People from large families have an increased risk of stomach cancer, suggests a study that followed more than 7,000 Japanese-American men for 28 years.

The study concluded that family size had a major influence on the development of stomach cancer linked to the bacterium Helicobacter pylori, and that younger siblings from large families were especially prone to the most common form of stomach cancer.

H. pylori lives in the mucous layer of the stomach and is associated with peptic ulcers and stomach cancer. It's estimated that half of the world's population carries H. pylori in the stomach. It can be transmitted orally from person to person or through contact with human feces.

The study found that men who carried certain strains of H. pylori in their stomachs and had seven or more siblings had more than twice the risk of developing stomach cancer, compared to men with the same H. pylori strains who had one to three siblings.

The findings are published in the Jan. 16 online issue of the journal Public Library of Science Medicine.

"This is a very carefully controlled study that clearly shows that there are factors in early childhood that affect the risk of developing cancer many decades later," study leader Dr. Martin J. Blaser, professor and chairman of the department of medicine, and professor of microbiology at New York University Medical Center and School of Medicine, said in a prepared statement.

"That early childhood events affect the risk of cancers occurring in old age is remarkable, and this may be a model for other cancers," Blaser said.

He said that younger children in large families may acquire H. pylori from older siblings at a time when the younger children's immune systems are still developing. This, in combination with the fact that the bacterium is already adapted to a genetically related person, means the younger children may have a more virulent H. pylori population in the stomach than if they'd acquired the germ from a non-relative.