Régime vs. diète
While both régime and diète can be translated as diet in English, they are not always interchangeable in French, especially when referring to a plan for losing weight. Régime is the common, everyday term, while diète carries a more medical or technical connotation.
Régime
A2This is the most common and widely used word for a slimming diet. It implies a set of rules and food restrictions undertaken, often temporarily, with the specific goal of losing weight. It is the standard term you would use in a casual conversation.
Je commence un régime lundi pour être prêt pour l'été.
(I'm starting a diet on Monday to be ready for the summer.)
Ce régime est très strict, il faut éviter tous les sucres.
(This diet is very strict, you have to avoid all sugars.)
Elle a perdu cinq kilos grâce à son nouveau régime.
(She lost five kilos thanks to her new diet.)
Après les fêtes de fin d'année, beaucoup de gens se mettent au régime.
(After the end-of-year holidays, many people go on a diet.)
Mon médecin m'a conseillé un régime pauvre en sel.
(My doctor advised me to follow a low-salt diet.)
Diète
B2This word is less common in everyday language for a simple slimming diet. It often has a more formal, technical, or medical meaning. A diète can refer to a specific eating plan prescribed by a health professional (like a dietitian) or a period of severe food restriction (sometimes even fasting) for medical or detoxification purposes.
Après son opération, il a dû suivre une diète liquide pendant une semaine.
(After his surgery, he had to follow a liquid diet for a week.)
La diète cétogène est parfois recommandée pour des raisons médicales précises.
(The ketogenic diet is sometimes recommended for specific medical reasons.)
Le nutritionniste a préparé une diète sur mesure pour l'athlète.
(The nutritionist prepared a custom diet for the athlete.)
Le médecin l'a mis à la diète complète avant l'examen.
(The doctor put him on a complete fast (nothing by mouth) before the exam.)
Pour purifier son corps, il fait une diète à base de jus de légumes.
(To cleanse his body, he is on a diet based on vegetable juice.)
Summary
In essence, use régime for general, everyday conversations about a slimming diet. It's the standard word for trying to lose weight by restricting what you eat. Use diète when referring to a more formal, specific, or medically supervised eating plan, or a severe form of food restriction, often for health reasons rather than just for weight loss.