Easy enough to do.
The participant takes their weight and also photographs themselves with a newspaper, to prove the date they started is consistent with everyone else.
The contest then runs for a pre-defined number of weeks, with photos including one newspaper shot every 4 weeks.
The best thing about one that could be done on here is that although we wouldn't offer cash, we would not require anybody to use some brand name companies fat burning supplement, because hey, you get to sign up for nothing

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I hate that kind of thing. Just sign people up for $10-$15 and stop forcing them to use potentially pricey rubbish, that will end up making you more money, without needing as many signups. Frankly if a company did what I said, paid the prize and lost $200 from lakck of signups, it wouldn't bankrupt them, as many people doing these things are rich supplement comapnies who could afford to lose $200 on a slightly unsuccessful challenge concept.
Though yes what you say is feasible, but would require more people on here to agree to it.