If trapped in a confined space with limited supply of oxygen, the best way to survive is to stay calm and take slow and shallow breaths.

"This would help to conserve available oxygen and enable people to think through solutions and how to get rescued," said Dr Goh E. Shaun, a specialist in emergency medicine and consultant at Raffles 24 Hour Emergency.

He was referring to the incident in Chiang Rai, where 12 boys and their assistant coach are trapped in a cave.

For nine days, until they were found 4km from the entrance of the cave, the 13 survived on snacks they had with them.

Dr Goh said that while a person cannot go without water for more than a week, he can potentially survive without food for a longer period of time.

"There have been cases of people surviving 21 days without food," said Dr Goh.

"The body contains stores to supply energy to critical organs, such as the brain, for several days even without intake of food.

"This is by prioritising critical organs, such as the brain and heart, and sacrificing fat and muscle and other organs," added Dr Goh.

To quench their thirst, the boys and their coach drank water that dripped from the limestone in the cave.They also limited their movement to conserve energy. Experts say the fact that they are young and fit have helped them survive the ordeal.

But they are not in the clear yet. As rescuers work out how to get them out of the cave, reports have emerged that two of the boys and their coach are suffering from exhaustion and malnutrition.

Dr Goh said the immediate treatment for patients who have gone without food and water for days is to address dehydration first.

He said a sudden intake of food or liquid nutrition may be too much for the body's current state to cope. This is because the body has become used to being in a starvation or catabolic state.

"If large amounts of food are introduced, the body will be unable to cope and it may... worsen electrolyte imbalances, causing death," said Dr Goh.

However, "a very small amount of water and food taken orally is generally safe", he added.