This can be a very tricky subject as there are standard guidelines on what a Certified Personal Trainer can and can’t discuss (or do) with their clients. These particular guidelines are based upon where you live as well as well you work. In some states you need at least a two (2) year degree, in others you need a Bachelor’s Degree, a certificate or be a Dietitian, and of course in other states (or gyms), you need nothing.
The best ways to advise your clients is to stay in a “Safe Zone” of nutrition. What this means is that when you’re advising, make sure you’re doing it with your client’s best interest at heart and that the program that you put them on, does not have a counter effect on what they’re currently doing and or eating.