Background
Cardiovascular disease and type 2 diabetes are examples of chronic diseases that cause significant morbidity and mortality in the general population worldwide. Effective interventions and public health policies are thus required to address the burden of chronic disease. Most chronic diseases are associated with underlying preventable risk factors, such as elevated blood pressure, high blood glucose or glucose intolerance, hyperlipidemia, physical inactivity, excessive sedentary behaviours, overweight and obesity, and tobacco usage. The development of chronic diseases may be prevented if these risk factors are addressed before they progress to overt disease. A simple, unidirectional schematic depicts the hypothesized pathways by which sugar-sweetened beverage (SSB) consumption leads to the development of overweight/obesity, prediabetes/type 2 diabetes, dyslipidemia, hypertension, dental caries, and fractures and any effects on academic achievement in children (Additional file
1). Overweight/obesity, prediabetes, dyslipidemia, and hypertension are risk factors for the development of cardiovascular/cerebrovascular disease in later adulthood. These mechanisms have not been conclusively established by research studies, and several conflicting theories have been put forward[
1‐
8]. However, as some of the SSB constituents (notably sugar but also caffeine, where added) are postulated to be involved in the mechanisms of disease development, they are included in our depiction of the disease pathways.
The role of SSBs in chronic disease has been a focus in the media in recent years. Well-known is New York City’s proposal to ban the sale of large-sized SSB products greater than 16 oz, including sodas, sweetened teas and coffees, energy drinks, and fruit drinks in restaurants, delis, sports arenas, movie theatres, and food carts[
9]. The proposed ban has been twice rejected but is being reviewed by the United States (US) Court of Appeals. Some districts have also banned the sale of soda in schools[
9], and the beverage industry has also self-regulated the sales of specific beverages to schools through the School Beverage Guidelines[
10]. In March 2014, the World Health Organization (WHO) released draft guidelines with recommendations on limiting sugar consumption (through food and beverage) to reduce public health problems like obesity and dental caries. They are recommending decreasing the total energy intake of sugar by day from 10% (recommended since 2002) to 5%[
11].
Some researchers hypothesize that SSBs are significant sources of caloric intake. With the adjusted prevalence of SSB consumption at 66% for children (2–11 years) and 77% among adolescents (12–19 years) according to the 2007–2008 US National Health and Nutrition Examination Survey (NHANES) data, the mean adjusted SSB energy intake was 178 and 286 kcal/day, respectively; intake of sports/energy drinks was 84 and 167 kcal/day, respectively, in these populations[
12]. Heavy consumption (≥500 kcal/day) occurred among 5% of children and 16% of adolescent consumers[
12]. Soda was the most heavily consumed SSB in adolescents, while fruit drinks were consumed the most in children[
12].
Several reviews have presented evidence syntheses on associations between SSB consumption and weight and include studies in children[
13‐
18]. Studies in three reviews have shown a positive effect of SSB intake and weight gain in children[
15‐
17]. Two reviews report mixed results among the subset of studies assessing SSB consumption in children[
13,
19]. The review by Forshee et al. assessed SSBs and body mass index in children and adolescents and found no association[
14]. A World Health Organization report concluded that the evidence to implicate high intake of sugar-sweetened drinks on weight gain is moderately strong and recommends restricting the intake of SSBs in children and adolescents[
19]. In addition, in a systematic review conducted as part of the Dietary Guidelines Advisory Committee deliberations, they concluded that ‘strong evidence supports the conclusion that greater intake of sugar-sweetened beverages is associated with increased adiposity in children’[
20].
One review reports a dose–response relationship between SSB and weight status but no corresponding weight loss when SSB consumption was reduced[
21]. Assessing studies in adults and children, a systematic review of epidemiological research analysed beverages by category (water, milk, soft drinks, sugary drinks, non-carbonated, fruit juices, carbonated beverages, hot beverages, and alcoholic beverages) and the authors concluded that the results were inconsistent and did not establish an association between beverage intake and subsequent weight gain[
22]. Within their beverage categories, however, the authors did not consistently separate SSBs from other drinks; for example, sweetened hot beverages were not differentiated from non-sweetened. Others have reviewed the biological plausibility of SSBs to uniquely affect the physiological energy balance regulatory systems (e.g. satiety and post-prandial regulatory systems) and concluded that known biological mechanisms did not support the concept that SSBs were somehow different from other sources of energy[
23].
Other reviews have synthesized evidence in adults and shown effect of SSBs on metabolic syndrome/type 2 diabetes[
15,
16,
24] and cardiovascular disease[
15]. It is reasonable to assume that effects on intermediate outcomes, such as insulin resistance, prediabetes, and dyslipidemia may occur in the paediatric population.
The role of SSBs in the context of total caloric intake is unclear. Although total caloric intake and caloric intake from other dietary sources are factors that may be thought of as confounders, some evidence suggests that they may be clustered with SSB consumption and general unhealthy eating habits[
15,
16,
25]. Conflicting evidence exists as to whether SSB intake is associated with increased energy intake[
13,
20] and may lower the intake of milk, calcium, and other nutrients[
13]. Furthermore, given that SSB consumption may alter taste preferences and quality of diet, caloric intake may in fact mediate the effect of SSB consumption and health outcomes (i.e. it may lie in the causal pathway).
The available systematic review evidence is conflicting and presents with several methodological issues, making firm conclusions difficult[
26]. The definition of what constitutes an SSB, for example, may vary and may not be explicitly described[
13,
17,
18,
27,
28]. Reviews may not have accounted for all variables that can confound associations between SSB consumption and health outcomes. Most reviews have addressed singular outcomes of interest to public health. Further, new primary evidence[
29‐
32] on the topic is rapidly accumulating since the last systematic review was published in 2011. To overcome identified methodological challenges, we will carry out a rigorous assessment of the bias of individual studies, which will include an evaluation of confounding and biasing factors using causal directed acyclic graphs (DAGs)[
33].
Objectives
The objective of the systematic review is to answer the following research questions:
In children, does the consumption of SSBs cause adverse health outcomes? If so, what potential moderating factors affect the causal association between SSB consumption and outcomes?
Acknowledgements
We thank Geoff Ball, Elizabeth Ghogomu, Gunnar Kraag, Sally Morton, Paul Poirier, Howard Sesso, Patricia Splett, Liz Stevens, Valerie Taylor, and Leanne Ward for their feedback on the protocol or aspects thereof; Becky Skidmore for devising searches and Jessie McGowan for peer-reviewing those searches; and Raymond Daniel for the assistance in preparing this manuscript. We have received funding from the Canadian Institutes of Health Research (FRN-132039) to conduct the systematic review. The funder was not involved in the design of the protocol or decision to submit this protocol for publication, nor will they be involved in any aspect of the conduct of the systematic review.
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Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
AdS, CH, KS, MTA, EM, PZ, BH, DM, and MT drafted the protocol. AS, LMB, SF, RG, SH, KOH, CP, ES, IS, and NW reviewed the draft critically and provided important feedback on the content. MT is the guarantor. All authors approved the final manuscript.