People in Gaza and Sudan face famine on a catastrophic scale, the UN and humanitarian organisations are warning.

In Gaza, 27 people – 23 of them children – have starved to death as a result of what international bodies say is Israel’s use of hunger as a weapon of war.

And in Sudan, the World Food Programme (WFP) is already receiving reports of people starving to death and is on course to be “the world’s worst hunger crisis”, a UN briefing to the Security Council warned in March of this year.

What is ‘starvation’?

Starvation, in a few words, is when the human body is deprived of food for so long that it suffers and in many cases dies.

“It’s a very cruel, slow death,” Dr Omar Abdel-Mannan, a British Egyptian paediatrician and neurologist who has volunteered in Gaza said. “You basically just waste away.”

For ethical reasons, scientists have been unable to pinpoint how long starvation takes to kill. However, based on observation, it is thought the human body can last up to three weeks without food.

When is someone classified as starving?

Starvation occurs over three stages.

The first starts as early as a skipped meal; the second comes with any prolonged period of fasting when the body relies upon stored fats for energy.

The third, and often fatal, stage is when all stored fats have been depleted and the body turns to bone and muscle as sources of energy.

What happens to the body?

In the early stages, as food is first denied, the body feeds on a starchy substance called glycogen, which is stored in the liver.

Initially, the body relies upon glycogen, before turning to fats and then muscles, causing the body to shrink and the starving person to assume a gaunt, hollow-cheeked look.

The brain is deprived of the energy it requires to function, so the starving person experiences irritability, mood swings and difficulty concentrating.

“Basically, the body just slows down, as it pulls energy from other organs to keep the brain and the heart going,” Abdel-Mannan explained.

The heart’s function will also be affected eventually, with a corresponding drop in blood pressure and pulse.

An adult heart typically weighs about 300g (11oz), but records show that it can shrink to 140g (5oz) in the later stages of starvation.

Eventually, if no infection takes hold in the body, the heart will fail.

Starvation usually causes the belly to bloat, as well as nausea and vomiting.

“In children, marasmus and kwashiorkor [severe protein deficiency causing fluid retention and a swollen abdomen] are the most common acute conditions from starvation/malnutrition and require specialist management to prevent early mortality,” Abdel-Mannan said.

The starving person’s digestive tract muscles are sometimes affected, and they can lose the ability to push food through the gut. Other potential complications, many severe, can arise, such as pancreatitis.

As the immune system shuts down, most starving people succumb to secondary infections like gastroenteritis, where the body expels what remaining food it can, rather than starvation, according to Abdel-Mannan.

Without the fat and cholesterol drawn from food, the production of testosterone, oestrogen and thyroid hormones is affected.

These hormones are needed to keep bones strong and the body’s cycles regulated. Without them, bones become weak, menstruation can be affected and the risks of hypothermia rise. Brittle hair or complete hair loss, can also occur.

How do you die from starvation? The pain of hunger, which you might expect, is relatively short-lived before the body works to shore up its defences.

However, the aching caused by hunger can lead to acute physical and psychological distress in the short term.

The damage to the human body is often so extreme that giving a starving person too much food or liquid nutrition in the first four to seven days can lead to a rush of glycogen, fat and protein production in cells that could prove fatal.

“Refeeding syndrome [where food is suddenly available] can also kill patients,” Abdel-Mannan said. “Food needs to be introduced gradually and under medical controls.”

Even if refeeding is successful, starvation survivors can feel the physical and psychological effects for a lifetime.

In infants, “at least those under the age of two, starvation can limit the brain’s development, limiting children from reaching their full potential cognitively and also leaving lasting negative effects on future health,” Abdel-Mannan said.

Why is this happening?

In both Gaza and Sudan, a belligerent party is being accused of keeping food from people as part of their arsenal.

Israel is limiting aid to some 2.3 million trapped and besieged Palestinians in Gaza, pushing 1.1 million people into “catastrophic hunger”, with 300,000 people trapped in northern Gaza facing famine.

In Sudan, the warring factions – the national army and the paramilitary Rapid Support Forces – are blocking the delivery of aid to people who live in areas they do not control, resulting in nearly 18 million people facing acute food insecurity, the WFP says.

Starving civilians is a violation of international humanitarian law.