Is increased physical activity an effective intervention for preventing obesity in children and adolescents?
Start by searching the two or three most important concepts, then add in additional concepts as needed to narrow the results to make the question more specific and to get the results to a manageable set.
You may not need to use all the PICO terms in your search.
The more concepts you AND into a search, the results become smaller.
For the comprehensive results required by IRs:
Use both keywords and subject headings in each database for each term/concept.
Be consistent and conduct searches systematically in each database: use the same strategy and keywords; subject headings will differ from database to database.