If most veg is calorie neutral....

[Scooterloo]

in that the body uses more calories processing the veg than the calories in the veg, can I eat as much veg as I want on 5:2?!

I'm not sure that anything is actually calorie neutral, but (green) veggies are pretty low calorie items anyway. Lots of people find a large salad with, eg. plenty of green leaves, radishes, celery, mushrooms (not officially a veg!), sliced peppers, a few olives, a handful of fresh herbs and a home-made vinaigrette dressing very filling, especially with a serving of protein (boiled eggs, piece of meat, tin of fish etc.).

Cooking, though, collapses plant cell walls, so cooked veg are much denser (so your whole bowl of fresh spinach becomes 2 spoons of wilted). Also, there's the added cooking fat that needs to be accounted for.

In my experience, it's always best to MFP everything at the beginning, until you find out what are your "almost-free" foods.

Its not just about the calories its also about giving your body a rest from digestion. ..I think!

No, veg is NOT "calorie neutral"- no food has yet been found - and scientifically proven - to require more calories to digest than it contains, e.g. see Wiki , MayoClinic

However, if you just ate non-starchy veg on FDs (fast days) without any sauce or oil etc, with no other accompanying food or fruit, you would be unlikely to exceed the 500 cal limit for women.

If you made veg into smoothies / juices, you could probably drink more than 500 cals worth.

The fibre is vegetables would bulk you out so that you would not be hungry. Though probably bored, they found feeding people veg only did eliminate a few problems health wise for people...

If you juiced or made smoothies you would get ride of the fibre.