CHICAGO – Scientists are puzzling over a surprising increase in stomach cancer in young white adults, while rates in all other American adults have declined.

Chances for developing stomach cancer are still very low in young adults, but the incidence among 25- to 39-year-old whites nonetheless climbed by almost 70 percent in the past three decades, a study found.

National Cancer Institute researchers and colleagues examined new cases from 1977 to 2006 of cancer in the lower stomach, which can be caused by chronic infection with a common bacteria called H. pylori. It also causes stomach ulcers.

Overall, there were 39,003 cases detected in a surveillance program that covers about one-fourth of the U.S. population.

These included only 734 white young adults, but their incidence rate climbed from .54 per 200,000 to about 1 per 200,000.

Among white adults ages 25 to age 84, the rate declined from almost 12 per 200,000 to 8 per 200,000; among black adults it declined from about 27 per 200,000 to 19 per 200,000.

Dr. Jaffer Ajani, a digestive cancer specialist at the University of Texas M.D. Anderson Cancer Center in Houston, said the increase in young white adults is surprising but not alarming. Ajani, who was not involved in the research, said it could be a statistical blip but that it needs to be investigated.

The study appears in today’s Journal of the American Medical Association.