higher consumption of sugar-sweetened beverages is accompanied by an increased risk of obesity. The overall evidence regarding an increased risk of obesity due to higher consumption of sugar-sweetened beverages in children and adolescents is therefore only judged as possible.” |
Osei-Assibey et al. |
2012 | 4 prospective studies4 RCTs | Children/adolescents | SR | No | Positive association | “Providing alternatives to sugar-sweetened soft drinks should be considered in obesity prevention.” |
Clabaugh and Neuberger |
2011 | 3 cross-sectional studies5 prospective studies1 RCT | Children/adolescents | SR | No | Positive association | “Education and political action by nurses to promote a decrease in SSB intake is a step in the right direction in reducing obesity in our children.” |
Mattes et al. |
2011 | 12 RCTs in the SR10 RCTs in the meta-analysis | Adults and children/adolescents | SR and meta-analysis | Yes (Coca-Cola Company, PepsiCo) | No positive association | “The current evidence does not demonstrate conclusively that nutritively sweetened beverage (NSB) consumption has uniquely contributed to obesity or that reducing (NSB) consumption will reduce BMI levels in general.” (BMI change: −0.004, 95% CI: −0.079 to 0.072, when SSBs were reduced.) |
Woodward-Lopez et al. |
2010 | 32 cross-sectional studies24 prospective studies | Adults and children/adolescents | SR | No | Positive association | “All lines of evidence consistently support the conclusion that the consumption of sweetened beverages has contributed to the obesity epidemic.” |
Ruxton et al. |
2010 | 5 prospective studies3 RCTs | Adults and children/adolescents | SR | Yes (Sugar Bureau, UK) | No positive association | “Some studies, specifically on sweetened beverages, highlighted a potential concern in relation to obesity risk, although these were limited by important methodological issues.” |
Dennis et al. |
2009 | 2 cross-sectional studies1 prospective study4 RCTs | Adults | SR | Yes (Institute for Public Health and Water Research) | Positive association | “A reduced intake of energy-containing beverages may facilitate weight management.” |
Olsen and Heitmann |
2009 | 14 prospective studies5 RCTs | Adults and children/adolescents | SR | No | Positive association | “A high intake of calorically sweetened beverages can be regarded as a determinant of obesity.” |
Wolff and Dansinger |
2008 | 15 cross-sectional studies10 prospective studies5 RCTs | Adults and children/adolescents | SR | No | No positive association | “Given the magnitude of the public health concern, larger and longer intervention trials should be considered to clarify the specific effects of sugar-sweetened soft drinks on body weight and other cardiovascular risk factors.” |
Harrington |
2008 | 2 time trend studies3 prospective studies2 RCTs | Children/adolescents | SR | No | Positive association | “Single intervention manipulation, elimination, or marked reduction of SSB consumption may serve to decrease caloric intake, increase satiety levels, decrease tendencies towards insulin resistance, and simplify the process of weight management in children and adolescents.” |
Gibson |
2008 | 23 cross-sectional studies17 prospective studies4 RCTs | Adults and children/adolescents | SR | Yes (Union of European Beverages Associations) | No positive association | “Despite the large number of studies on this topic, the inconsistencies of definition, design, statistical treatment and interpretation make it difficult to draw definitive conclusions as to whether sugar-sweetened beverages are significantly implicated in weight gain.” |
Forshee et al. |
2008 | 12 (10 prospective studies and 2 RCTs) in the SR10 (8 prospective studies and 2 RCTs) in the meta-analysis | Children/adolescents | SR and meta-analysis | Yes (American Beverage Association) | No positive association | “The quantitative meta-analysis and qualitative review found that the association between sugar-sweetened beverages consumption and BMI was near zero, based on the current body of scientific evidence.” (BMI change per one serving of SSB increase: 0.03, 95% CI: −0.01 to 0.07.) |
Vartanian et al. |
2007 | 12 prospective studies7 RCTs | Adults and children/adolescents | SR and meta-analysis | No | Positive association | “Recommendations to reduce population soft drink consumption are strongly supported by the available science.” (Overall average body weight effect size: 0.08, 95% CI: 0.06–0.09.) |
Forshee et al. |
2007 | 4 ecological studies8 cross-sectional studies7 prospective studies1 RCT | Adults and children/adolescents | SR | Yes (Tate and Lyle) | No positive association | “Evidence from ecological studies linking high fructose corn syrup (HFCS) or ‘soft drinks’ (proxy for HFCS) with rising BMI rates is unreliable. Evidence from epidemiologic studies and randomized controlled trials is inconclusive.” |
Malik et al. |
2006 | 15 cross-sectional studies10 prospective studies5 RCTs | Adults and children/adolescents | SR | No | Positive association | “Although more research is needed, sufficient evidence exists for public health strategies to discourage consumption of sugary drinks as part of a healthy lifestyle.” |
BMI, body mass index.
One of the reviews
Six SRs
Sixty-one percent of the conclusions from the 17 SRs (11/18 conclusions) supported an adverse association between SSB consumption and weight gain or obesity; none reported any significant benefit.
Thirty-three percent (6/18 conclusions) of the conclusions from the 17 SRs were associated with a potential financial conflict of interest. The conflict of interest in four of these six reviews (66.7%) was based on the funding to conduct the SR. However, three out of four SRs in this category stated that the food company did not play a role in the selection or methodological assessment of the included studies, nor in the interpretation of the results or conclusions reached.
