The profession is regulated here. A dietista-nutricionista holds a university degree and registers with the regional Colegio Oficial de Dietistas-Nutricionistas, and that colegiado number is what separates them from an unregulated health coach. If you qualified abroad, recognition of the degree is handled before practising, not afterwards.
In practice, nutrition is a private service. The public system through your centro de salud reserves dietetic referral for specific clinical situations, so most residents either pay per consultation or use a private policy. Practices sit inside multidisciplinary clinics rather than standalone offices, mostly in the Eixample, along Gran VĂa and in Ciutat Vella, and many practitioners split the week between two addresses, so the barrio you visit can depend on the day you book.
Consultations are sold as a first appointment plus reviews, sometimes as a prepaid pack of several visits, so ask whether the plan and the follow-ups are included before you commit. Bring recent blood work and your insurance card if you have one. Confirm the consultation language when you book, because this work depends on being precise about symptoms, habits and food vocabulary, and where a medical condition is involved the diagnosis stays with your doctor.