At just 6 stone 10lbs, 19-year-old Tatiana Moxey is extremely underweight and desperate for people to stop asking her if she's got an eating disorder. Weighing in at 23½ stone, jazz singer Sandra Andrew is a late-night food lover, enjoying home-cooked West Indian food through the day before feasting on junk food all night long. In the feeding clinic, under the supervision of Dr Christian Jessen, Tatiana and Sandra take part in an extreme diet swap to fatten up skinny Tatiana and slim down overweight Sandra. Meanwhile, size 14 journalist Anna Richardson immerses herself into the world of serial dieters and attempts to drop a dress size or two by following their faddy diets and lifestyle regimes. This week she looks behind the glamorous façade of modelling and exposes the body-abusing methods some models use to stay skinny as she embarks on an extreme apple-only 'model' diet. And it's no wonder we're obsessed with the size of our bottoms: Britain has the biggest bums in Europe; so Gillian McKeith is on a nationwide campaign to reduce the size of our rears. And with the help of 101 women, the latest treatments, hi-tech gadgets and machinery that claim to achieve a perfect rear are put through their paces. This week a group of Nottinghamshire lasses are lassoed to horse-riding simulators to gee up their chances of success. And if you wise up to calorie content you can still indulge without the bulge. Gillian McKeith heads to Billingsgate Market to find out if the fishmongers of London know a high-calorie breakfast when they see one.