Is one allowed to enter into a weight loss contest, which entails that all contestants put money into a pot, and whomever losses the most weight by the appointed date takes all the money in the pot?
My assumption is that you are asking if this is considered like mesachek b’kuvia. One is allowed to enter in such a contest. The reason it would not be mesachek b’kuvia is because the money was already put into the pot, and at the time that the winner gets the money, no one has to really pay anything or losing anything at that point. Therefore it is permitted.
If however one of the people will want to back out of the deal in the middle, because they are afraid of losing the money, then the halacha might be different, depending on how the deal was made.