You could narrow the topic of obesity in certain groups of people even more by adding a sub-topic.

Obesity in Certain Groups of People

funnel

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:

How can we prevent youth from becoming obese?