Hiya - I've been away so catching up on posts. Dr J sez you can fast on a DD if you want but he set the 20% because most people wouldn't be able to stick to an alternate day fast. The reason it doesn't affect your metabolism is because you eat on the up day - it would only put your body into starvation mode if you did it for longer than a day.
I know there are people on here who have studied nutrition - I haven't so just have Dr J's book to go on in saying this.
There's loads of really interesting scientific information in the book too - all the background tests, the asthma study and so on. I found it really interesting and a good antidote if people try to tell you it's an unhealthy diet.