While you may not be prescribing supplements, as a personal trainer you can influence your client's ability to make healthy choices in their diet. The more knowledgeable you are about nutrition, the better you can help them. It also makes you more marketable. Share any areas of expertise. For body builders, you can talk about the importance of incorporating the right amount of protein and carbohydrates. For weight loss, you can talk about cutting out sugar and increasing vegetables and high fiber foods. Share whatever you have found through research and experience that has worked for you and your clients.