Chief Theresa Spence, leader of the Attawapiskat First Nation in Canada, has not eaten since Dec. 10. She vows she won't eat again until Prime Minister Stephen Harper meets with her to discuss the treaty rights of her people. Spence's act of protest outside Harper's Ottawa office, one of the focal points of a larger indigenous movement across Canada called Idle No More, is one with a long and storied history, but one that can have grave consequences.

“Spence is a very, very strong woman. She’s still laughing at jokes, she’s still taking her walk around the camp every day,” Thomas Louttit, an elder from Moose Factory, Ontario, told a reporter for Postmedia News on Thursday. “Of course, she is getting weaker every day. But her spirit is strong.”

Hunger strikes are hardly just a modern affair. There are reports documenting the practice, and laws enacted against it, in the histories of countries from Ireland to India. British suffragettes, environmental protesters and political prisoners have all found it to be an effective tactic.

As long as a person is well-hydrated and otherwise healthy, usually he or she can go quite a long time without food. At age 74, Mahatma Gandhi fasted for 21 days as part of his campaign for Indian independence. Reports of other, longer fasts abound: one 1997 letter in the British Medical Journal mentions someone fasting for 40 days without dying.

In the early 1980s, Irish Republican prisoners in Northern Ireland fasted for two months or more to protest prison conditions. Ten of those hunger strikers died -- one, Bobby Sands, was elected to Parliament on his deathbed -- while 13 others survived, including one man who fasted for 70 days. Many of the survivors suffer health problems with their eyes, nerves and digestive tracts to this day.

Even if a person survives a hunger fast, the prolonged lack of food can cause health problems in the body later on. For the first few days of fasting, a person is usually still getting energy from glucose, the byproduct of food in the digestive tract. After a few days, their body starts to break down fat

How long a person can survive without food, even with proper hydration, can vary greatly, and is influenced by genetics, weight and general health.

“The body's ability to alter its metabolism is poorly understood, but it occurs at least in part through changes in thyroid function,” medical doctor Alan Lieberson wrote in Scientific American in 2004. “This may help explain the evolutionary persistence of genes causing diabetes, which in the past could have allowed individuals to survive periods of starvation by enabling more economical use of energy.”

But as a person fasting grows weaker, any physician attending them may be torn between his or her duty to provide care and the need to respect a striker's wishes.

The World Medical Association, a confederation of international physicians' organizations, recommends that physicians ascertain that the hunger striker does not have a mental health problem that undermines their ability to make decisions, and that their decision to fast or refuse treatment is not coerced.

If there's no evidence of coercion, “it is ethical to allow a determined hunger striker to die in dignity rather than submit that person to repeated interventions against his or her will,” the WMA says.