Scientists have produced the first direct evidence that fat accelerates aging, possibly speeding the unraveling of crucial genetic structures inside cells that wither with age.
A team of researchers from the United States and Britain found that the more people weigh, the older their cells appear on a molecular level, with obesity adding the equivalent of nearly nine years of age to a person's body.
The findings suggest that many health problems associated with being overweight -- heart disease, cancer, diabetes, arthritis -- may result from fat cells hastening the natural aging process.
"We've known obesity increases your risk of many diseases, and of dying early. What's novel here is that it seems that fat itself actually accelerates the aging process," said Tim Spector of St. Thomas Hospital in London, who led the study, which was published online yesterday by the Lancet medical journal. "This may not be apparent because these people may not have as many wrinkles. But underneath it looks like they are aging at a faster rate."
That could help explain, for example, why an alarming number of obese children are developing the most common form of diabetes, which had been known as "adult-onset" diabetes; prior to the surge of obesity among the young, it almost invariably had been seen only in adults.
"It might just change the whole of the body's metabolism in a way that increases aging and increases the risk for all the aging diseases," Spector said.
Other researchers said the findings are provocative and could lead to fundamental new insights into the effects of fat on a molecular level at a time when public health experts are alarmed about the number of obese people.
"We know obese people live, on average, less time. Here we are going into the DNA sequence of these people and showing this condition is associated with a biomarker of aging," said Eric Ravussin of the Pennington Biomedical Research Center in Baton Rouge, La. "I think it's going to stimulate a lot of research."
The study comes amid intense debate over the impact of obesity. More than two-thirds of Americans are overweight, including about one-third who are obese, raising concern the nation could be facing an epidemic of weight-related illnesses. Federal health officials, however, have been criticized recently for producing conflicting estimates of the impact of obesity, including a report that some said suggested people who were overweight but not obese may actually have a lower death rate. CDC Director Julie L. Gerberding and other public health experts said that finding was misinterpreted to suggest weight could be beneficial, and they remain convinced obesity represents a major public health threat.
Skeptics continue to challenge that assertion and question the new findings, saying the researchers had failed to rule out the possibility that other factors may be responsible for the results. People who are overweight, for example, may not get enough exercise, which could account for premature aging.
"It is impossible to determine if the 'aging' association with obesity is due to obesity itself or some other factors that co-vary with obesity, such as diet, physical activity, fitness, or other lifestyle factor," Glenn A. Gaesser, of the University of Virginia in Charlottesville, wrote in an e-mail.
But Spector said the results are consistent with recent findings that, contrary to the long-held belief that fat cells are inert blobs, they churn out a host of substances that can be toxic to the body.
"So it may be the body has to repair itself much faster and that accelerates the aging process," Spector said in a telephone interview. "We don't fully understand all the mechanisms of how obesity causes ill health, but this may be a central one that underpins all of them."
Spector and colleagues at the University of Medicine and Dentistry of New Jersey studied 1,122 women ages 18 to 76, including 119 who were obese. The researchers took blood samples so they could examine structures inside their white blood cells called telomeres.
Telomeres are the caps at the ends of chromosomes -- the molecules that carry genes. Every time a cell divides, telomeres shorten. In the natural aging process, telomeres eventually get so short that cells can no longer divide, and they then die. As more and more cells reach the end of their telomeres and die, the inexorable process produces the effects of aging.
Spector found a direct relationship between body weight and telomere length, with telomere length decreasing with increasing body weight. The lean women had significantly longer telomeres than the heavy women, whose telomeres were significantly longer than those of the obese women. Obesity was defined using a standard measurement based on height and weight known as a body mass index, or BMI. Anyone with a BMI of 30 or higher is considered obese.
In addition, the researchers found that the higher levels of a hormone in the blood produced by fat cells called leptin, the shorter the telomeres.
The researchers found a similar relationship with smoking, with the length of telomeres shortening with the number of cigarettes the smokers in the group smoked.
Rudolph L. Leibel of Columbia University said the findings were provocative but did not necessarily mean people who are overweight, or have short telomeres, are destined to die young.
"It may be that in a biological sense the aging process is accelerated in these individuals, but that in and of itself doesn't necessarily permit you to predict what the outcome will be," he said. "Maybe the telomere shortening is comparable to gray hair. Somebody who has gray hair is more likely to be older, but it doesn't cause aging."