Systematic Reviews for the 2020 Dietary Guidelines Advisory Committee
Systematic reviews are one of three scientific approaches that the 2020 Dietary Guidelines Advisory Committee uses in its review of current scientific evidence. USDA’s Nutrition Evidence Systematic Review (NESR) team is supporting the Advisory Committee in conducting original systematic reviews. In addition, NESR is working with the Advisory Committee to identify opportunities to leverage relevant, timely, high-quality, and transparently documented existing NESR reviews.
A NESR systematic review is a research project that answers a clearly formulated scientific question. It uses rigorous and transparent methods to search for, evaluate, analyze, and synthesize all relevant research studies to answer the scientific question. This allows the Advisory Committee to look at the total body of scientific evidence that has been published on a particular topic. Thus, one study is not used to answer a question, rather the question is answered based on all of the available and relevant peer-reviewed scientific studies.
NESR Systematic Reviews for the 2020 Advisory Committee
The NESR team will be using its rigorous, protocol-driven methodology to support the 2020 Advisory Committee to conduct systematic reviews. NESR’s general methodology for answering a systematic review question involves:
Our Methodology page explains more about what a systematic review is, and provides general information about the methodology we use to conduct our systematic reviews.
Prior to starting each new project, the NESR team evaluates its methodology and tools to ensure that our process aligns with current best practices in the field of systematic review. Therefore, below you will see the specific state-of-the-art tools NESR will be using to assess risk of bias and grade the evidence for systematic reviews conducted by the 2020 Advisory Committee.
Risk of Bias Assessment
Use of a risk of bias tool is key to ensuring that risk of bias assessments are done consistently across studies, and that the results of the assessment are transparent. NESR is using the following risk of bias tools for systematic reviews done to support the 2020 Dietary Guidelines Advisory Committee:
- “Cochrane risk-of-bias tool for randomized trials” (RoB 2.0) (August 2016 version) for randomized trials, including parallel group trials, cluster-randomized trials, and cross-over trials
- “Risk of Bias in Non-randomized Studies-of-Interventions” tool (ROBINS-I) for non-randomized trials
- “Risk of Bias for Nutrition Observational Studies” tool (RoB-NObs) for observational studies.
Grading the Evidence
NESR has predefined criteria that are used to evaluate and grade the strength of the evidence supporting each conclusion statement. The criteria are based on the following grading elements: risk of bias, consistency, directness, precision, and generalizability of the evidence, and are detailed in the NESR Grading Rubric.
Using Existing NESR Systematic Reviews
NESR has also collaborated with expert groups to conduct a number of systematic reviews on questions relevant to the 2020 Advisory Committee’s work. NESR will work with the 2020 Advisory Committee to identify existing NESR systematic reviews that are relevant to one or more of its systematic review questions. An existing review is determined to be relevant if it addresses a similar population, intervention and/or exposure, comparator (i.e., the alternative being compared to the intervention or exposure), and outcomes. In addition, the existing review should have used similar definitions for key terms and applied similar inclusion and exclusion criteria for selecting studies to include in the review.
If a relevant NESR systematic review is identified, NESR will work with the 2020 Advisory Committee to determine if it is timely. The Committee will consider the date range of the literature search used in the existing NESR review to determine if it is timely, or if an update is warranted. If the Committee determines that an update is warranted, NESR will conduct a literature search to identify articles published since the end of the date range used in the existing NESR review. NESR’s methodology for searching for and screening articles is described on our Methodology page. In addition, NESR will also determine whether any articles included in the existing systematic review have since been retracted.
When an existing NESR review is being used in its original form to answer a question, the Committee will carry forward the conclusion and grade from that review.
When an existing NESR review is being updated to answer a question, the Committee will consider any new evidence identified via the literature search as it relates to the conclusions of the existing review, and determine whether revisions to the original conclusion statement and/or grade are warranted.
It is important to note that all NESR reviews – existing and original – will consider all relevant research studies to answer the scientific question.
Nutrition Evidence Systematic Review Team
The NESR is a team of scientists from USDA’s Center for Nutrition Policy and Promotion (CNPP) who have expertise in systematic review methodology. The team also has advanced degrees in nutrition, public health, epidemiology, psychology, library science, or a related field.