One way narrow a broad topic from the health sciences is to add a specific population, intervention (treatment)*, or health outcome.
*In the above example, a gluten-free diet is an intervention/treatment for people with celiac disease.
The library has access to many scholarly articles related to nutrition through its database subscriptions. However, If you don't narrow your results to "full-text" (the articles you can read right away) when searching in Academic Search Complete, you may come across articles from the nutrition journals listed below that you can't access. Instead of an online subscription, the library subscribes to the print version of these journals, which you can find on the 2nd floor of the library.
What you learn from your background research may spark an idea for a more specific investigation in the scholarly research. Since most scholarship tries to measure a concept in relationship to another concept, you will want to search for your topic in relationship to something else. To find scholarly articles, the library has a number of databases that have this kind of content.
For example, if you want to learn about the effect of a gluten-free diet on the gastrointestinal symptoms of adults with celiac disease, you will want to find scholarly articles.