Obesity in the U.S. Body weight is commonly measured by a formula called the body mass index (BMI). Normal weight is a BMI of less than 25. People with a BMI of 25 or greater are considered overweight. Obesity is defined as having a BMI of 30 or greater and severe obesity is a BMI of 40 or greater.

According to the American Obesity Association, 64.5 percent of Americans are currently overweight (about 127 million people). More than 30 percent of those who are overweight are obese and almost 5 percent are severely obese.

Obesity eventually takes a toll on the body. The heart and lungs must work harder to provide oxygenated blood to the whole body. Joints may break down from carrying excess weight. Even sleep can be impaired because overweight people are at risk for developing sleep apnea (a condition characterized by temporary periods of breathing cessation). Being overweight increases the risk of several different kinds of chronic health problems, like high blood pressure, high cholesterol, type 2 diabetes, stroke, heart disease and certain types of cancer.

Measuring Body Fat A simple measure of body weight from a scale is often of limited value in determining health risk. Some people may weigh more because they are very active or lift weights. But that excess weight doesn't necessarily mean an increased risk for chronic health problems. Body mass index was developed to take into account some of the variations in height and body composition. However, even that score has some limitations. Instead, experts say the amount of lean muscle mass and body fat are more important factors in overall health predictions. A higher percentage of body fat is associated with an increased risk for obesity and weight-related chronic medical conditions.

Body composition analysis determines how much of a person's weight is made up of muscle and how much is fat. The gold standard for measuring body composition is underwater weighing (also known as hydrostatic weighing). The technique requires a water tank and an underwater scale. First, the patient is weighed outside the tank. Then, with minimal clothing on the body, the patient sits on a seat mounted with a scale. Next, the seat is placed into the water tank, totally submerging the patient into the water. Underwater weight is then recorded and the patient is lifted out of the water. Doctors must also measure lung capacity to subtract the weight of the air in the lungs while the patient is holding his/her breath underwater. Body fat percentage is determined by plugging the results into a formula.

Underwater weighing is highly accurate. But since the technique requires special equipment, it is usually only used at larger hospitals and research centers. And the procedure can't be used on everyone. Children, many of the elderly and people who are very sick may not be able to cooperate or hold their breath long enough to get an accurate estimate of underwater weight.

Researchers at St. Luke's Hospital in New York City are testing a new type of device for body scanning, called the 3-D Photonic Image Scanner. The technique and device are actually borrowed from the fashion industry, where the equipment is used to scan the body to create a perfect fit for clothing.

The image scanner uses the light of lasers from four angles to measure and map the contours of the body. The information is processed by a computer, which creates a 3-dimensional image of body contour. The information allows doctors to measure total body volume, as well as body volume in specific areas of the body, like the abdomen, arms and legs.

Doctors say body shape, which can indicate pockets of stored fat, is a very reliable indicator of risk for future chronic disease. For example, central body fat has already been established as a risk factor for high blood pressure, coronary artery disease and type 2 diabetes. The 3D scanner may help doctors track patients as they lose or gain weight to follow the effectiveness of a treatment.

Information about the 3D scanner is available on the manufacturer's website at http://sales.hamamatsu.com/en/products/system-division/3d-scanning.php

For general information on obesity and its health consequences: American Obesity Association, http://www.obesity.org Centers for Disease Control and Prevention, http://www.cdc.gov/nccdphp/dnpa/obesity NAASO, The Obesity Society, http://www.naaso.org