You may want to start your research by reading up on a topic. You may want to consult an encyclopedia--for example the Multicultural handbook of food, nutrition and dietetics (RM216 .M85 2012) to locate concepts and vocabularies.
You might want to read a literature review or meta-analysis article in a database such as PubMed.
Here is a short video to illustrate searching PubMed.
You can search many resources, especially library databases, more successfully by typing in keywords rather than phrases or sentences.
Why? Because computers do not think. But computers are good at matching your words with words in the records in the database.
High blood pressure AND coffee AND food
gives different results than
Hypertension AND caffeine AND diet