If you are making specific recommendations you are opening yourself up to liability that is not covered on any fitness instructor insurance. When anyone with liability insurance does something outside of their scope of practice, the insurance company abandons them completely. Aside from the possible dangers of allergies and medication interactions, doing things outside of your scope of practice could also come back to haunt you if you are ever taken to court over any type of claim. The plaintiff’s lawyer will be able to show that you work outside of your scope. It may be a rarity, but if it happens, the headache would be pretty bad. When there are so many other ways to help clients, why would you go outside your scope of practice?
You can do many things nutritionally, but if you charge a fee, you should be able to show that what you are permitted to provide nutritional guidance in your state. This is a gray area in many states, but courts tend to look to scope of practice. If your certification does not state that you can give nutritional advice or states that you cannot, you will not do well legally.
I would speak to a lawyer in your area. Not checking is not a defense in court.