Seeking to better their health and live longer, millions of Americans have purged their diets of red meat, butter, ice cream and sinfully rich sauces. But a new bit of scientific guesswork contends that eating less saturated fat and cholesterol will only add three or four months to a life span. That is the conclusion of three California researchers who used mathematical models to predict what would happen if all Americans heeded the advice of health groups and the federal government and altered their eating habits to get 30 percent of their calories from fat instead of the current average of 37 percent.
According to the group's figures, the number of deaths resulting from fat-related illnesses -- heart disease and cancers of the breast, colon, rectum and prostate -- would decrease only modestly, increasing life spans by about three months for the average woman and four months for the average man.
Their report, published in today's Journal of the American Medical Association, is likely to fuel a debate over how to manage the incremental risks and benefits of everyday life.
The American Cancer Society immediately said the study should not lead anyone to return to a high-fat diet, and advocates of low-fat diets questioned the findings.
Dr. Margo Denke, assistant professor of internal medicine at the Center for Human Nutrition of the University of Texas Southwestern Medical Center, told the Associated Press that low-fat diets don't necessarily help everyone but can help those prone to heart disease stave off death or debilitating symptoms.
Dr. Denke also criticized the study for failing to take into account that for every American who dies of heart disease, four others have heart disease symptoms that could be caused by too much fat in their diets.
Dr. Warren S. Browner, an epidemiologist at the University of California at San Francisco and the lead author of the study, said the projection of three to four months would surprise Americans, who he said had been led to believe that taking steps to lower dietary cholesterol would lengthen their lives considerably.
"My guess is that people anticipated it would have a bigger effect . . . maybe five or six years."
Thomas J. Moore, author of the 1989 book "Heart Failure," which accused the U.S. medical establishment of exaggerating the risks of a fatty diet, said the study's conclusions added to the evidence that there was no link between low-fat diets and longevity.
But he criticized the report for relying on mathematical models, which are not as scientifically convincing as clinical trials that follow tens of thousands of volunteers over the course of years.
To find dietary habits relatively uninfluenced by health-food trends of recent years, the study's authors, Dr. Browner, Janice Westenhouse and Jeffrey A. Tice, turned to surveys of American eating habits done in the late 1970s and gathered data on diseases.