Generally speaking, the average woman needs 2000 calories; men 2300. For the sake of simplicity, we’ll just use 2300, which will include basal metabolic requirements (just living breathing, repairing, digesting etc) and basic everyday mundane activity, but excludes exercise.
We need 2300, but are only eating 800, which leaves a deficit of 1500, and the body has to take that from its fat stores. Multiply that 1500 by 7 equals a weekly deficit of 10500. Divide that by 3500 (calories in a pound of fat) gives a weekly weight loss of 3 pounds. Now we introduce a bit of magic. If I swim 50 lengths and cycle 11.44 miles I expend roughly 900 calories, which cancels out the 800 I am eating PLUS there's an extra 100 to come off daily total, 700 weekly. Bonus! But as far as the body is concerned I am eating nothing per day. So, a few more sums. NOW the daily deficit is 2300! Multiply by 7 equals 16100, divide by 3500 equals a 4.6 pound loss per week. But add in the 700 bonus calories takes us to a total of 4.8 pounds. And that’s not including a14% metabolic advantage (more of that in a separate post). Now, how this pans out in reality depends on weather and motivation, but I would expect the least loss to be in the order of 4 pounds.
We’ll see next weigh in.
Update.
Yesterday, after my 20 hour fast I only ate 400 calories and was not feeling hungry at my cut off time of 6pm. How did I do? Well, I kept going, and this morning went for a 50 length swim, followed by a 11.44 mile bike ride. Got home and broke my 21.5 hour fast at 12.30pm. NO hunger WHATSOEVER!
Counting calories
Low carb high fat 
All in the mind ?
BMI