Bobby Sands took 66 days to fast to death; Francis Hughes, 59, and Raymond McCreesh, 61. And yesterday, Patrick O'Hara died after 61 days. The critical period, roughly 60 days, is a constant not of the Irish soul but of the human body. It is determined by the amount of fat and muscle tissue an average-sized person can sacrifice to keep the heart pumping and the brain alive.

Women -- whose bodies contain a higher proportion of fat -- and overweight individuals of either sex can survive a bit longer. But "a normal man . . . has just about enough fuel power, if he drinks water and stays quiet, to last about 60 days," said Dr. Oliver E. Owen, a professor of medicine at Temple University Hospital who has done research on starvation.

Most of that fuel power resides in fat or adipose tissue, which is designed solely to store energy for use in times of need. In the average person, fat makes up 20 percent of body weight, or about 31 pounds for a 155-pound man. That amount of fat contains four times the energy found in total body protein, the substance of which the muscles, heart kidneys and many other organs are chiefly composed.

The body adapts to fasting by slowing its rate of fuel consumption and even switching the brain to an alternate source of energy. In the early days of a fast, according to Owen, "the body is not very economical. In the first few days, it begins to waste a variety of fuels," breaking down not only fat but also protein contained in "lean tissue" -- muscles and vital organs like the intestines, spleen and kidney -- to supply the brain with its preferred fuel, glucose or sugar.

But as a person continues to fast, the body quickly adjusts. One of the first changes, occurring within 48 hours, is that the brain -- the organ with the highest energy demands -- begins to consume mostly ketones, a breakdown product of fat, rather than glucose, which can only be manufactured from protein.

"It is the existence of these ketones which makes it possible for the body to perserve lean tissue," said Dr. Theodore B. Van Itallie, special advisor to the surgeon general on human nutrition. Because the brain still requires some glucose, a small amount of lean tissue is burned, but most energy now comes from expendable fat.

As starvation continues, there are other adjustments."Metabolic rate goes down . . . physical activity dimlnishes and some psychological changes occur," said Van Itallie. "People becomes sort of involuted, depressed. They lose interest in the outside world."

The pulse slows, the blood pressure falls below normal, and the fasting person goes into a kind of low gear, burning up far less energy than would be predicted for someone the same size who was eating normally, according to Owen.

"The body gets cold -- the body temperature falls," he said. "That makes sense, because for every few degrees of temperature rise you have, you augment your metabolic rate."

Ordinarily, Van Itallie said, there is no pain. After the first few days, when the brain has switched to ketones, hunger subsides and some people may even feel euphoric.

He said the neurologic symptoms reportedly developed by some of the hunger-strikers after prolonged fasting -- blindness, hearing loss and trouble with speech -- are probably related to deficiencies of sodium, calcium, magnesium, phosphorus and other substances needed for the normal functioning of nerves.

The blindness is probably caused by lack of vitamin A, a vitamin required by the retina that is stored in fat, according to Dr. George L. Blackburn, associated professor of surgery at Harvard Medical School. Blackburn said hunger-strikers first develop night blindness, which then progresses to total blindness.

The final phase of starvation begins when all the body's fat stores have been exhausted, and protein from muscle and vital organs must be broken down to feed the brain. Blackburn likened the situation to the action in the movie, "The African Queen," when Humphrey Bogart gradually dismantles his wooden boat to keep the boiler going.

"When you start getting down to the keel, you sink," he said.

In a hunger striker, death occurs when about half of the "lean tissue" or protein has been destroyed. Owen said that in many cases, the muscle wall of the heart grows thin, the remanining fibers begin to fracture, and the heart weakens as a pump. In other cases, the hunger striker dies of pneumonia, because of decreased immunity to infection and difficulty coughing and breathing deeply.

But by then, according to Van Itallie, the striker has slipped into a coma because "all of the biological processes get slowed down so much" from lack of fuel. "Consciousness does require a certian level of electrical activity in the brain," he said.

Because of this natural sedation, there is little suffering. "No way to die is good," said Van Itallie, "but I think that to some extent it is a painless way to die, a gradual way. I think people may become euphoric or may hallucinate, or just gradually fade away without any distress."

He added, "The suffering is more for the people around them."