In the 1940s, in one of the world's great acts of civil disobedience, Mahatma Gandhi went 21 days without food, surviving only on occasional sips of water.

Gandhi was in his mid-70s at the time and thin as a reed. But he survived this hunger strike, by no means his first, without any obvious harm. In fact, he would live another four years before his life was cut short by a religious fanatic.

By contrast, Rita Chretien, the 56-year-old B.C. woman who survived nearly 50 days in the Nevada wilderness, existed, it is believed, on a small amount of left-over food and melted snow, raising the question: How long can a person get by without food or water? 

It is an issue inquiring minds have been studying for 1,000 years or more, about as long as there have been hunger strikes. (In fact, for months now, Indian scientists have been examining, for the second time, an 82-year-old mystic, Prahlad Jani, who claims to have not eaten anything in almost 70 years.)

There are no precise answers.

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Aminatou Haidar, sometimes called the Gandhi of the Western Sahara, underwent a 32-day hunger strike in 2009 to protest against her expulsion from her homeland because of her activism. (Reuters)

In general terms, the human body can go two to three days without water and, it is often said in survival guides, 30 to 40 days without food of any kind. (Many of these guides also discourage people from scavenging for wild plants or shrubs because of the adverse effects these can have.)

The female advantage

Water is clearly the most important requirement. It takes an average of eight to 10 cups of water to replenish what our bodies lose each day.

A lack of fluid causes problems with kidney function within just a few days, particularly if a person is active.

For Chretien, the three main priorities would be thirst, cold and then food, said Gordon Giesbrecht, a professor of thermo-physiology at the University of Manitoba who is known as Professor Popsicle for his experiments enduring cold-weather environments.

"The fact that this was a colder environment would allow her to live longer without water," compared with trying to survive in a desert, said Giesbrecht.

Starvation and metabolism

When it comes to food, most clinicians now believe that women can survive longer than men without nourishment because women tend to have more body fat and less muscle mass, which requires more calories to maintain.

Still, starvation or near-starvation takes its toll on the body and its internal organs, as concentration camp and famine survivors have clearly shown.

Exactly how much food or water these individuals were able to ingest has always been open to speculation.

In purely scientific terms, the 1981 hunger strike by IRA political prisoners in Northern Ireland probably produced the most usable evidence of what the modern human body can endure.

In the end, 10 prisoners died after undergoing between 46 and 73 days without food. Most had been in reasonable health.

But as Dr. Alan Lieberson reported in Scientific American magazine a few years ago, "the duration of survival without food is greatly influenced by factors such as body weight, genetic variation, other health considerations and, most importantly, the presence or absence of dehydration."

In Chretien's case, having a van for shelter during the long, cold nights might have also helped her mental state, Giesbrecht said.

Toxic byproducts

Deprived of food, as any dieter knows, the body literally consumes itself. In extreme cases, the liver starts to break down body fat and then protein in a process known as ketosis within three to four days.

Metabolism slows and the body begins to exude a sour odor as it goes on to mine muscle and organs, chiefly the kidneys and liver, for energy.

Ketosis produces toxic byproducts, which can be excreted but ultimately lead to the potentially lethal condition called ketoacidosis.

That is how starvation works. Near-starvation is a different story and has no doubt occurred often throughout human history because of famine or illness.

A result of that is that, in times of need, the human metabolism can slow somewhat to conserve energy, though many researchers feel that this ability varies from individual to individual.

In fact, some scientists believe that the genes that give rise to diabetes, a huge problem today because of our rich diets, are part of an evolutionary makeup that allowed our ancestors to survive long periods of scarcity by making more economical use of the body's energy.