I would refer her to the Paleo Magazine community. They give excellent nutritional advice and it is extremely valid. They also address such issues as weight(fat)loss and proper weight gain(muscle gain).
The beauty of it is that if you feed someone one real wholesome food, they will not go wrong. Their body will autoregulate itself to whatever weight it is suppose to be and also whatever healthy body composition is meant to be in.
You should try to get her to a Registered Dietician.
You can stay within your practice by encouraging her to eat real food (such as whole grains, fresh fruits and veggies) and teach her to read food labels. Emphasize that a product that claims to be “natural”, “gluten free”, “low carb” or “paleo” among many other misleading labels are not necessarily healthy at all. Teach her to read the back of the label so she knows what a serving size is, and exactly what the food contains. Also, the healthiest foods will probably not have labels at all!
I think a Registered Dietician is a good call for your client. That way a program can be developed specifically for her with regard to her specific nutritional needs. There should be no guessing when it comes to her individual requirements. She should avoid the pitfalls of the numerous diet plans out there that may severely restrict or eliminate important food groups and nutrients– without knowledge of the client and her needs.
If that were my client, I would simply have her increase her calorie intake by about 150 – 200/calories per day – preferably from good quality protein sources & start her out on a sensible progressive strength training program. Take it from there & monitor her progress. I give you this advice under the assumption that she is a healthy adult, not suffering from any health issues or eating disorders. If that were the case, then yes, I would consider this “out of my scope of practice.” I would then refer her out to other professionals – RD’s, MD’s, etc. But if she doesn’t have any of those issues, I think you’ll find this a good place to start. Good luck Melanie!
Without knowing pretty much anything about your client and assuming she is healthy, I would keep it as simple as possible:
-Refer her to a RD. Increasing her caloric intake would be beneficial but knowing the correct amount is the key to being successful.
-Concentrate more on strength training and lifting heavier weights.