Note that while reference sources are excellent to use and cite in your research projects, they are NOT considered "scholarly" or "peer-reviewed"
STEP 1: Use general concepts as search terms in reference sources. Ex: Nutrition AND geriatric
STEP 2: Skim through articles list to select relevant articles. Assess: article title, encyclopedia title and date, and then open an article to judge if it meets your needs.
My search returned 28 articles, including this first article, "Geriatric Nutrition" from The Gale Encyclopedia of Nursing and Allied Health.
Step 3: Read for definitions, facts, issues, organizations, history, key stakeholders and concepts and language you can use as search terms.