The BMI is a mathematical formula based on a person's height and weight and is a helpful indicator of obesity and underweight in adults. The BMI is the most popular tool for defining healthy weight, overweight and obesity. The BMI is more highly correlated with body fat than any other indicator of height and weight. The relation between fatness and BMI is influenced by age and gender. For example, women are more likely to have a higher percent of body fat than men for the same BMI. At the same BMI, older people have more body fat than younger adults. BMI ranges are based on the effect body weight has on disease and death. BMI is used to screen and monitor a population to detect

risk of health or nutritional disorders. On an individual basis, other data must be used to determine if a high BMI is associated with increased risk of disease and death for that person. The 1998 Clinical Guidelines on the Identification, Evaluation and Treatment of Overweight and Obesity in Adults, developed by the National Heart, Lung and Blood Institute, recommend the following classifications for BMI:

	Obesity Class
Underweight		BMI less than 18.5
Healthy Weight		BMI between 18.5 - 24.99
Overweight		BMI between 25 - 29.99
Obese	I	BMI between 30 - 34.99
Obese	II	BMI between 35 - 39.99
Morbid Obesity	III	BMI greater than 40

Formula to calculate your BMI BMI = [Weight in pounds ÷ Height in inches ÷ Height in inches] x 703 or BMI = Weight in kilograms ÷ [Height in meters]2 Lean body mass refers to your arm, leg, back, neck and abdomen muscles. It also includes your heart muscle, and the tissues of your other internal organs as well as water, and bone The quantity of lean body mass you have is the most important factor in determining your metabolism (the rate at which you burn the calories). The greater your lean body mass, the higher your metabolic rate and the more calories you will burn while sitting or lying down. A higher metabolic rate makes it easier to maintain your weight. A regular program of strength training ("resistance" training) can increase the amount and the strength of your muscles. This, in turn, will also increase your metabolic rate. Fully-grown adults gain both lean body mass and storage fat when they gain weight. The amount of fat gained usually far exceeds the amount of lean body mass gained (about 60 - 80 % fat and 20 - 40 % lean body mass). For instance, if you gain ten pounds, about 6 - 8 pounds would be fat and 2 - 4 pounds would be lean body mass. This would mean increases not only in your fat tissue but also in your muscles, stomach, intestines and other organs, bone and water. These percentages would vary if you are involved in a training program specifically aimed at increasing muscle mass. When you lose weight, you lose water, lean body mass and storage fat. To prevent dehydration, it is important to drink water. From a health standpoint and to maintain your metabolic rate, it is better to preserve as much lean body mass as possible while you reduce your body fat. Experts have determined that:

• During the early weeks of weight loss, at least 75 % of the weight you lose should be fat loss and not more than 25 % should come from lean body mass.
• As you continue to lose weight, especially if certain types of exercise are included in your weight loss plan, fat loss should be about 90% of the weight you lose and lean body mass should be about 10%.

Everyone needs a certain amount of body fat for stored energy, heat insulation, shock absorption, and other functions. As a rule, women have more body fat than men. Most health care providers agree that men with more than 25 percent body fat and women with more than 30 percent body fat are obese. In recent years, body mass index (BMI) has become the medical standard used to measure overweight and obesity. Health care providers are concerned not only with how much fat a person has, but also where the fat is located on the body. Women typically collect fat in their hips and buttocks, giving them a "pear" shape. Men usually build up fat around their bellies, giving them more of an "apple" shape. Of course some men are pear-shaped and some women become apple-shaped, especially after menopause. If you carry fat mainly around your waist, you are more likely to develop obesity-related health problems. Women with a waist measurement of more than 35 inches or men with a waist measurement of more than 40 inches have a higher health risk because of their fat distribution.