This is a great mash-up idea! Although I have never heard of a nutrition focused concentration in the field of psychology, these areas are not mutually exclusive. There are many people in the field of psychology/ counseling whom understand the importance of nutrition (and other facets of healthy living) in the understanding of their clients well being. There is a progressive movement of people in the field who believe psychotropics do more harm than good, and that food/ exercise/ meditation are more appropriate means of helping people heal. I do not see any reason why you could not combine these fields successfully. Albeit, if you have visions of working in the medical or school systems this would not be acceptable. If however, you are interested in private practice, you could advertise your methodology and would attract clients who are interested in the types of services you offer.