So when Alison heard that Amy, 31, was having gastric bypass surgery, she was stunned. The thought of Amy getting her size 12 figure back while she slobbed around in size 20 jogging bottoms was devastating.

“Amy and I had always been overweight together, we made a joke of it,” recalls ­Alison. “We were ­partners in crime and egged each other on to have another cake or biscuit. When Amy told me she was going to have a gastric band fitted I was jealous she’d leave me behind.

“I was utterly distraught at being fat on my own. Amy had always been there with me. I worried I would lose my best friend. What if Amy became ­different when she was slim?”

And after Amy lost seven stone, Alison decided it was time for her to go under the knife too, despite the dangers. Now the sisters from ­Plymouth, Devon, have lost a staggering 13 stone between them in the past six months. Amy has trimmed down from a size 22 to a size 10-12 and Alison has shed four dress sizes to be a size 12.  Alison, who works in sales, says: “We put on weight together, so why not slim together? “We’d never been big as children, but after I’d had kids and put on six stone I’d resigned myself to being fat.”

Amy’s problems ­began when she was diagnosed with ­polycystic ovaries as a teenager and the treatment she received made her gain weight. But the problem became worse because of their bad eating habits. “Alison would pop over most afternoons and we’d go through a packet of HobNobs or two with cups of tea. And she’d invite me and my hubby Nick over for a huge Chinese takeaway at weekends. We’d laugh and say ‘What’s another half a stone?’ as we piled our plates with fried rice or dunked another biscuit.”

But behind the jokes Amy – who at her heaviest weighed 18st – was struggling to cope with her rocketing weight. “I was very depressed,” she recalls. “I had lots of problems carrying my son Harrison and he was born a month early. But most of all I hated my body and pushed Nick away because I was so disgusted with my looks.

“Being a mum was a lot harder than I’d imagined and my midwife thought I had slight postnatal depression. “I went to counselling and when I noticed they’d written ‘morbidly obese’ on my notes it was a wake-up call. I knew I was overweight, but I’d never thought of health implications. I suddenly realised if I didn’t lose the weight I might not be around to see Harrison grow up. But I didn’t know how I would do it.”

The answer came to hairdresser Amy as she was watching TV a few weeks later. She was watching a programme about a woman who had gastric bypass surgery – a procedure that helps people lose weight by making their stomach smaller. Surgeons create a small pouch, bypassing your small intestine so you can only really eat small meals and your body absorbs less food. “It seemed like the ideal solution,” recalls Amy. “My lack of willpower was my downfall in all my other diets but with this I literally wouldn’t be able to eat as much.”

The only glitch was the couple didn’t have a spare £10,500 to pay for the op. “Nick was adamant. We couldn’t afford it, that was that,” says Amy. “Then a few weeks later he said he had a surprise – he had cashed in his pension early. I couldn’t believe it! It was ­impulsive but he said all he wanted was for me to be ­happy.”

But Alison was horrified and begged her not to go ahead with the op. “I was worried sick,” she recalls. “I couldn’t believe she was going to take such a drastic step. It’s such a massive operation I asked her over and over if she’d thought about it properly. She assured me she had.”

Amy had the operation in July 2008 at the South West Bariatric Centre in Somerset’s Nuffield Hospital. The procedure made her stomach permanently smaller. “I was nervous as they wheeled me down to the theatre,” says Amy. “I kept thinking that this was the first day of my new life as a slim person. Afterwards, despite the pain, I felt like a new woman straight away.”

And after shedding a huge three-and-a-half stone in just six weeks, she looked like one too. “The difference in my daily diet was immense. I used to have a huge bowl of cereal and a couple of rounds of toast for breakfast, a pile of sandwiches, crisps and chocolate for lunch and a whole pizza with chips and garlic bread for dinner before some cheese and crackers.

“After the operation I ate hardly anything. I could only manage a tiny handful of cereal in the morning, a quarter of a sandwich for lunch and a mini chicken fillet with some veg for dinner. I wondered if Alison was ­jealous,” recalls Amy. “She never said it outright but when I’d call her and tell her how much weight I’d lost she changed the subject.”

Alison wanted to be happy for her sister, but the truth was she was envious. “I was jealous of what Amy achieved,” admits Alison. “And I knew there was no way we could afford the op.” But in October 2008, as Amy grew thinner, Alison decided enough was enough. At 18st 7lb she saw her GP who referred her to a weight-loss programme, and she was then booked in for the same op as Amy – on the NHS.

Amy says: “I was really shocked when Alison said she was going to have a gastric bypass too. She’d always been so dead against it. But she could see the huge difference it had made to me and wanted that for herself.” Last July, almost a year after her little sister went under the knife, Alison had part of her stomach removed too. And as petrified as she was, she got through it with Amy’s support.

Six months on, at 12st 7lb Alison has lost six stone and is a size 12. Amy has shed a massive seven stone and got down to 11st 11lb. She wears size 10 bottoms and size 14 tops. Now the sisters don’t bother meeting for lunch, as neither can manage more than a saucer-sized plate of food.

So instead they’ve learned to love shopping instead. “The only downside of the op is that it costs me a fortune in new clothes,” laughs Amy. Their children are impressed by the changes: “I used to break out in a sweat collecting my kids Emily, 12 and Sam, 11 from school,” admits Alison. Now they tell me how great I look and how proud they are of me – and Auntie Amy too.”

● Before bypass surgery is advised, doctors will ask you to try non-surgical treatments like diet and exercise first.

● The procedure is recommended only to people with a BMI of 40 or above.

● Losing weight after surgery can help high blood pressure and high cholesterol.

● But surgery is not a guaranteed fix as some people don’t lose any weight while others who do put much of it back on.

● Weight loss will also depend on what you eat and how much exercise you do.

● The surgeon uses staples to make a small pouch in the top part of the stomach, splitting it into two parts. Then the small intestine is cut away and reattached to the pouch, bypassing the rest of your stomach. This means you can only eat small meals and food has a shorter route to pass through your digestive system so the body absorbs fewer calories.

● Between one in 300 and three in 300 people die during the procedure or in the month after.

● After the op you might feel sick after eating, particularly if you try to eat too much. Too many sugary foods can make you feel sick, faint or have diarrhoea. Also, you might need to take multivitamin tablets.

● There are other complications such as gallstones if you lose weight too quickly, painful or bleeding ulcers and one in 20 people even get open sores in the part of the stomach that is operated on.

Source: Daily Mirror