You should have no worries, if you work hard you will hopefully be working in the career you want before forty. Fair play to put in the work and go back to studying at 31. I would treat your study and job search over the coming years as though you are in actual job. By that I mean you treat your study as important as you would tasks at work, the same with when you seek a job in health and nutrition - the job search in and of itself will be a job. Also I would start immediately trying to build contacts, ask for shadowing or work experience days in the job your aiming for - find and read materials around the type of work you would be doing. Just keep a clear vision of what you want and be positive, it's a very attainable goal and I wish you all the best.