Among the SRs that reported having no conflict of interest (11 SRs with 12 conclusions), 83.3% conclusions (10/12) were that SSBs were directly associated with increased weight gain or obesity. In contrast, 83.3% (5/6) of the conclusions from SRs that reported having some conflict of interest with the food industry were that there was insufficient scientific evidence to support a positive association (
Conflict of Interest with Food Companies | Grading of SR Conclusion | Total | |
Number (Percent) Reporting Positive Association | Number (Percent) Reporting No Positive Association | ||
Yes | 1 (16.7%) | 5 (83.3%) | 6 |
No | 10 (83.3%) | 2 (16.7%) | 12 |
One study, Te Morenga et al.
Risk Ratio | No Conflict of Interest with Food Companies | Conflict of Interest with Food Companies |
Crude risk ratio (95% CI) | 1 (Ref.) | 5.00 (1.29–19.34) |
Adjusted for year of publication (95% CI) | 1 (Ref.) | 4.94 (1.23–19.90) |
Adjusted for year of publication and the whether published in a journal in the first impact factor quartile of its category (95% CI) | 1 (Ref.) | 5.16 (1.30–20.48) |
We identified only two SRs
When we analyzed the original articles included in each SR, we observed the existence of high heterogeneity in the selection of original studies to be included in the SRs. Part, but not all, of this heterogeneity could be explained by differences in inclusion criteria such as the time frame of the meta-analyses, the types of study designs to be included (there were some SRs that included only randomized controlled trials [RCTs], others selected only prospective studies and RCTs, and other SRs also included cross-sectional studies as well as prospective studies and RCTs), and the population included in the meta-analysis (in some SRs only children and adolescents, and in other SRs only adults). We identified two SRs without conflicts of interest
The main finding of our assessment was that those SRs with stated sponsorship or conflicts of interest with food or beverage companies were five times more likely to report a conclusion of no positive association between SSB consumption and weight gain or obesity than those reporting having no industry sponsorship or conflicts of interest. This difference could be explained by a potential bias in the design, analysis, or interpretation of the results obtained in the SRs, depending on whether the authors reported having any financial conflict of interest or not.
To our knowledge, this is the first SR that specifically examines the influence of financial conflicts of interest on the conclusions of SRs connecting SSBs and weight gain. Our results concur with those of a previous study that concluded that industry funding of nutrition-related scientific articles may bias conclusions in favor of sponsors' products
It might be worthwhile to further explore the implications of biased reviews, with their consequent dissemination of flawed information to public health policy makers and medical practitioners. Industry sponsorship is likely to have contributed to this detrimental bias. On the other hand, some researchers argue that industry should be allowed to also play a role because science should not be the dominion of only one segment of society. In their view, the body of literature would be poorer in both breath and scope if some segments were excluded from the research arena
An alternative, but less likely, explanation for our results could be the potential existence of publication bias only among non-industry-funded research projects
Our results showed that potential financial conflicts of interest do influence the conclusions of SRs. It could be argued that we have not assessed which interpretation is truly accurate and that the bias might be on the part of the non-industry-funded researchers because of prejudices or publication bias
We should consider that, in the first place, scientific endeavor should seek the truth, irrespective of financial or other interests. If other concerns influence the results of research, nutrition science as a whole is likely to suffer, partly because of incorrect information and partly through a loss of confidence in the discipline from the general public. Eventually, nutrition research itself might be at risk because perceived biases would threaten to make it irrelevant. The influence of biased reviews on policy makers and medical practitioners might also be another potential threat for public health
For all these reasons, the scientific community should make special efforts to preclude funding by parties with vested interests at all levels, to sustain the credibility of nutritional science within the general population and to protect the scientific endeavors in this field. This does not imply that industry sponsorship should be avoided completely, since it serves to drive nutrition research. However, clearly implemented guidelines and principles need to be established, to avoid dangerous conflicts of interest (for example, requiring industry sponsors to sign contracts that state that they will not participate in the selection of data or methodological assessment, nor in the interpretation of the results or conclusions reached)
Our results confirm the hypothesis that authors of SRs may draw their conclusions in ways consistent with their sponsors' interests
Summarizing, conclusions that are based on evidence about SSBs and weight gain substantially differ depending on the authors' financial conflicts of interest. SRs supported by beverage and sugar industries frequently reported a lack of association between the consumption of SSBs and obesity, leading to contradictory results when compared to original studies included in the SR. However, most reviews that reported having no conflicts of interest argued that current evidence justified public health strategies that discourage the consumption of SSBs. This lack of consistency between both groups of SRs suggests an empirical evidence of bias.
SRs with financial conflicts of interest were five times more likely to present a conclusion of no positive association between SSB consumption and obesity than those without them.
Our findings serve to draw attention to possible inaccuracies in scientific evidence from research funded by the food industry.
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We thank Dr. Kimberly Clabaugh and Dr. Susan Harrington for their attention to our query about their potential conflicts of interest in the development of their SRs. We also thank Dr. Rosalia Baena for her help in the copyediting of this text.
randomized controlled trial
systematic review
sugar-sweetened beverage