Very difficult to get health or diet related JV's I would suggest facebook ad's as this is the biggest community of people on the planet and no doubt a vast number 1 in 5 are overweight ( i got that stat from the news)

Make sure you have a killer offer and a great advert though before you start spending money or you will flop.

Check out google's keyword tool for the most popullar keywords like:
lose weight fast
weight loss diet
lose fat
fat loss
quick weight loss

See how much money people are paying for these keywords and check out the ads. If the ad's appear constantly then the ad's must be working so create a simlar ad and try and find phrases with more targeted phrases and create your test facebook campaign.

doing it this way, you are relying on people on facebook to click on impulse while they are sitting at their computers whilst playing farmville and eating a whole packet of biscuits.

Just an idea.
Martin