The best thing is to get a degree in Nutritional Science and become a licensed nutritionist.
If you choose to get a certificate you want to make sure you understand what your legal scope of practice includes. This is something it is helpful to talk about with your insurance company before paying for any training. I see that Venice beach talks about this on their site. They seem to be saying that if you use their services you can claim an exception to the legal rule. I think I have heard of a couple of such organizations structuring themselves in this way. I don’t know much about ISSA.
Personally I still think the answer to your question is that it is BEST to get fully trained rather than quickly trained, both to display professionalism, and to diminish possible liability issues.