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Lichtenstein AH, Yetley EA, Lau J. Application of Systematic Review Methodology to the Field of Nutrition: Nutritional Research Series, Vol. 1. Rockville (MD): Agency for Healthcare Research and Quality (US); 2009 Jan. (Technical Reviews, No. 17.1.)

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Application of Systematic Review Methodology to the Field of Nutrition: Nutritional Research Series, Vol. 1.

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1Introduction

Systematic reviews represent a rigorous approach to synthesize and evaluate scientific evidence.(1) This approach to summarize available data minimizes potential reporting bias through comprehensive and reproducible searches using clearly defined and described selections and reporting protocols. The systematic review approach enhances rigor by assessing the methodological quality of the included studies and overall strength of the body of evidence. Transparency of the process is ensured through detailed documentation of the decision making process. An analytic framework helps to clarify key questions and delineate the connecting logic between them. The tables used to summarize study characteristics and findings stand alone as independent scientific publications that can be used to document the state of the scientific evidence, provide input into program and policy decision-making processes, identify knowledge gaps and research needs, and serve as the foundation for later updates as new data emerge. The objectivity of systematic reviews comes from the approach used to review the literature with its requisite documentation and also from the involvement of individuals trained in systematic review methodologies who are unlikely to have a vested interest in the particular nutrient/disease relationship outcome and pre-defined procedures for ensuring independence of the scientific review decisions from persons who may carry preconceived ideas or personal biases into the process. Examples include investigators whose studies may be considered in the systematic review process or persons and groups who may have vested interests in the outcome of the review such as sponsors, users, consumer advocacy, and industry groups.

There is a long history for the use of systematic reviews in the medical community to develop clinical and public health practice guidelines (2, 3), set research agendas (1) and formulate scientific consensus statements.(4, 5) The use of systematic reviews to address nutrition related issues is more recent. (610) Nevertheless, there is a wide range of nutrition applications for which a systematic review process has been used or is being considered (Table 1). Although many of these applications are similar to those used in the areas of medicine and public health, characteristics unique to nutrition related topics (e.g., essentiality, habitual exposure) necessitate the development of a more complex set of research questions and approaches to the decision-making process than have traditionally been encountered in other fields.(11) It should be noted that as systematic reviews are increasingly being performed and published for nutrition related topics, the term systematic review has been subjected to various modifications to include evidence-based review, systematic evidence-based review, and evidence-based systematic review. In this article, we use the term systematic review, which is the common usage in medicine and other disciplines.

Table 1. Examples of current and potential uses of systematic reviews in nutrition applications.

Table 1

Examples of current and potential uses of systematic reviews in nutrition applications.

Understanding the basic components of the systematic review approach and how it can be adapted to address a wide range of nutrition related questions is critical to maximizing its utility and gaining wider acceptance. It is important to appreciate that the systematic review approach is flexible and can accommodate unique challenges posed by questions related to food and nutrition. It is equally important to understand that the focus of a systematic review is to provide answers to specific questions. These questions may be just a few among many needed to address an overarching topic. The answers to these questions do not constitute recommendations. Users of systematic reviews (e.g., government agencies, expert panels) must combine the results of a systematic review with other information and expert judgment to formulate clinical or public health policies. The intent of this article is to describe the steps used to perform systematic reviews and measures to ensure the integrity of the reviews to minimize bias, identify areas unique to the discipline of nutrition that should be factored into an evidence review process prior to undertaking the task, and discuss the strengths and limitations of systematic reviews for users of these reviews in setting recommendations and guidelines and other nutrition applications. We also identify areas for future consideration.

Examples of Recent Systematic Reviews of Nutrition-Related Topics

Three examples of systematic review applications are summarized for nutrition related topics:

1.

Effectiveness and Safety of Vitamin D in Relation to Bone Health, (10)

2.

Effects of Soy on Health Outcomes, (12) and

3.

Health Effects of (n-3) Fatty Acids on Arrhythmogenic Mechanisms in Animal and Isolated Organ/Cell Culture Studies (Table 2). (13)

Table 2. Systematic review steps and examples from nutrition-related topics1.

Table 2

Systematic review steps and examples from nutrition-related topics1.

These examples were selected because they serve to illustrate the comprehensive and flexible nature of the systematic review process. Although similar steps were followed, they were conducted by two EPCs (Tufts Medical Center Evidence-based Practice Center and University of Ottawa Evidence-based Practice Center). The inherent flexibility of the systematic review methodology is illustrated by the topics that address issues related to a single nutrient, vitamin D and bone health, complex nutritional interventions, soy protein/isoflavones and health outcomes, and multiple experimental models and outcomes, n-3 fatty acids and animal/isolated organ/cell culture. They also include study design foci that address issues related to animal/in vitro, n-3 fatty acids, combination of observational and intervention human studies, soy and health outcomes, exclusive reliance on randomized clinical trials, and several questions for vitamin D and bone health.

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