I’m not sure who you are certified through as a personal trainer, but ACE has good advice on the matter.
To stay within the scope of a personal trainer it seems you would have to stay within the USDA Dietary Guidelines for Americans (2010).
And it also seems that personal trainers do not have the formal training to recommend meal plans and/or supplements, but can help in other ways.
Here’s an excerpt from a blog post from ACE:
The personal trainer can educate a client about healthful eating, have the client keep a food record for several days and go through this with the client to educate him/her on areas that he/she could be eating better, educate the client on hydration, when sports drinks can be beneficial for energy during endurance events, techniques for carbohydrate loading, appropriate protein consumption, nutritious snacks/energy food during exercise, timing of post-workout meals for enhanced recovery, and more. All of these topics are out of the very comprehensive USDA Dietary Guidelines for Americans (2010) and through research published in reputable scientific journals. Until supplements are regulated by the FDA, with guidelines from the USDA, it will be very difficult for the fitness professional to recommend them without stepping outside the scope of practice.
The entire blog post can be found here:
Hope this helps.