From this analysis it's clear that weight loss through exercise isn't particularly fast. What's important to realise is that this is a common route for people gaining weight too. A lot of people gain weight over time. Think of middle age spread. Over 5 years you could easily gain 2.5 stone. The reverse is also true. You could lose 2.5 stone over this time if your lifestyle were set up to lose weight.
So, do you think that explains why the UK is more obese than ever? Is inactivity a bigger problem than what we're eating?