Doctors have told Katie Christensen that her daily dose of vitamin B12 would be enough to keep the whole population of Oxford healthy.

Miss Christensen, 28, from Witney, has a rare blood disorder and has to take huge quantities of the vitamin every day - or she would die.

But she does not let the condition hold her back, and on Sunday, she completed the Great North Run half-marathon, between Newcastle-upon-Tyne and South Shields, in two hours, 32 minutes, beating her previous best time by 14 minutes.

Her brothers, Toby, 31, and Simon, 36, also ran with her in the North East and she and Toby hope to raise up to £400 for the Oxford Children's Hospital Campaign. Simon is raising money for leukaemia research.

Miss Christensen, who works at marketing information firm AC Nielsen, in Headington, said: "When I was born at the John Radcliffe Hospital, I became very weak soon after my birth.

"My parents took me back to hospital, where I was put on a multi-vitamin drip, which made me better, so they sent me home.

"Very soon, I became poorly again so I had to return to the JR, where the specialists found out what was wrong with me.

"My blood lacks the binding protein which transports vitamin B12 around the body. Vitamin B12 affects the levels of haemoglobin in the blood and the size of the red blood cells which carry oxygen in the blood.

"Thanks to the specialists at the JR, I was diagnosed with Transcobalamin 2 Deficiency, the first case found in the UK at that time, and put on oral B12 on a daily basis to flood my system with this essential vitamin.

"I still take B12 every day in huge quantities, enough for the city of Oxford, someone once told me. I take about 10,000 times what you would normally need.

"I've been doing it all my life and I will continue to do so, because without my daily dose, I would die.

"The amount I have to take has increased as I have got older.

"Quite simply, without the JR, I wouldn't be here, which is why I'd like to give something back.

"I did the Great North Run last year and it was such a great atmosphere, I wanted to do it again.

"It's a lot of fun running between Newcastle and South Shields, but it's a tough run."