It used to be an out and out no but current advise from cambridge is moderate exercise is fine on SS. This is (apparently) because there is a better long term success rate with those that introduce a healthy lifestyle as well as loosing weight.(my poor CDC has gone back to them 3 times as each time it has been raised on here I have gone back and made her check again!!) The guidance that came back was:
If you haven't exercised recently then don't do so in the first 2 weeks but after that its fine to introduce it slowly, such as brisk walking/swimming building up to running /fast swim over a few weeks( they also said Couch potato to 5K was OK as it builds you up slowly). If you are already a regular exerciser than carry on on but just take it a bit easy in the first 2 weeks while you adjust.
I hope this helps - I wish CDC s had to keep up with current advise because so many seem not to.
I've been dabbling with C25K and the shred DVD but stopped a couple of weeks ago as I had a stinking cold
