Nutrition Evidence Library -- About


Nutrition Evidence Library (NEL): Answers food and nutrition questions with systematic reviews

What is the Nutrition Evidence Library?

The NEL is a systematic review entity within the USDA’s Center for Nutrition Policy and Promotion (CNPP) that uses state-of-the-art methodology to search, evaluate, and synthesize food- and nutrition-related research. The NEL is staffed by Federal nutritionists and librarians with expertise in systematic review methodology. CNPP’s mission is to improve the health of Americans by developing and promoting dietary guidance that links scientific research to the nutrition needs of consumers. The NEL supports CNPP’s mission by conducting systematic reviews to inform Federal nutrition guidance, policy, and programs. It is important to note that the NEL is not a repository of all food and nutrition research, nor is it a search engine or database.

What is a systematic review?

A systematic review is a research project that answers a clearly formulated scientific question. It uses systematic and explicit methods to identify, select, and critically assess all relevant research studies, and to collect and analyze data from the studies that are included in the review.

This is an image depicting the systematic review methodology. It starts with a question mark to indicate that a systematic review begins with a question. An arrow then goes to boxes with an “x” and a “check mark” and then thumbnails of three journal articles, indicating that articles are either included or excluded based on specific criteria. Arrows then point to the word “evidence” that is compiled to reach an answer, which is depicted with an “A.”
This is an image of a question mark.A systematic review question is very precise. It has a clearly defined population, intervention or exposure, comparison, and outcome. It is answerable.

This is an image of boxes with an “x” and a “check mark.”Specific criteria are defined for including and excluding literature. This helps minimize bias and ensure that the optimal literature is identified to answer the question.

This is an image of thumbnails of three journal articles.A body of evidence is carefully summarized by systematically and objectively extracting relevant data from all literature meeting the inclusion criteria.

This is an image of the letter “A.”The body of evidence is synthesized and interpreted to answer the research question. Objective criteria are used to evaluate the strength or level of confidence in the evidence.

How are NEL systematic reviews conducted?

This is a flowchart showing the six steps of the Nutrition Evidence Library systematic review process. The steps are: Develop systematic review questions; search, screen, and select studies to review; extract data and assess the risk of bias of the research; describe and synthesize the evidence; grade the evidence and develop conclusion statements; and identify research recommendations.The NEL uses a six-step systematic review methodology. Its development was informed by the Agency for Healthcare Research and Quality [1], the Cochrane Collaboration [2], the Academy of Nutrition and Dietetics [3], and the 2011 Institute of Medicine systematic review standards [4]. The NEL’s state-of-the-art methodology is rigorous and protocol-driven. It is designed to maximize transparency, minimize bias, and ensure that systematic reviews are relevant, timely, and high quality. This complies with the Consolidated Appropriations Act of 2001 (Data Quality Act), which mandates that Federal agencies ensure the quality, objectivity, utility, and integrity of the information used to form Federal guidance.

NEL systematic reviews are completed in a collaborative manner. The NEL works with expert groups, who are convened to review the state of the science on nutrition and health. These groups include domain experts, nutrition scientists, clinicians, epidemiologists, methodologists, communicators, and/or end users of a review.

How are NEL systematic reviews used?

NEL systematic reviews help inform Federal dietary guidance. For example, a series of systematic review questions were answered by the 2010 and 2015 Dietary Guidelines Advisory Committees. These systematic reviews, along with other sources of evidence, are examined by the advisory committee and inform the advice the advisory committee provides to the Federal government as USDA and HHS prepare to develop the next edition of the Dietary Guidelines. The Dietary Guidelines are the cornerstone of U.S. governmental efforts to promote health and help prevent disease through diet.

Where can I find completed NEL systematic reviews?

NEL systematic reviews are transparent and accessible to the public. Documentation from each step of the systematic review process for all reviews completed by the NEL is available on the NEL Systematic Review Projects webpage. This includes research questions, literature selection criteria (inclusion/ exclusion criteria), search strategies, lists of included and excluded studies (with rationale), assessment of the risk of bias of included studies, evidence summaries and tables, key findings, graded conclusion statements, and research recommendations.


[1]    Methods Guide for Comparative Effectiveness Reviews. AHRQ Publication No. 10(14)-EHC063-EF. Rockville, MD: Agency for Healthcare Research and Quality; January 2014. Available from:
[2]    Higgins J, Green S, editors. Cochrane Handbook for Systematic Reviews of Interventions Version 5.1.0: The Cochrane Collaboration; 2011 [updated March 2011].  Available from:
[3]    American Dietetic Association. Research and Strategic Business Development. Evidence Analysis Manual Chicago, IL: American Dietetic Association; 2012 [updated January 2012].  Available from:
[4]    Institute of Medicine. Finding What Works in Health Care: Standards for Systematic Reviews. Washington, DC: The National Academies Press; 2011. Available from:

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