What should a trainer know to work with Obese clients?

I am hoping to open a discussion on what is needed to be qualified to work with an obese client. Training considerations, drug interactions, psychology, energy systems, biochemistry..etc. I know there are certifications that cover the basics of this stuff, but my opinion is that if a person is going to work with an obese client then that should be your niche and you should be dedicated to that. I train athletes and my personal library reflects that and so does my education.