WASHINGTON (Reuters) - It`s official - obesity is a major cause of heart disease, according to the American Heart Association. The organisation has added obesity to its list of major risk factors for heart disease, which is headed by smoking and also includes high blood cholesterol and lack of exercise. "Obesity itself has become a lifelong disease, not a cosmetic issue, nor a moral judgment, and it is becoming a dangerous epidemic," Dr. Robert Eckel, vice-chairman of the AHA`s nutrition committee, said in a statement. "We want to send a message to both healthcare providers and the public that the time has come to take obesity seriously." The AHA had until now listed obesity as a "contributing factor" to heart disease. About 53 percent of Americans are overweight according to a standard measurement known as body mass index (BMI), that compares height to weight. More than 22 percent of U.S. adults aged 20 to 74 are medically obese. BMI takes a person`s weight in kilograms and divides it by their height in meters squared. A person who is five feet, seven inches (1.70 m) tall and weighs 153 pounds (69.5 kg) has a BMI of 24, near the high end of normal. That same person has a BMI of 25, technically overweight, at 159 pounds and is obese, with a BMI of 30, at 191 pounds. Doctors know it is hard to fight obesity. Behaviour and genetics both act to encourage people to put on weight, and to discourage losing it. But studies have also shown that a modest weight loss of five to 10 percent of body weight, just 20 pounds in a 200-pound person, can reduce the risk of illnesses such as heart disease and diabetes. "Healthcare providers and the public need to accept that obesity is a chronic disease, just like high blood pressure," Eckel said. "Today, our understanding of obesity and its impact on coronary heart disease is in its infancy, comparable to our understanding of cholesterol`s role in heart disease in the mid-1970s," he said. Obesity is defined as having too much body fat, but the AHA says that for most purposes being overweight and being obese are the same thing.