Re: @AC
I had understood that no food provides a net calorie burn.
Well, to be a pedant, if it provided no net calorie burn, a) we're unlikely to eat it and b) we're very unlikely to describe it as food. Eating grass has little to none nutritional benefit for us, since we lack the advanced stomachs to use it, and we wouldn't normally refer to "grass" as a food.
There is one vegetable that is known for being so chewy and fibrous that it takes a lot of calories to masticate it fully, and since it is so rough we are hardly able to break it down when raw - the humble celery. This is often cited as a negative calorie food, but whilst chewing a whole stick of celery will only give you maybe 10 calories, the energy expended chewing is only about a pitiful 2 calories.
I guess if you chewed for an hour or more..