In Levitt's style

My unprofessional observation that reminded me of the best seller:

If biodiesel catches on, there will be competition to pick up used oil at fast foods outlets, so the task of disposing of used oil will go from an expense to a revenue. This in turn could lower the cost of french fries dramatically and result in more obesity, especially among lower income people, heart disease etc. and more people who use their cars to get everywhere, because it is a greater effort with their extra weight.

Counter to this, French fries are already pretty cheap.