As an old NICU nurse that now teaches a theory class ... There are lots of theories/models that can apply to the neonatal population and to issues related to nutrition. The question is, which theories appeal most to you?
Start there. Which nursing theories best fit with your approach to nursing practice in general? Work from that starting point to "sketch in" how each of those theories could be applied to your research interests. Choose the one that fits best and feels most comfortable to you.
If you can't identify any nursing theories that fit with your general approach to nursing, then your problem is NOT in your research topic, but rather in your incorporation of nursing theory into your way of thinking about your profession. You'll need to do some foundational work to develop a nursing perspective of practice before you will be ready to approach the task of doing a research project from a nursing perspective.
If you can identify nursing theories you can relate to -- but are just having trouble applying one to your research project, that's a different issue.
Which issue are you having?