|
Overweight
The term overweight is generally used to indicate that a human (or other mammal) has more body fat than is typical or required for the normal functioning of the body. Being overweight is a fairly common condition for many people, especially in the United States and other developed nations where food supplies are plentiful and lifestyles often do not involve a lot of exercise. Recent studies have indicated that as much as 640f the adult US population is overweight, and this number is increasing.[1]
A healthy body normally requires some percentage of body fat, which is known as essential fat (approximately 20-25 percent for women and 15-18 percent for men). This is required for the proper functioning of the hormonal, reproductive, and immune systems, and is located throughout the body. In addition to essential fat, most bodies (including healthy bodies) have some amount of storage fat, which is used primarily for storing excess energy for future use. This storage fat is usually located just underneath the skin, but also exists in smaller amounts around some internal organs. Some amount of storage fat is normal and beneficial, providing thermal insulation and shock absorption for sensitive areas and a reserve of energy for the body when it might need it, but the accumulation of too much storage fat can impair movement and flexibility, may lead to obesity, and can alter the appearance of the body in ways that some may find undesirable or unattractive.
The average weight for male adults in the United States is about 76 to 83 kg (168 to 183 lb). The average weight for male teenagers in the United States is about 45 to 64 kg (99 to 141 lb) [citation needed]. For female adults the average weight is 54 to 64 kg (120 to 140 lb). For female teenagers the average weight is 45 to 57 kg (100 to 126 lb) [citation needed]. Weight is, of course, related to height, so people taller than average tend to be heavier as well.
A quantity called Body Mass Index (BMI) is used to express weight relative to height. This is determined by dividing the weight of a person in kilograms by the square of the height in meters. Optimally, this number should be in the 20s; over 40, and the person is morbidly obese. Nevertheless, if the person is an athlete or a bodybuilder with big muscles and a low percentage of body fat, he or she is given the benefit of the doubt; these people have high BMIs but low body fat, and the number by itself would not be a good indicator of good health.
The degree to which a person is overweight is generally described using an indication of the amount of excess body fat present. There are several common ways to measure the amount of fat present in an individual's body:
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.
Body Mass Index (BMI): This is an adaptation of simple weighing which attempts to take into account the subject's general body size by dividing the weight by the height squared (the units for BMI are kg/m2, but are rarely referenced, and BMI numbers are typically written and used as unitless numbers). This provides a slightly more accurate representation than simply measuring raw weight, but still ignores many factors which can affect the results, and is generally not accurate for many individuals. This method has historically been used a great deal by researchers and statisticians due to the fact that most sample data collected contains height and weight information but often does not include any more accurate measurements of body fat, so BMI is a convenient approximation which can be used with the data available and is generally effective when dealing with large populations. More recently, BMI has come to be used more frequently by individuals as well, but in this context it should only be considered a rough estimate at best.
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.
Bioelectric Impedance: 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.
Despite the inherent inaccuracies, the most common method for discussing this subject used by researchers and advisory institutions is body mass index (BMI) numbers. Definitions of what is considered to be overweight change from time to time and sometimes from country to country, but the current definition proposed by both the US National Institutes of Health and the World Health Organization designates anyone with a BMI of 25 kg/m2 or more to be overweight.
It should be noted however that BMI does not account for differing amounts of muscle mass, genetic factors, or many other individual variations, and thus many individuals can have BMIs less than 25 and still be considered overweight, while others may have BMIs significantly higher without falling into this category[2]. Many of the more accurate methods mentioned above for determining body fat content can provide better indications of whether a particular individual is overweight or not.
If an individual is sufficiently overweight that excess body fat could present substantial health risks, he or she is considered to be obese. It is possible for someone to be overweight without being obese (according to the NIH and WHO, a BMI between 25 and 30 is considered to be overweight but not obese). Again, the designation of obesity is subject to a great deal of interpretation and many individual factors, so an individual with a BMI well below 30 may be considered to be obese depending on their particular condition, while in some cases a BMI above 30 may not actually indicate obesity (although likely still does indicate being overweight).
Source: Wikipedia
|
|
|
|
 |
|
|
|
|
|
