I think the keyword does not go well with .lu and the TLD is not the cheapest. It depends on your intentions. If you are looking to resell good luck. I believe you will have a hard time finding a buyer.
If you want to monetize, how ? I assume you paid 50 euros or more, how are you going to recoup the annual fee ?
For this TLD French words would be better.
In French diet = régime
verbs: mincir, maigrir
Some names are still available to register: (dns.lu)
The domain name regime.lu is still available.
The domain name maigrir.lu is already registered.
The domain name mincir.lu is still available.