A person involved in a 40-day water-only religious fast ended up spending last week in Nelson Hospital's intensive care unit.

Nelson Marlborough District Health Board spokeswoman Katherine Rock said on Friday the patient's condition was improving.

"The specialist who treated the person said prolonged, severe fasting can be very dangerous to a person's health," she said.

Rock would not discuss the person's gender, age or any public-health followup with the group concerned, which is believed to be Christian.

It is understood other people from the group had also been fasting, but had since stopped.

Mission Fellowship senior leader David James said half a dozen pastors from several Nelson churches met last week, and none knew of a 40-day fast that had been taking place.

Fasting and prayer were parts of the Christian life, but he didn't know of any extended fasts in recent years, he said.

About a decade ago, people from five city churches took part in a 21-day fast, James said, but in a variety of ways.

Some drank only water, others gave up one meal a day, and some stopped eating one kind of food, such as meat.

"There are many variations," he said.

Nelson GP spokesman Graham Loveridge said he had not heard of any religious fasting recently, but going without food for more than a few days was dangerous.

"Shorter-term fasting is very common in the community," he said.

"Some people are on two and five-day diets and will fast for a day for whatever reason, and there's little harm . . . in that sort of thing unless they have diabetes."

However, no food for any more than two days would pose potential problems, he said, "and 40 days is excessive".

"By the time you get into the intensive care unit, you're very unwell, and the chances of coming out with some long-term consequences are significant," Loveridge said.

Anyone contemplating a fast for longer than two days should seek medical advice first.

The faster's ability to make rational decisions would be vastly reduced after about 10 days, he said, meaning someone might decide to continue a fast when it was making them unwell. They might also suffer hallucinations.

There have been several cases reported overseas in recent years of people dying during fasts undertaken for religious or political reasons.

One of the most well-recorded was in 1981, when members of the Irish Republican Army imprisoned in Northern Ireland went on a hunger strike to protest their incarceration. Former IRA commanding officer Bobby Sands was elected to the British House of Commons while on the hunger strike, helping to raise awareness for their cause.

The prisoners drank small amounts of water but refused food altogether. Sands died of starvation in a prison hospital on the 66th day of the strike.

Nine more prisoners died of starvation after Sands. Thomas McElwee lasted the longest, at 73 total days - a full week longer than the next-closest prisoner.