Heredity, rather than family life style during early childhood, is the dominant factor in determining whether a child becomes a fat adult, according to a definitive new study released yesterday.

It has long been known that fatness runs in families, but long debated whether it is the genes we inherit or the way we live -- and eat -- that matters the most.

The conclusion in favor of heredity comes from a unique, large-scale survey of 540 adults in Denmark who were adopted as infants. American and Danish researchers found a "strong relation" between how fat they were as adults and the size of their natural parents. But, to the scientists' surprise, they found no such relationship to the degree of fatness of the adoptive parents with whom they had grown up.

The team, headed by well-known University of Pennsylvania obesity specialist Dr. Albert J. Stunkard, concluded that while "genetic influences are important determinants of body fatness . . . childhood family environment alone has little or no effect."

The researchers reported that genes appeared to "exert their effect across the whole range of body fatness -- from very thin to very fat." Interestingly, said Stunkard, "mothers had more influence on body weight of children than did fathers," with the mothers' effect stronger on daughters than on sons.

The strength of the findings may play a significant role in resolving the long-running "nature versus nurture" debate about the relative importance of genetic and environmental contributions to obesity.

The study's strong support of genetics "appears to resolve the controversy," said Columbia University professor Dr. Theodore B. Van Itallie. "The study's findings are unequivocal," he wrote in an editorial in the New England Journal of Medicine, which published the research yesterday.

Anticipating that some might use the new research as an excuse for giving up on efforts to control weight, he cautioned against "a defeatist view of the problem."

"As members of a sedentary and food-laden society, obesity-prone persons who wish to control their weight must learn to maintain a relatively high level of activity and to eat defensively," said Van Itallie, who is codirector of the obesity research center at St. Luke's-Roosevelt Hospital in New York. Unfortunately, "it may require much more effort" by those whose genes naturally push them toward fatness, he added in an interview.

Stunkard and his colleagues were optimistic that their demonstration that genetics play a key role in obesity will help guide efforts to prevent obesity and provide a "basis for the understanding and eventual control of this disorder."

"Genetic predisposition" to obesity appears to be stronger and early environmental influence much weaker than had been widely believed, Stunkard said in a telephone interview. But while traits such as hair or eye color are determined at conception, he stressed that in this case "genes are not destiny."

"This means that your genes don't cause you to become obese. They make you more vulnerable to environmental effects or put you more at risk to the effects of high-fat foods," said Stunkard, a professor of psychiatry and director of the obesity research group at the Hospital of the University of Pennsylvania.

Similarly, individuals who are predisposed toward thinness may not have license to eat all the calories they wish without risking getting fat. However, he speculated that it was also possible that societal pressures to be thin could trigger reverse problems in those with a genetic tendency to be thin. Such pressures might contribute to problems such as anorexia nervosa, a condition characterized by a refusal to eat and severe weight loss, particularly among young women.

The study was based on surveys of 540 adult Danish adoptees of both sexes who were an average of 42 years old. Stunkard collaborated with scientists in Copenhagen and at the University of Texas Health Center at Houston.

The researchers used the Danish Adoption Register, a gold mine for researchers that has been used in genetic studies of schizophrenia, alcoholism and criminality. "It is unique," said Stunkard, in providing not only information about adopted children and their adoptive parents, but information on the biological parents as well.

In the study, fatness was determined by a measure called the "body-mass index," in which a person's weight is divided by the square of the person's height. Based on self-reporting, an approach that Stunkard said had been validated by other studies, the researchers looked at four weight classes -- thin, median weight, overweight and obese.

In the United States, about 25 percent of adult women are considered obese, compared with about 18 percent of men, said Stunkard. Earlier studies had suggested that 80 percent of the offspring of two obese parents become obese themselves, as compared with no more than 14 percent of the offspring of two parents of normal weight.

A National Institutes of Health panel warned last year that obesity is a "killer disease" affecting 34 million Americans. They are at added risk of heart disease, stroke, diabetes and several types of cancer.