Body Weight

Technically, body weight is the measure of the gravitational force on your body, which is based on the mass of your body multiplied by the acceleration of gravity. Simply put, it's the number you see on the scale, which indicates how heavy you are in pounds or kilograms. Although most of us aren't too concerned about the science behind the scale's reading, we use this number to determine how big or small we are.

In the United States, people attribute great significance to body weight. Since 1959, the Metropolitan Life Insurance Company has issued desirable height-weight charts. Although some have been revised, these charts still are prevalent. Americans continually consult these charts to determine what they should weigh and spend an estimated more than $40 billion each year in efforts to reduce their weight.

The term "overweight" describes excessive weight for one's height, generally according to some predetermined "average" standards such as those outlined in these charts. While weight is an indication of heaviness, it does not indicate body composition and body fat, however, which are critical to health status.

Body Composition and Body Fat

The body is made up primarily of bones, muscles and fat. Muscles and bones are called lean body mass; these tissues weigh more than fat.

Two types of fat exist: essential fat and storage fat. Essential fat is required for normal physiologic functioning and is stored in the bone marrow, the heart, lungs, liver, kidneys, intestines, spleen, muscles and other major tissues and organs. Storage fat lies beneath the skin and serves as insulation and nutritional reserve; it also protects internal organs from trauma.

Due to hormones and the ability to bear children, women typically have up to four times more essential fat than men (12 to 15 percent versus 3 to 5 percent). Men and women, however, tend to have similar levels of storage fat (12 percent in males and 15 percent in females).

So body fat refers to the percentage of the overall body that is composed of essential and storage fat. For healthy men, the optimal range is approximately 10 percent to 18 percent; for women, 16 percent to 25 percent is ideal. Obesity is defined as overfatness, or greater than 25 percent body fat for men and 30 percent for women.

Body fat can be determined several different ways. One of the most common is skinfold measurements where the thickness of fat tissue just under the skin at various sites on the body is measured. Underwater weighing, considered highly accurate, uses special equipment to calculate body fat. Other ways to measure body fat include bioelectrical impedance that sends a harmless electrical current through the body, circumference measurements, ultrasound and special scales that read body fat just by standing on them. All methods have varying degrees of accuracy, and results from a single test should be used only as an approximation.

Most health clubs and some universities conduct body fat measurements; you also can buy devices over-the-counter today to measure your body fat.

Overweight versus Overfat

So what is the difference whether you focus on body weight or body fat, since they both are numbers? Interestingly, it is possible to be overweight according to a weight chart but have a healthy body fat composition. For instance, a short, well-muscled female athlete may weigh more than the number listed on a chart for her height and frame size. Therefore, the conclusion would be that she is overweight, however, her body fat may only be at 16 percent, which is within healthy limits. Because of her dense muscle tissue, her weight is higher, but her body composition is healthy. So there is no reason for her to lose weight.

Similarly, a man can have a normal weight according to the height/weight charts yet still have an unhealthy level of body fat (more than 25 percent) and technically be considered obese. Therefore, thinness doesn't automatically indicate health if a body has an excess amount of fat and a minimal amount of muscle.