Ironman: Mysterious ailment knocks Doe out of Taupo event

After nine months of fighting a mysterious illness, Ironman New Zealand hopeful Kieran Doe has had to cry off Saturday's race in Taupo.

The flying Doe, a crowd favourite and second seed behind defending champion Cameron Brown for the 25th anniversary race, has reluctantly called a halt to his career in an attempt to recover from a stomach and bowel problem.

Doe, 27, the winner of Ironman Canada two years ago, has been forced to train at no more than 80 per cent with each of his long training sessions hit by frequent problems.

"It's really disappointing. This is the big race of the year for me in front of my sponsors, family and supporters," said Doe. "Initially, this problem started last year and would happen in some intensive races. Now it happens during my hard and long training sessions as well.

"It is a hard decision to withdraw but the only one I could really make ... We've tried lots of things with medical people but we can't get to the bottom of it."

Brown, chasing an eighth successive win in full-distance races here, said he had been expecting Doe's decision and admitted his withdrawal "changes the dynamics of the race".

He could now be a little more conservative as he would not be chasing the tearaway Doe.

Other challengers include rookie ironman Terenzo Bozzone, who has often joined Doe in testing training sessions here and in the United States.

Doe will be in Taupo fulfilling sponsor commitments and supporting Bozzone in his much-anticipated debut in the ultimate triathlon challenge.

The race starts with a 3.8km swim at 6.45am followed by a 180km cycle and a 42.2km run. The winner is expected around 3.15pm.