You could narrow the topic of obesity in certain groups of people even more by adding a sub-topic.
|an age group||Obesity in youth|
|a location||Obesity in the U.S.|
|an additional topic||Obesity prevention|
|a population||Obesity in women|
|a timeframe||Obesity in the last 5 years|
It is helpful to write out your topic as a sentence or a question. Let's say the question that most interests you is: