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 64% of the adult US population is overweight, and this number is increasing.

A healthy body normally requires some percentage of body fat, which is known as essential fat (approximately 20-25% for women and 15-18% 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.

Measurement and Classification

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.

BMI, however, 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. 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).

Health Implications

While there are several known health issues associated with obesity, the health implications of being simply overweight (but not obese) are substantially less well known. A significant amount of medical research on obesity in the past has led to the general conclusion, maintained by many in the medical community, that being even slightly overweight may be detrimental to one's health in a variety of ways, and should be avoided if possible. However, some recent studies have suggested that mortality rates for individuals who are classified as overweight but not obese (BMI between 25 and 30) may actually be lower than for those with an "ideal" weight (BMI 18.5 to 25), while yet others show little or no significant difference.

One proposed explanation for this discrepancy, suggested by some data, is that the mortality risks associated with being overweight have reduced over the past few decades, possibly because medical treatments for many of the conditions associated with weight have gotten better, and thus being somewhat overweight may not present as much of a risk to one's health as it once did. However, the findings of these studies, while significant, are far from conclusive, and more research to verify and investigate the reasons for these findings has yet to be performed. The studies also only measured mortality rates, not health of the individuals in question; thus it is quite possible that other factors such as chronic illness and reduced quality of life may be found to still be issues for those with increased weight, even if mortality is not an issue.

Causes

Being overweight is generally caused by the intake of more calories (by eating ) than are expended by the body (by exercise and everyday living). Factors which may contribute to this imbalance include:

• Limited exercise and sedentary lifestyle
• Genetic predisposition
• A high glycemic diet (i.e. a diet that consists of meals that give high postprandial blood sugar)
• Weight cycling, caused by repeated attempts to lose weight by dieting
• Underlying illness (e.g. hypothyroidism)
• An eating disorder (such as binge eating disorder)
• Stressful mentality
• Insufficient sleeping
• Psychotropic medications
• Smoking cessation
• Gluttony

The amount of body fat is regulated to some extent subconsciously by the brain (by controlling caloric intake through appetite and food preferences). Although the exact mechanisms by which this occurs are not entirely known, one common theory suggests that each person may possess an inherent "set point" weight which the brain attempts to maintain, and that this set point may vary for each individual depending on a variety of factors including genetic predisposition, environment, and past experience.

This leads to the conclusion that some individuals may be predisposed to naturally maintaining different body weights than others, and thus it may be easier for some people to avoid being overweight, while others may find it much more difficult. It also suggests, however, that an individual's set point may be changeable with appropriate environment and conditioning.

Treatment

A large number of people undergo some form of treatment to attempt to reduce their weight, usually either in an attempt to improve their health, to improve their lifestyle, or for cosmetic reasons. The generally recommended treatment for being overweight is a modified or controlled diet in conjunction with increased exercise. For those who are significantly obese, more severe methods such as drugs or surgery may be recommended, but should generally only be pursued on a doctor's recommendation after a thorough examination of the patient and analysis of previously attempted remedies (for more information on this, see the appropriate section of Obesity).

Studies suggest that reducing calorie intake by itself (dieting) may have short-term effects but does not lead to long-term weight loss, and can often result in gaining back all of the lost weight and more in the longer term. For this reason, it is generally recommended that weight-loss diets not be attempted on their own but instead in combination with a larger program of increased exercise and long-term planning and weight management.

The health benefits of weight loss are also somewhat unclear. While it is generally accepted that for significantly obese patients, losing weight can reduce health risks and improve quality of life, there is some evidence to suggest that for merely overweight patients, the health effects of attempting to lose weight may actually be more detrimental than simply remaining overweight. Moreover, for all individuals, repeatedly losing weight and then gaining it back (so-called "yo-yo dieting"), is almost always considered to do more harm than good and can be the cause of significant additional health problems.

References

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See also

• Underweight
• Obesity
• Body image

Health | Nutrition

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