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Dr. Vincent Felitti, founder of Kaiser Permanente's Department of Preventive Medicine and director of its obesity-treatment program, was seeing some good results. His patients were losing 50, 80, even hundreds of pounds. He might have considered the program a success, if not for the fact that the participants who were doing the best  those who were both the most obese and losing the most weight  kept dropping out.

Felitti was baffled. Why, invariably, did so many patients quit just as they approached their healthy goal weight? Ella, for instance, a middle-aged woman who entered the program in the mid-1980s morbidly obese at 295 lb., had managed to whittle her frame by 150 lb. over six months. "Instead of being happy, she was having anxiety attacks and was terrified," Felitti says. (See "The Year in Health 2009: From A to Z.")

He asked Ella what she thought was going on. "Finally, the story comes out," he says. "She had been molested as a child, both within her family and outside it. She tried to escape by marrying at 15, at her mother's urging. It was a disastrous marriage  her husband was crazy jealous. They divorced in two years. She remarried. Her new husband was also jealous. He was convinced that when she was out hanging the laundry, she was sexually posturing to attract the neighbors."

When Ella was overweight, Felitti learned, her husband was less suspicious. And her fear of his rage  perhaps he saw her new slimmer weight as a provocation?  was probably spurring her anxiety. (See a special report on the science of appetite.)

Felitti wondered if there was something similar barring weight loss in other patients  or causing obesity itself. In the late '80s, he began a systematic study of 286 obese people, and discovered that 50% had been sexually abused as children. That rate is more than 50% higher than the rate normally reported by women, and more than triple the average rate in men. Indeed, the average rates of sexual abuse are themselves unsettling: according to a large 2003 study conducted by John Briere and Diana Elliott of the University of Southern California, 14% of men and 32% of women said they were molested at least once as children.

In recent years, studies by both Felitti and others have largely confirmed the association between sexual abuse  as well as other types of traumatic childhood experience  and eating disorders or obesity. A 2007 study of more than 11,000 California women found that those who had been abused as children were 27% more likely to be obese as adults, compared with those who had not, after adjusting for other factors. A 2009 study of more than 15,000 adolescents found that sexual abuse in childhood raised the risk of obesity 66% in males in adulthood. That study found no such effect in women, but did find a higher risk of eating disorders in sexually abused girls.

Discoveries by Felitti and colleagues have also helped give rise to broader work linking stressful experiences early in life  as early as in the womb  to effects on health and behavior later on, such as an increased risk of heart disease or becoming addicted to drugs. Scientists are finding that such effects are not only long-lasting, but can even be inherited by future generations. (Watch a video about obesity and social networks.)

In decades of experiments with rats, for instance, neuroscientist Michael Meaney at McGill University in Canada and his colleagues have shown how such environmentally induced traits can be passed down  then undone, also by environment. Meaney studied rats with differing maternal styles  some were naturally nurturing (they licked and groomed their pups constantly), others were less attentive and even neglectful (mother rats placed in stressful environments like isolation had greatly decreased capacity for nurture). What researchers found was that these behavioral traits were passed down to future generations: pups born to neglectful mothers endured stressful childhoods and grew up to become neglectful mothers themselves. But when babies born to stressed or less attentive mothers were instead placed with nurturing, affectionate mothers, that early experience changed the pups. They adapted quickly to the new mothering style and grew up to tend carefully to their own offspring. These pups' adaptation was then passed to successive generations as well.

When Felitti first presented his Kaiser Permanente data connecting obesity with child molestation at a national meeting on obesity in 1990, most colleagues dismissed him immediately (one even claimed that obese people made up such stories to justify their "failed lives"). David Williamson, an epidemiologist at the Centers for Disease Control and Prevention (CDC), was the lone exception. He said that a large epidemiological study was needed to determine whether there were any implications of Felitti's findings for public health.

Felitti knew that he had just the right data set: Kaiser Permanente has the largest medical-evaluation facility in the developed world, diagnosing some 58,000 patients annually. Even if only a minority agreed to discuss their childhoods and allow anonymous use of their medical records, that would be a huge sample. And so the Adverse Childhood Experiences (ACE) study was born, as a collaboration of Felitti and another CDC researcher, Dr. Robert Anda.

For the past several decades, the ACE study has recorded reports of negative childhood experiences in more than 17,000 patients. Adverse experiences include ongoing child neglect, living with one or no biological parent, having a mentally ill, incarcerated or drug-addicted parent, witnessing domestic violence, and sexual, physical or emotional abuse. The researchers then searched for correlations between these experiences and adult health and the risk of disease.

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