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.

Fussy eaters put up a food fight in the feeding clinic as Superskinny Yasmin Smith is forced to swallow a gargantuan diet of snack foods and diet drinks, while Supersize Darryl Weston weighing in at a whopping 33 stone and one pound suffers on Yasmin's lactose, wheat, sugar and fat free diet. Dr Christian Jessen referees the match. Journalist Anna Richardson takes the nip and tuck route to slimming by investigating the world of cosmetic treatments, but she meets with unforeseen complications during the procedure. Gillian McKeith is sent to Coventry as part of her nationwide campaign to turn bountiful bottoms into bums of steel. This week can the personal services of a gym bunny reduce their rears? And there's no need to battle the bulge if you know the calories of your favourite foods. With Cornish pasties, chicken salad and ham rolls on offer, do the market traders of St Albans know a high calorie lunch when they see one?

This week Dr Christian Jessen tackles the eating habits of Amy-Jo, a superskinny undereater, who is due to spend five days in the 'feeding clinic' with 38½-stone Andy. The participants' attitudes towards food couldn't be more different, but they are both putting their health in danger. Amy-Jo may look perfect on the outside but her extreme exercise regime and diet, which restricts carbs, could lead to her body crumbling inside. To shock her into a new way of eating, she must gorge on the gigantic meals and colossal snacks of the biggest supersize of the series. Meanwhile Andy must learn to survive on the limited calorie intake of Amy-Jo. The aim is for them both to learn how they are affecting their bodies Journalist Anna Richardson tries another extreme diet, taking the star-studded route to dropping a dress size: living on tiny jars of baby food. She also meets former Liberty X star Michelle Scott-Lee, who spills the beans on the pressures for celebrities to be thin. Gillian McKeith has a shock in store for a team of large-bummed female rugby players from York as she continues her quest to reduce the nation's rears. Gillian also tests the knowledge of a bunch of London taxi drivers. With cheesy jackets, ploughman's and scampi and fries on the canteen menu, can they recognise a high calorie option when they see one?

Tipping the scales at just over six stones, 41-year-old Elizabeth Young thinks she's found the elixir to eternal youth: an extremely disciplined diet. She exists on a radical diet of Brazil nuts, rice, garlic and not much else. In the feeding clinic, Dr Christian Jessen, begs to differ and has the challenge of persuading her that her lack of eating is doing the exact opposite; it's prematurely ageing her. Dr Jessen's prescription is a short, sharp shock, a week of eating the food of 34 stone 9lbs supersized milkman Stefan Ginesi, the fattest contributor of the series. The Northern Irish 'carb junkie' lives on potatoes and Irish stew and is keen to lose weight to start dating and get involved in a relationship. Into the fourth week of her diet, journalist Anna Richardson goes nil by mouth when she swaps food for diet pills. On the way she meets Denise Townsend, whose prescription pills left an unsightly and permanent residue in her pants… and her toilet. And more alarmingly she encounters two young pill poppers whose cocktail of diet pills is potentially seriously damaging their health. Corporal Gillian McKeith gives a posse of Portsmouth girls their marching orders as they attempt to reduce their rears with the help of Masai technology, a trainer designed to get the wearer to copy the tribe's long-distance walking technique. And do the high-rollers at the dogs know a high calorie takeaway when they see one? With pizza, a burger and an Indian on offer which one is odds on favourite for a feast? Gillian McKeith reveals how to indulge and avoid the bulge.

Dr Christian Jessen comes face to face with twenty-six-year-old Kevin Calladine from Brighton who hasn't had a decent meal since he left home. Short on time and money, his weight has plummeted to just 8 stone 5lbs, two and a half stone underweight, which is no great surprise because Kev lives on a diet of coffee and cigarettes for breakfast, pre-packed sandwiches for lunch and a liquid dinner of orange juices. Meanwhile, supersized super-eater Julie Mead from Essex weighs in at 26 stone 6lbs. Julie hopes some motherly love will help fatten Kev up while she learns to curb her excessive food consumption. Journalist Anna Richardson continues her weight-loss crusade, hoping that hypnosis will help her drop a dress size. Surfer girl Gillian McKeith dons her wetsuit to get some Welsh rare-butts to reduce their rears. And how to have your cake and eat it: with three posh puddings on offer – Pavlova, Belgian chocolate cake and apple tart – can the horsey set guess a high-calorie pudding when they see one? Gillian offers some low-calorie alternatives.