Anyone on the Atkins Diet must "follow the programme to the letter" to avoid the risk of side effects, the company behind it has warned.
Maurice Gleeson feared he would need an operation
Followers of the fashionable low carbohydrate, high protein diet were advised that mixing it with other weight-loss programmes could be dangerous.
In particular, it could be "deadly" to mix high carbohydrate foods with high fat foods, Collette Heimowitz, vice president of Atkins Health and Medical Information Services, told the BBC.
She made the comments in an interview with the Real Story programme, following an experiment in which three doctors followed the diet for a month on behalf of the show.
'Worse and worse'
One of the doctors was hospitalised and given morphine to control chronic stomach pains. He had to be fed intravenously.
Maurice Gleeson, 42, a psychiatrist in London, was taken to hospital after suffering from acute constipation.
Dr Gleeson said: "The pain just started getting worse and worse. I could not find any position that was comfortable so in the end I called an ambulance.
"I was concerned I would need an operation to unblock the bowel."
The former A&E doctor said that if there is complete obstruction of the bowel it can lead to death.
"I was very, very, very cold because with the pain I was sweating profusely and then shivering. I was very, very shut down - my fingers were blue," he said.
"I was almost crying on several occasions because I have never had pain like this before. I was completely convinced that was due to the Atkins diet."
Dr Gleeson fell ill after a month on the diet, which has been tried by four million Britons in the last five years.
Dr Sarah Brewer found the diet very successful
He kept a video diary for the duration of the diet and admitted to deviating from the low carb, high fat regime on a number of occasions. He drank small quantities of alcohol and ate ten chocolate biscuits.
Then, when the pains in his intestine became so severe, he ate bread and drank mango juice "to try and get things moving".
A few hours later, he had to call the ambulance.
'When you cheat'
Commenting on Dr Gleeson's ordeal, Ms Heimowitz said he had cheated and had not actually been on the diet.
She said: "When you cheat with alcohol, when you cheat with juice, when you cheat with bread, you're switching back to a carbohydrate metabolism.
"It's a different metabolism, so the risk factors associated with high carb', high fat is the most deadliest of combinations and you're not on the programme if that's what you're doing.
"The only reason why taking in high fat is safe is because you're burning it as a primary source of fuel."
Another doctor who tested the diet, Gill Jenkins, an air ambulance doctor and GP, found that her levels of "bad" cholesterol had risen above a clinically safe level.
But medical journalist Dr Sarah Brewer found the programme very successful.
Her bad cholesterol fell slightly and so did her fat tissue.
She lost 11lbs in weight and has decided to continue with the Atkins diet.
Ms Heimowitz said only highly motivated individuals should follow the diet.
She said people who suffer from side effects should see a doctor and follow their advice.
The company was making aggressive efforts to educate doctors about the Atkins diet, Ms Heimowitz said.
Real Story: BBC One, Monday 1 December at 1930 GMT. Also streamed live on the Real Story website.