Hunger pains can be overpowering within hours, so imagine the determination needed to repeatedly defy that instinct, in the face of constant badgering from others placing meals in front of you.

Key points: A psychiatrist says the psychological state of hunger strikers deteriorates quickly

A psychiatrist says the psychological state of hunger strikers deteriorates quickly She says current practices for encouraging people to eat don't work

She says current practices for encouraging people to eat don't work A scientist says the body can last a surprisingly long time without food

Imagine persisting, day after day, for two months — all the while wasting away.

That is the reality for Canberra prisoner Isa Islam, who has not eaten since December 9 after choosing to hunger strike.

His stated reasons were allegations against prison guards that the ACT Government said it was not aware of and that had never been reported to authorities.

Negotiations were ongoing as of the weekend while Islam's health continued to deteriorate in hospital.

His situation begs the question: how does one's mind and body cope without food for nine weeks?

Isa Islam, jailed over a stabbing in Canberra's north in 2009, has been in hospital since January. ( Supplied: RiotACT )

Attempt to regain control

Psychiatric academic Professor Louise Newman had been at the forefront of providing mental health support to asylum seekers in Australian detention centres.

The University of Melbourne researcher said that, while there was no one-size-fits-all for the motivations of hunger strikers, most felt their act was a way to resist unjustness and regain some control in an otherwise powerless and voiceless circumstance.

"The one thing that one can do if you're a prisoner of whatever variety, or in an unbearable situation, is to control your own body," Professor Newman said.

Severe depression, difficulty sleeping

She said this desire for control was a shared trait of many types of starvation, including restrictive anorexia.

A well-known hunger strike in 1981 by Irish prisoners, 10 of whom died. ( ABC News: James Glenday )

"But there's no doubt that it takes a huge amount of willpower to — at least in the initial stages — withstand food and stick to the hunger strike," she said.

"And you have to be determined to do that because of the power of hunger."

Professor Newman said the feeling of hunger was known to subside after unbearably intensifying over the first couple of days. But while the mind stopped thinking about food, it still suffered psychologically.

Professor Louise Newman has been at the forefront of mental health support for hunger-striking asylum seekers. ( Twitter: Louise Newman )

"After a couple of days, hunger actually isn't an issue," she said.

"It's more weakness and fatigue. They become very depressed and they might still be very determined to carry out their protest but they have social withdrawal, irritability and disturbed sleeping.

"Many people continue to sip water, which keeps them alive, but people often describe changes in mood and severe depression and a lack of clarity of thinking, where it becomes hard to focus and concentrate."

In her experience, Professor Newman had seen more people die from suicide while hunger striking than from starvation, with the former a far more common ending.

She said the psychological deterioration of children was particularly difficult to watch.

"They're not consciously thinking 'I must go on a hunger strike', they're just in such despair," she said.

"When you're seeing children with life-threatening withdrawal in that way it's very disturbing … it's an awful situation."

Pressuring to eat can attract the opposite

Hunger-striking is common among asylum seekers in Australian detention centres, including Nauru. ( Department of Immigration and Citizenship )

In the years of helping hunger strikers, Professor Newman had learned the human capacity to persevere one's goals through hunger and illness was so strong that "badgering" them to eat often encouraged the opposite.

"That paradoxically actually increases people's desire to keep doing what they're doing," she said.

"Saying 'you must eat, you must eat' is done with good intent because staff are worried. But it's like a child — when you pressure a child and say 'you must do that', the more resistant they become.

"A better approach would be to say 'yes, you have the availability of food and water' and just leave it there and not encourage and badger them into eating."

Environmental activist GD Agarwal died age 86 after an 15-week hunger strike against the Indian Government. ( Supplied )

Professor Newman called for more staff training in prisons and detention centres — though she said the latter required more comprehensive education — to help them understand the dangers of over-pressuring someone to eat.

"People want to feel like it is their choice to eat again," she said.

"And it is their choice, which is why we don't, and legally can't, force-feed anyone unless they become unconscious."

Professor Newman also acknowledged serious mental illnesses sometimes contributed to the decision to hunger strike.

Brain quick to switch off hunger response

But, even if psychological determination prevailed, how did the body survive months without food?

Professor Stefan Broer, a scientist with the Australian National University specialising in starvation, said he was not surprised a Canberra prisoner had been able to survive so long without eating.

He said the brain was quick to "switch off the hunger response" and adapt to starvation.

Professor Broer studies the physiological process of starvation in humans and animals. ( Supplied: Australian National University )

"They would have to be drinking water and on a drip with sodium chloride, but a normal healthy person can easily last 40 to 50 days without food."

"An obese person could possibly live a year because they would still have lots of fat reserves."

This is made possible by the process of ketogenesis, Professor Broer explained, which is when the brain can no longer be fed by glucose so the body instead makes ketone bodies from fat.

Body continues to eat itself

But even when hunger stopped, Professor Broer said the body would continue to eat itself and eventually turn to muscle when fat was exhausted.

People who have starved themselves for long periods must eat minimally for the first few days, a nutritionist says. ( ABC News: Giulio Saggin )

"There would be extreme weakness, and when there is no other muscle left the heart is targeted and the organs fail," he said.

If someone who had starved themselves for weeks returned to eating with extremely careful portion control, Professor Broer explained, there was little chance of permanent damage.

The striker would be quick to regain their lost body weight — which could be upwards of 30 per cent — and be more susceptible to rapidly gaining even more weight when their normal diet returned.

"They might return to full health, but there is no doubt that the whole experience would be very unpleasant, especially the severe mood changes," he said.