I think the important thing here is that career interest and opportunities does not have to mean climbing the corporate ladder in the literal sense of advancing to positions with more and more responsibility.
To put it a different way, Development within a specific individual contributor position is a totally viable form of career development. You mentioned yourself that you're already doing this. Growing and learning (developing your career) can happen totally independently of changing titles and climbing the ladder.
Are there journals you read? Continuing education opportunities you pursue? How do you stay on top of new advances? You mentioned you're a nutritionist. I'm sure there's new research released all the time which could help you improve your skills. If your employer wants a formal "career interest plan" there's no reason why you can't include these things in it.
Besides improving yourself, you have the opportunity to improve others. You mentioned you are in a management position. Are you mentoring and leading those who report to you? Is there an opportunity to participate in leadership development programs within your employer? If not, what about external opportunities? It's always possible to get better at managing your direct reports. With the years of experience you have, you're a valuable asset for those who are less experienced. Now is the time to develop your mentoring and knowledge transfer skills, and that can be part of a "career interest" plan.