Economic conflicts of interest may bias systematic conclusions evaluating the relationship between soft drink consumption and weight gain or obesity, according to a study published by researchers from the University of Navarra in PLOS Medicine. Maira Bes-Rastrollo, of Preventive Medicine and Public Healthfirst author of work led by Miguel Angel Martinez-Gonzalez states that systematic reviews where the authors acknowledge the existence of a potential economic conflict of interest are five times more likely to find the relationship between soft drink consumption and weight gain inconclusive than those without a conflict of interest.

Systematic reviews identify, analyze and summary all programs of study of research available on a topic. They use standardized methodology to be comprehensive and reduce potential bias, and are often used for health policy design as the best evidence available. For their study, which itself used systematic review methods, the researchers identified 17 published systematic reviews that had evaluated the association between soft drink consumption and weight gain or obesity. In six of these, the authors reported being affiliated with or receiving funding from the food or beverage industry.

Contradictory results
The researchers found that while 83.3% of the "clean" (no conflict of interest) systematic reviews concluded that soft drink consumption was a potential risk for weight gain, 83% of the reviews in which the authors admitted links to industry determined that the scientific evidence was insufficient to support a positive association between soft drink consumption and weight gain. "These findings, Bes-Rastrollo stresses, raise concerns about the accuracy of the results coming from research funded by the food or beverage industry, although it is true that this study cannot assess which interpretation available is correct. However, this concern is supported by randomized controlled trials in both children and adults, in which a association risk association between soft drink consumption and weight gain is observed.

The researchers from the University of Navarra point out that "for authors who declare that they have no conflicts of interest, systematic reviews funded by the beverage or sugar industry usually show a lack of association between soft drink consumption and obesity, leading to contradictory results when compared with the original programs of study included in the systematic review". They continue: "Our findings serve to draw attention to possible errors in the scientific evidence from the food industry-funded research ".

In this study, the German Human Nutrition high school also collaborated.