Body weight

Although many people prefer the less-ambiguous term body mass, the term body weight is overwhelmingly used in daily English speech and in biological and medical science contexts to describe the mass of an organism's body. Body weight is measured in kilograms throughout the world, although in some countries people more often measure and describe body weight in pounds (e.g. United States and sometimes Canada) or stones and pounds (e.g. United Kingdom) and thus may not be well acquainted with measurement in kilograms. Most hospitals in the United States now use kilograms for calculations, but use kilograms and pounds together for other purposes. (A kg is about 2.2 lbs, and a stone (14 lb) is about 6.4 kg.)

The term is usually encountered in connection with:

food and feeding behaviour
normal and abnormal growth and development
the physiological and hormonal control of ingestion and digestion
foraging for food in animals
hunger and other motivations to eat
problems in regulating body weight, often resulting in obesity
eating disorders such as anorexia nervosa and bulimia nervosa
effects of disease
athletic competitions where the participants are classified according to their body weight

Classification

The degree to which a person is overweight is generally described by Body Mass Index. The range for overweight is a BMI of 25 to 29.9 with BMI's >= 30 classified as obesity. There are however several other common ways to measure the amount of adiposity or fat present in an individual's body.

Body Mass Index

Body Mass Index, or BMI is a measure of a persons weight taking into account their height. It is given by the formula: BMI equals body weight divided by the square of the persons height; with weight in kilograms and height in meters. BMI numbers however are typically used and written as unitless numbers. BMI provides a significantly more accurate representation of body fat content than simply measuring a persons weight. It is highly correlated with both percentage of body fat and body fat mass. It does not take into account some factors such as pregnancy, bodybuilding, or children which effect body composition however is an accurate reflection of fat percentage in the majority of the adult population.

Simple Weighing

The weight of the individual is measured and compared to an estimated ideal weight. This is the easiest and most common method, but by far the least accurate, as it only measures one quantity (weight) and often does not take into account many factors such as height, body type, and relative amount of muscle mass.

Skinfold Calipers or "pinch test"

With this method, the skin at several specific points on the body is pinched and the thickness of the resulting fold is measured. This measures the thickness of the layers of fat located under the skin, from which a general measurement of total amount of fat in the body is calculated. This method can be reasonably accurate for many people, but it does assume particular patterns for fat distribution over the body which may not apply to all individuals, and does not account for fat deposits which may not be directly under the skin. Also, as the measurement and analysis generally involves a high degree of practice and interpretation, for an accurate result it must be performed by a professional and cannot generally be done by patients themselves.

Bioelectrical impedance analysis

This method involves passing a small electrical current through the body and measuring the body's resistance to the electrical flow. As fat and muscle conduct electricity differently, this method can provide a direct measurement of the percentage of body fat present as compared to muscle mass. In the past, this technique could only be performed reliably by trained professionals with specialized equipment, but it is now possible to buy "home kits" which allow individuals to do this themselves with a minimum of training. Despite the improved simplicity of this process over the years, however, there are a number of factors which can affect the results, including hydration and body temperature, so a fair amount of care must still be taken when applying this test to ensure that the results are in fact accurate and applicable.

Hydrostatic Weighing

Considered one of the more accurate methods of measuring body fat, this technique involves completely submerging the subject underwater and using special equipment to measure his or her weight while submerged. This weight is then compared with "dry weight" as recorded outside the water to determine overall body density. As fat is less dense than muscle, careful application of this technique can provide a reasonably close estimate of fat content in the body. This technique does, however, require expensive specialized equipment and trained professionals to administer it properly.

DEXA (dual energy X-ray absorptiometry)

Originally developed to measure bone density, DEXA imaging has also come to be used as a precise way to determine body fat content by using the density of various body tissues to identify which portions of the body are fat. This test is generally considered to be very accurate, but requires a great deal of expensive medical equipment and trained professionals to perform.

The most common method for discussing this subject and the one used primarily by researchers and advisory institutions is body mass index (BMI). Definitions of what is considered to be overweight vary by ethnicity. The current definition proposed by the United States National Institutes of Health (NIH) and the World Health Organization (WHO) designate whites, hispanics and blacks with a BMI of 25 kg/m2 or more as overweight. For Asians, overweight is a BMI between 23 and 29.9 kg/m2 and obesity for all groups is a BMI >30 kg/m2

BMI, however, does not account extremes of muscle mass, some rare genetic factors, the very young, and a few other individual variations. Thus it is possible for an individuals with a BMI of less than 25 to have excess body fat, while others may have a BMI that is significantly higher without falling into this category. Some of the above methods for determining body fat are more accurate then BMI but come with added complexity.

If an individual is overweight and has excess body fat it could cause health risks.