An alien sugar molecule that gets into human cells from eating red meat and dairy products triggers an immune system response that could lead to the inflammation believed to be involved in heart disease, cancer and other illnesses, scientists reported yesterday.

"This is the first example of a non-human molecule from red meat and dairy products that appears to be absorbed and incorporated into humans," said Dr. Ajit Varki of the University of California at San Diego, head of a research team that detailed its findings in the prestigious Proceedings of the National Academy of Sciences.

"Many molecules derived from animal foods can be digested, absorbed, and incorporated into human tissue," Varki said in an interview. "However, these are all molecules that are also present in humans, or are some kind of vitamin that the animals originally obtained from their own dietary sources."

Other studies have shown that vegetarians have a lower risk of heart disease and cancer, while people who eat large amounts of red meat have an increased risk. Researchers have long blamed cholesterol and saturated fat.

"Our study suggests that there may be additional factors that would encourage limitation of intake of these foods," said Dr. Elaine Muchmore, another UCSD scientist on Varki's research team.

Many foods high in saturated fat -- including beef, pork, and lamb -- also contain a lot of the alien molecule, hinting that it might be the real culprit in causing health problems, the researchers suggested.

The molecule, called N-glycolyneuraminic acid (Neu5Gc), has a notorious medical dossier. Large amounts cause "serum sickness," a severe allergic reaction, because the human immune system recognizes Neu5Gc as non-human and tries to destroy it.

Serum sickness used to be a risk in people given rabies or tetanus "antitoxins" made from the blood serum of horses, which contains Neu5Gc. The disease is rare today because those antitoxins now are made from human blood.

Varki, Muchmore and an associate became guinea pigs to show that Neu5Gc is incorporated into certain proteins made in the cells of people who eat meat and dairy products. They drank Neu5Gc extracted from pork and then analyzed their own body tissue to track its path.

"It is conceivable that gradual Neu5Gc incorporation into the cells of the body over a lifetime, with the subsequent immune response, could contribute to the inflammatory processes involved in various diseases," Varki said.

The presence of Neu5Gc may hinder use of pigs or other animals as organ donors for humans, Varki said, because the human immune system may attack the Neu5Gc.

But Dr. John J. Fung, chief of transplantation surgery at the University of Pittsburgh Medical Center, noted that genetic engineering techniques can remove undesirable genes in prospective donor animals, including the gene that makes Neu5Gc. Fung and Dr. Yu-Teh Li, an expert at Tulane University, saw no public health implications of the Neu5Gc study aside from reinforcing long-standing recommendations against eating too much red meat.

Li observed that Varki had discussed the findings at an Australian scientific conference last July. "As a side note," he said, "I distinctly remember that after presenting the work on Neu5Gc, I saw Dr. Varki consume a large, juicy Australian steak for lunch."