There's more to it than diet and exercise.
Having constantly high blood sugar will keep your insulin levels elevated which will eventually render you insulin resistant, or, will knacker your pancreas.
That's how people get Type II diabetes.
EDIT: I'm sure if Stephen Fry is putting effort into going for long walks he has probably altered his diet too. Another problem with calorie restricted diets is that you lose weight in the ratio of 60/40 fat/muscle but because they raise your insulin sensitivity you become more likely to put the weight back on. It has been found that the weight usually goes back on 80/20 fat/muscle. This is why people yo-yo diet.
(, Sun 27 Jun 2010, 12:11, Reply)