Chinese salted fish linked to cancer

Science News, June 29, 1985 by Janet Ralof

Chinese salted fish linked to cancer

In the United States and most developed countries, the incidence of nasopharyngeal carcinoma (NPC) has dwindled over the past 50 years. Not so in southern China. There, this cancer, affecting a silver-dollar-sized region at the back of the nose, is the leading cancer among men and ranks third or fourth among women. A new epidemiologic study now strongly suggests that its prevalence in China results from a single dietary factor--consumption from early childhood of salted, partially rotted fish common to the Chinese diet.

Brian Henderson, director of the University of Southern California (USC) Comprehensive Cancer Center and a collaborator on the study, says, "To my knowledge, this is the first human food to be proved epidemiologically as a cause of a human cancer.' Henderson suspects that a chemical formed during the fish's decomposition leads to this cancer.

Southern China's high NPC rate had historically been attributed to inhalation of indoor air pollutants emitted by unvented wood stoves during cooking. But Hong Kong radiotherapist John Ho had his doubts. For one thing, although southern Chinese women tend to do all the cooking, their nasopharyngeal cancer rate is only half that of men's. Moreover, a disproportionate share of Ho's NPC patients were boat dwellers who cooked only in the open air.

Ho decided that what most differentiated the boat dwellers from their landbased kin was the proportion of their diet made up of salted, partially rotted fish. Having seen signs of a similar link between that disease and dietary consumption of the salted fish in Los Angeles's Chinese population, Henderson and USC cancer epidemiologist Mimi Yu teamed up with Ho to study it further.

Their just-completed epidemiologic study, which they plan to publish soon, involved 250 Hong Kong NPC patients and an equal number of matched controls. It shows "that consumption of this food during early childhood can explain over 90 percent of all cases,' Yu told SCIENCE NEWS. Most critical, she says, is the role of the fish as "the most popular weaning food in that population.' Exposures begin early and remain high. In fact, the Hong Kong data show that if the fish is eaten during infancy and childhood, there is no apparent decrease in risk by omitting it from the adult diet. Reinforcing the apparent significance of this early exposure, the researchers say, is the young age--usually 15 to 30--at which the cancer appears in the Chinese.

"We've also demonstrated a dose response,' Yu says. Their data show that by age 10, those who had eaten the fish daily had roughly a 40-fold greater risk of developing the cancer than did those who had eaten it rarely. It's "a magnitude of risk approaching that between lung cancer and smoking,' she says.

Lacking refrigeration, most Chinese families rely on salt-cured fish, Yu notes. Ironically, because the fish is intentionally allowed to rot for a few days before salting, its softness makes it a favorite for flavoring the rice of weaning children. Interviews also suggest that when Chinese families have little fish, they preferentially feed sons--perhaps explaining NPC's higher incidence in males.

Yu expects their follow-up study in Guangzhou (formerly Canton) to be completed within a year. Back at USC she's feeding the fish--collected from Hong Kong street stalls--to rats to see if they might serve as animal models in which to study the fish's cancer causation.