Use AND between all of your search terms.
Example: cancer AND lung AND treatment AND diet
Why? Without the AND, these databases search your words as a single phrase, "cancer lung treatment diet."
Use * (asterisk) to search for variations of a word.
Example: diet* will search for diet, dieting, dieted, dietary, dietitian, etc.
The more terms you connect with AND, the fewer results you will retrieve. The more terms you connect with OR, the more results you will retrieve.