About 59 million Americans are suffering from a disease. This illness sneaks into peoples’ lives, and it grows on them as they perform daily routines such as eating lunch. But once this disease takes hold of a body, it could put a person at risk for heart disease and other serious medical complications. It could also cause a person to develop social problems, including poor self-esteem. This disease–which has reached a record high number of people in the U.S.–is called obesity, or having an excessively high level of body fat.

What is the cause of the nation’s expanding waistline? “Obesity is a very complex disease that involves many factors,” explains Myles Faith, an obesity researcher at the University of Pennsylvania. So far, scientists have identified obesity-causing factors ranging from the way people eat to how they spend their pastimes to genetics (heredity). And with approximately 300,000 people dying from obesity-related complications each year, an all-time high, scientists are determined to help find solutions that will stop this disorder.

DEFINING OBESITY

It takes more than a glance to determine if a person is obese. That’s why scientists measure a person’s Body Mass Index (BMI), a mathematical formula that calculates a person’s weight adjusted for his or her height. (For more on the BMI, see Web Extra, p.20.) If the BMI falls within a specified range, (25 to 29.9 kg/[m.sup.2]), an adult is considered overweight. For adults, if the BMI is above the overweight range (over 30 kg/[m.sup.2]), the adult is likely considered obese.

For people under the age of 20, scientists categorize the BMI differently. They use a chart of BMI ranges that’s specific to the person’s age and sex. And these ranges do not include a category for obesity. That’s because growing boys accumulate fat differently from growing girls. Additionally, developing bodies have ever-changing fat levels. “So we talk in terms of ‘underweight,’ ‘normal weight,’ ‘at risk of overweight,’ and ‘overweight,’” explains Michele Maynard, an epidemiologist (scientist who studies rates of diseases within a population) at the Centers for Disease Control and Prevention (CDC).

Scientists have found strong evidence that people who are overweight as kids and teens are likely to remain overweight or become obese in adulthood. Of particular concern: Since the 1980s, the number of overweight U.S. children between ages 2 and 11 has more than doubled. Even more staggering: In adolescents between ages 12 and 19, the number has more than tripled (see graph, p. 19). “We’ve also been seeing children and adolescents with increased frequency of type 2 diabetes (blood-sugar disorder) and high blood pressure,” says Maynard. “These are conditions once primarily associated with adults.” Why are so many bodies now tipping the scales?

BIG QUESTIONS

Like a machine, the body needs fuel to operate. When you eat, your body converts that food into energy. And what your body does not use for fuel is stored as reserve–fat. “Obesity is caused by an imbalance between energy input and energy output,” says Maynard. That means, when anyone consumes more calories (energy units in food) than he or she is able to use, or expend, the body is likely to gain fat.

But every person’s metabolism (chemical reactions in the body that change food into energy) is different (see diagram, p. 20). And the mystery of why some people tend to pack on fat easier than others has led scientists to search for clues. “We believe that genes (units of hereditary information) play a major role in obesity,” says Shirly Pinto, a biologist at Rockefeller University.

In 1994, scientists at Rockefeller University made a breakthrough: They discovered that fat cells produce an appetite-controlling hormone (function-controlling chemical), which they named leptin. This hormone keeps people at a healthy weight. It signals a brain region responsible for maintaining weight levels–the hypothalamus–information on how much fuel the body needs. In other words, leptin sends out the “I’m full” message. When you have fewer fat cells, less leptin is produced, signaling you to eat more. And with more fat cells, leptin levels increase, signaling you to eat less.

How does leptin relate to obesity? “In most cases, obese individuals have leptin levels that are higher than normal,” says Pinto. “In a sense, they should stop eating. But they don’t.” Scientists believe that some obese people may have genetic differences that cause their bodies to be resistant to leptin’s messages.

Since leptin’s discovery, scientists have found other hormones that are involved in how the body perceives food. Could hormone-regulating, fat-fighting medicines result soon? It’s too early to tell. How these natural hormones work alone–much less, how they interact with each other–is still unclear. “Scientists are just starting to piece this puzzle together,” says Pinto.