() perdre du poids, which one fits?

Question

I got a quiz question from the "a besoin de" lesson:

Cette année, Michaël ________ perdre du poids.

I was using "doit" here, but the correct was "a besoin de"

I couldn't find a full explanation why the second one is correct but the first one not.

Does the meaning change in this case (I could imagine that doit would be closer linked to a real need, e.g from a medical perspective, while besoin would be more linked to his wish to lose weight, but no idea if that's the case).

Accepted Answer

Hi Matthias,
"Avoir besoin de" expresses "need" but perhaps best thought of as "necessity" 
Devoir (to have to) perhaps more like "obligation"
To take your medical point  --  Michaël, for health reasons is less obliged, but has more an imperative to lose weight.
I see this as being the key diffence between the two terms.
Hope this helps
Jim

Answer

Jim's right. Here's the corresponding lesson: Devoir vs Avoir besoin de to express "to need to" in French

Both of the two options are grammatically correct. The appropriate one is determined by contect.