When you do SS on Cambridge, you do it for 4 weeks at a time and every 5th week you do Add A Meal. You still have your packs 3 or 4 depending upon your height and add a very small meal of protein and green veg. It's a very small meal and doesn't knock you out of ketosis and most people find they enjoy having this small amount of food and then get straight back into SS the following week.
It's something that you have to do on Cambridge, if you choose not to Add a meal, then you have to get your Dr's sig to carry on SSing without it.
Ah thats cool, I think I'd be able to get my GPs signature as he had to give it to me for LL and that was for a 14 week solid period. But actually I think I would enjoy eating real food, its almost like a mini-goal probably.
Thanks for answering my q, look forward to joining the CD bunch!