The latter (who may also be called a médecin nutritionniste) is a medical doctor who did an extra diploma in nutrition, usually as part of their university studies.
He or she may be a GP but potentially practises any specialism, such as endocrinology (hormone specialist) or gastroenterology.
On the other hand, the former only has a qualification in diet, generally a state diploma such as a BTS or DUT.
In some cases, a diététicien might take longer to discuss your eating habits with you and provide a more tailor-made diet, but he or she cannot prescribe any medications or blood tests and is less qualified to consider how any health conditions might be affecting weight gain or loss. In particular, you should see a nutritionniste if there are complications such as diabetes.
While sessions with a nutritionniste are reimbursed by the French social security system, within reason (if you see them very regularly, Cpam may refuse to reimburse all the sessions), those with a dietician are not. However, some mutuelles do reimburse part of the cost.
Note, however, the usual rules apply as with most specialist doctor visits, ie. reimbursement may be at a lesser rate if you are not referred by your own GP (médecin traitant) and some doctors practise dépassements d’honoraires (they charge above the set state-reimbursed fee).
One factor that muddies the water is some diététiciens refer to themselves as diététician-nutritionniste, a meaningless addition.
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