1. You will feel hungry
It's sad but true. It's impossible to lose weight - to fast - without feeling hungry. The body, looking on fat as insurance against lean times, prefers you to forage rather than draw down the fat reserves. Hunger is its message: ignore it - tomorrow you get to eat again.
2. Weight loss does not equal fat loss
A pound of fat contains 3,400 calories (you need about 2,000 a day to operate). If you don't eat a thing, you will burn - over a day - about half a pound of fat.
A litre of water weighs a kilogram, 2.2 pounds. If you drink you will gain weight; if you eat you will gain weight; if you go to the toilet you will lose (a little) weight.
After a recent holiday my daily weight fluctuated over a week between 11 stone 3 lb and 10 stone 12 lbs, a difference of five pounds (plus or minus one kilogram). Losing fat is watching that kind of interval steadily reduce in its endpoints.
3. The 5:2 diet is not a road to anorexia
Once the fat had burned off at the six month point, my weight plateaued at c. 11 stone and refused to stay much lower. On an eating day you would be frankly appalled to watch me at dessert.
4. Fasting and exercising
About half-way through the process last year, I joined the local gym and began hour-long workouts three days a week. I soon discovered that it's:
a. Not a good idea to fast the day before a hard gym session: no energy.
b. Not a good idea to eat nothing after a gym session: no protein repair.
So I schedule gym sessions after a day of eating. When I return home I have a protein-rich salad (c. 5-600 calories) if it's a fast day, eating nothing else. I estimate a gym session burns around 400 calories.
5. It's more about cell-repair than dieting
Best not to forget that Mosley's programme promoted intermittent fasting as helping to fight cancer and diabetes - fat loss was was a desirable side effect.
So, we carry on.