If everyone adopted a diet deriving no more than 30 percent of its calories from fat, the average American's life span would increase only three to four months, according to a new statistical analysis.

But experts who have studied the results of low-fat diets challenged the validity of the assumptions that were considered in reaching this conclusion in the analysis, being published today in the Journal of the American Medical Association.

They said that because the effect of dietary change was averaged over the entire population, the true value was obscured for people destined to develop heart disease or fat-related cancers if they continue to consume large amounts of fat.

The leading author of the new report, Dr. Warren S. Browner, an epidemiologist and statistician at the University of California, San Francisco, says he is not surprised to find such small gains in average life expectancy.

Browner, who also is an internist at the Veterans Administration Medical Center in San Francisco, notes that most people who died from heart disease and cancer were elderly, "so we cannot expect a gigantic effect on average life expectancy."

Even if all deaths from coronary heart disease were eliminated, Americans would gain only 3 1/2 years in life expectancy, he says. And if all cancer deaths were prevented, life expectancy would rise only two years.

Dr. Jacques Rossouw, a cardiovascular epidemiologist at the National Heart, Lung and Blood Institute, says that, for those people who would die of coronary heart disease, the increase in life expectancy for individuals who reduce their fat intake could be three to four times the average rise.