A huge, rugged man who loves food and friends, learning and life, astrophysicist Charles Hyder seemed an unlikely candidate to give up everything for peace.

But Hyder, who sought peace for two decades to no avail, is now in Lafayette Park across from the White House in the 119th day of a fast he vows he will continue until death to prevent "a nuclear holocaust."

The 56-year-old Hyder gave up all his possessions, traveled to Washington from his home in the West and took up residence in the park last March amid a ragtag band of protesters. On Sept. 23, he says, he stopped eating and began to drink only water in a quest to rid the country of nuclear arms and to banish war from the world. He has dropped more than 100 of his 300 pounds, though he is not sure exactly how much he weighs.

Cantankerous these days as he sits among his protest signs, Hyder has little patience for questions about his size. "Weighing is irrelevant to what I'm doing," the 6-foot-2 Hyder said brusquely.

He has less patience for those who question the authenticity of his fast or the depth of his commitment. "You have to think I'm a liar to ask that question," he told a reporter who wondered if he was prepared to die.

Indeed, another protester, a man in fatigues who wandered by to scream at Hyder, told him that the peace movement is "not looking for liars."

But two experts in nutrition, interviewed about Hyder's case, said such a lengthy fast is quite conceivable for a person who started at 300 pounds. "It is not only possible," said Dr. Theodore Van Itallie, professor of medicine at Columbia University, "but in the past it has been used to treat" obesity.

Dr. C. Wayne Callaway, a nutritionist at the George Washington University Medical Center, said a person of average weight could not survive for so many days, but that at 300 pounds, a person has "substantial fat stores" in the body to provide energy.

However, both said Hyder may be nearing a critical stage because the body begins using muscle, including the heart muscle, to provide needed protein. "As he approaches normal weight," said Van Itallie, "he is at higher risk of depletion of heart protein . . . that could set the stage for a lethal cardiac disturbance."

Hyder appears to be uninterested in such talk, explaining that he has worked in the peace movement for 18 years and found that "everything that wasn't lethal . . . hadn't worked."

He said he learned that the only thing the system responds to are "dead bodies," and that the bodies from a nuclear holocaust would come too late. "So," said Hyder, "I must offer my life up before the fact."

He chose fasting, he said, because other forms of death were too quick to gain public attention for his message. "You can't do it if you go out like a roman candle," Hyder said. "The light was bright, but what was it about?"

Hyder said he would end his fast only if the United States commits to three specific conditions -- dismantling all nuclear warheads by 2000, banning the use of military operations in international relations and getting the Soviets and the rest of the world to join in these commitments.

Hyder has just begun to win slowly the attention he craves, at least from the U.S. media.

He boasted that he is "big stuff in Japan" and has been covered "three times on Russian national television." But he added, "I have been covered one time at 4 o'clock in the morning on U.S. national television. What a joke."

Still, Hyder is becoming known, and letters are starting to arrive at the Virginia apartment where Hyder's close friends and Laurie Hyder, his ex-wife, are caring for him during his more frequent days away from the park.

Chika Yamada, a 21-year-old student in Tokyo, wrote that "many Japanese young people . . . are respecting what Dr. is doing." Ed B. Richardson of Beloit, Wis., urged Hyder to "stay alive for this cause. You have proven to so many it's not your problem alone, but ours. Help us please some other way."

A woman from Kensington sent $100, and from Middletown, Ohio, came word that "my heart is there with you" and the wish that "Reagan and all his buddies take notice."

There were letters from two of Hyder's five children. Niels, 22 and the youngest, wrote, "I think about you constantly, Dad . . . . I still have much trouble with this whole thing, but I still agree with the cause."

And from Paul Hyder, the oldest at 32, came an article he had written for a peace movement newsletter. "I don't see killing myself as a solution for anything . . . . Yet, he does, and I will not refuse him the right or the dignity of his beliefs." The son wrote that the peace movement "lacks a focal point. His death may provide the kick in the pants needed to pull all this together."

Scientific colleagues have begun to join his cause, though many disagree with his method. Twenty-four physicists from Harvard University, the Massachusetts Institute of Technology and the Harvard-Smithsonian Center for Astrophysics joined in a one-day fast as a gesture of support and signed a letter saying they "respect and care for him as a colleague."

Astrophysicist Robert Noyes, one of that group, who worked with Hyder in the 1960s, described him as a "bright and well-respected" scientist who specialized in studying the sun. Noyes said that he "tried to dissuade" Hyder from his fast but that Hyder told him "he felt he had to play nuclear hardball."

In the 1970s, Hyder worked as part of National Aeronautics and Space Administration team on a mission to launch a satellite that later studied sunspots. His most recent work was with the National Center for Atmospheric Research in Boulder, Colo. -- a job he said he left in 1984 to devote all his time to peace.

He gave away his possessions and shuttled between Colorado Springs, where one son lives, and Albuquerque, where his other children reside, and lived out of a station wagon, according to Laurie Hyder. He gave away the car in November 1985 and moved to Washington.

Now he sits a few hours each day in the park. Bundled in layers of clothing, a jaunty striped scarf at his neck and a blue cap with a pompon fastened securely on his head, Hyder tells his story to all who will listen.

He says he is running out of energy. "I wake up tired and just get tireder," he joked. But said he he does not miss his work, or his food, or anything else. "I am profoundly occupied out here," he said. "There's more to do here than there is in any single life to get done."