David Blaine's ordeal will continue even if he survives his 44-day fast - his first meal could kill him.
Blaine's stunt is nearly at an end
Showman Blaine has now been suspended in a box by the Thames in London for more than six weeks.
Experts say that the prolonged period without food makes could cause a heart attack if he does not eat the right things when he comes out on Sunday.
In addition, drinking too much fluid could lead to dangerous swelling around the lungs, dieticians say.
If Blaine's 44-day fast is genuine - and not an elaborate illusion - his body will be in extremely poor condition when he emerges from the box.
Starved of nutrients, and once all body fat has been used up, his body will have been quite literally consuming itself in order to survive.
He has been reported to be in worsening health in the final days of his fast, suffering heart palpitations and blurred vision.
Blaine will have suffered significant muscle wastage during the stunt, say experts - and even his heart is likely to have been weakened.
However, while the natural instinct is to start eating again quickly to replenish the body, dieticians say this is extremely dangerous.
Catherine Collins, from St George's Hospital in London, said: "If the diet is not adequate, you run the risk of suffering a heart attack.
"During starvation you cannibalise body muscle and parts of your organ tissue, like the liver, kidney and heart.
"As you break down the cells, it releases the electrolyte contents of the cell, such as potassium and magnesium.
"When you start to eat again, you start to make brand new cells to replace the ones you have cannibalised, and each has its own requirement of potassium, magnesium and phosphate.
WHAT'S HAPPENING TO BLAINE?
1. Brain has primacy - all systems geared to its survival
2. Sweet taste due to ketones
3. Heart wall starting to erode
4. Appetite will have gone
5. Very low blood pressure
6. Muscles weakened by inactivity
"If your diet does not contain enough of these, the body extracts them from the blood.
"If the blood electrolyte levels fall too low, this imbalance, on top of the fact that the heart is already wasted, can induce a heart attack."
She recommends either a slow start to feeding, or to introduce food quickly - but with large quantities of the vital salts.
The shrunken nature of Blaine's stomach could also cause problems, he said - he could have to be fed by tube at first because he would feel "full" after the tiniest meal.
Ms Collins added: "He needs to be careful with fluids, because, with the heart under strain, too much fluid would create an overload and he could get oedema - swelling of the limbs or even around the lungs, which is dangerous."
In addition, Blaine's immune system is likely to be extremely weak, she said, with even minor infections causing far more significant problems.