Good afternoon Jessica! I’d thought I would weigh in on your question since I do have the ISSA Fitness Nutrition certification. I cannot personally speak for NESTA though. I did the course because it was a requirement that I had to meet and was very happy with the course overall. The first half of the book was dedicated to the science behind it all, and then the second half was how to effectively coach people, and the basics behind nutrition programming and planning. I really feel it gave me a tremendous edge at work because so many clients will have questions on nutrition. I think for the most part most fitness nutrition certifications will cover the same basic principles, and all will give you that extra knowledge needed to help people out. With nutrition certifications I do not believe, and I could be wrong, there are any that are NCCA accredited, or accredited by any organization, so picking a reputable fitness organization would be your best bet. Both the ISSA and NESTA are accepted at a lot of the same health clubs and NESTA does have an NCCA accredited personal trainer course, which I believe may give them a little more credibility. Another important thing to remember is why do you want a nutrition certification, is it to just help people out with nutrition questions, or to broaden your base of knowledge, or maybe its both of these. If that is the case then I believe either ISSA or NESTA would do just fine. Or if you maybe want to actually become a nutritionist I would highly recommend finding an accredited college program to enroll in. Also find out the laws in your state regarding “practicing” nutrition with people, some states are more strict than others. For the most part healthy individuals or the athletic population will be the people you deal with most, those with any medical issues are pretty much off limits.
I hope this can help answer your question some, you will not be disappointed by all of the knowledge you will learn from one of these courses, I believe either or would serve you very well! Good luck!