April 12 (Bloomberg) -- Scientists discovered a gene linked to a 70 percent increase in the risk of becoming obese, a clue that may explain why some people find it harder to shed weight than others on the same diet and exercise regime.

Sifting through the genes of more than 38,000 people of northern European descent, researchers from a dozen institutions studied three varieties of the FTO gene: AA, AT and TT. They found people who had the AA version faced the greatest obesity risk, while those with the TT version had the lowest.

Obesity causes complications including diabetes and heart disease and is becoming more common worldwide. The genetic discovery may give health-care professionals better insight into the growing epidemic and help with designing effective weight- loss programs. Further research may illuminate the gene's purpose and lead to new obesity treatments, researchers said.

``We know that the gene is expressed in lots of parts of the body, the brain, the fat,'' Timothy M. Frayling, lead author of the study published in the journal Science, said in a telephone interview today. ``We don't know if it's altering your appetite, your tolerance for exercise or your metabolic rate.''

The percentage of people who are obese has grown rapidly around the world as people have changed their diets to include fattier, sugar-filled foods and cut back on physical activity, due to changing work and transportation patterns. The World Health Organization estimates the number of obese people will almost double in eight years, to at least 700 million in 2012 from 400 million in 2005.

Obesity Rates

About 33 percent of adult Americans are obese, more than twice the percentage 20 years earlier, according to the U.S. Centers for Disease Control. The effect of years of poor diets and insufficient exercise also is taking its toll on children, with the percentage who are obese tripling. In the U.K., about 20 percent of women and men are obese.

``Obesity is one of the most challenging problems for public health in the U.K.,'' Mark Walport, director of the Wellcome Trust, a charity which helped fund the study, said in a statement. The discovery of a gene that influences the development of obesity ``will open up a wealth of new avenues to understand and treat common diseases.''

Obesity is measured using the body mass index, or BMI, which calculated from a person's weight and height. Someone with a BMI exceeding 25 is defined as overweight, and those more than 30 are considered obese.

Researchers now are expanding the study to include people of South Asian and African-American descent and to figure how the gene works, Frayling said. Further study results are due later this year, Walport said.

Diet and Exercise

Among the 38,759 people in the study, half had the AT version of the FTO gene, 35 percent had the TT variety and 16 percent had the AA version, Frayling said. The obesity risk among people with the AA variety was 70 percent higher than those with the TT one, while people with the AT version had a 30 percent higher risk.

Those with AA were on average 3 kilograms (6.6 pounds) heavier than people with TT, while those with AT were 1.5 kilograms heavier. Some people with AA were slender while others with TT were obese, Frayling said. The study doesn't change the message that diet and exercise are important. It will help explain why some people on a diet and exercise program will have greater difficulty shedding weight, he said.

Researchers from the University of Oxford and Peninsula Medical School in the U.K. were involved in the study. The Wellcome Trust is funding research into genetic links to diseases including diabetes, rheumatoid arthritis and high blood pressure.

To contact the reporter on this story: Frances Schwartzkopff in Copenhagen at fschwartzkop@bloomberg.net

Last Updated: April 12, 2007 14:00 EDT