I know that a nut can either be a seed that we eat, a crazy person, and hardware item, but what is a NUT in the bariatric world?

Nutritionist

I wondered that when I first registered at BP, but the context gave it away.

It's a misapplied abbrev -- at least, I hope it is -- as anyone can call himself a Nutritionist. There is no standard of education and training to qualify to use the title. What matters is that a surgery practice have on staff or refer patients to a Registered Dietitian, which is a position licensed by the state only after passing an exam and having completed a course of study. Some "Nutritionists" have studied at least something, but it's still not the same.

I've never researched the difference, but your question set me to Googlin'. Here's one result:

http://eatrightdc.org/dietitian-vs-nutritionist/