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Body Mass Index – BMI – BMI & Obesity, Weight Loss
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Body Mass Index – BMI – BMI & Obesity, Weight Loss
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Body Mass Index – BMI
Body mass index From Wikipedia, the free encyclopedia Jump to: navigation, search
A graph of body mass index is shown above. The dashed lines represent subdivisions within a major class. For instance the “Underweight” classification is further divided into “severe,” “moderate,” and “mild” subclasses. Based on World Health Organization data here.Body mass index (BMI) or Quetelet Index is a statistical measure of the weight of a person scaled according to height. It was invented between 1830 and 1850 by the Belgian polymath Adolphe Quetelet during the course of developing "social physics".
Contents 1 Calculation 2 Usage 3 Accuracy 4 BMI categories 4.1 Thresholds 4.2 BMI-for-age 4.3 International variations 5 Applications 5.1 Statistical device 5.2 Clinical practice 5.3 Problems 6 References 7 See also 8 External links
Calculation BMI is defined as the individual's body weight divided by the square of their height. The formulas universally used in medicine produce a unit of measure that is not dimensionless; its unit is kg/m2. Body mass index may be accurately calculated using any of the formulas below.
SI units US imperial units UK mixed units
Usage As a measure, BMI became popular during the early 1980s as obesity started to become a discernible issue in prosperous Western society. BMI provided a simple numeric measure of a person's "fatness" or "thinness", allowing health professionals to discuss the problems of over- and under-weight more objectively with their patients. However, BMI has become controversial because many people, including physicians, have come to rely on its apparent numerical "authority" for medical diagnosis - but that has never been the BMI's purpose. It is meant to be used as a simple means of classifying sedentary (physically inactive) individuals with an average body composition.[1]. For these individuals, the current value settings are as follows: a BMI of 18.5 to 25 may indicate optimal weight; a BMI lower than 18.5 suggests the person is underweight while a number above 25 may indicate the person is overweight; a BMI below 15 may indicate the person has an eating disorder; a number above 30 suggests the person is obese (over 40, morbidly obese).
In physiology the term “weight” is used interchangeably with “mass”. For a given body shape and given density, the BMI will be proportional to height e.g. if all body dimensions increase by 50%, the BMI increases by 50%.
Generally, a BMI chart displays calculated BMI as a function of weight (horizontal axis) and height (vertical axis) using “contour lines” for different values of BMI or colors for different BMI categories.
Accuracy The BMI is meant to broadly categorise populations for purely statistical purposes. As noted, its accuracy in relation to actual levels of body fat is easily distorted by such factors as fitness level, muscle mass, bone structure, gender, and ethnicity. People who are mesomorphic tend to have higher BMI numbers than people who are endomorphic, because they have greater bone mass and greater muscle mass, respectively, than do endomorphic individuals.
Similarly, an ectomorphic individual could conceivably receive an unhealthily low reading, when in fact their body type makes them naturally thin no matter what they eat.
People with short stature tend to have lower BMI. Therefore they should use a lower cut-off value for obesity diagnosis.[2] The same applies to older people, whose reduced muscle mass can hide additional body fat without increasing BMI.
BMI categories A frequent use of the BMI is to assess how much an individual's body weight departs from what is normal or desirable for a person of his or her height. The weight excess or deficiency may, in part, be accounted for by body fat (adipose tissue) although other factors such as muscularity also affect BMI (see discussion below and overweight). Human bodies rank along the index from around 15 (near starvation) to over 40 (morbidly obese). This statistical spread is usually described in broad categories: underweight, normal weight, overweight, obese and morbidly obese. The particular BMI values used to demarcate these categories varies based on the authority, but typically a BMI of less than 18.5 is considered underweight and may indicate malnutrition, an eating disorder, or other health problems, while a BMI greater than 25 is considered overweight and above 30 is considered obese. These ranges of BMI values are valid only when applied to adults. But these measurements are not exact as someone with a very low BMI may just be naturally very thin.
Thresholds Given the reservations detailed below concerning the limitations of the BMI as a diagnostic tool for individuals, the following are common definitions of BMI categories:
Starvation: less than 15.0 Anoretic(n.)/Anorexic(adj.): less than 17.5 Underweight: less than 18.5 Ideal: from 18.5 to 25 Overweight: from 25 to 30 Obese: from 30 to 40 Morbidly Obese: over 40 The U.S. National Health and Nutrition Examination Survey of 1994 indicates that 59% of American men and 49% of women have BMIs over 25. Extreme obesity — a BMI of 40 or more — was found in 2% of the men and 4% of the women. There are differing opinions on the threshold for being underweight in females, doctors quote anything from 18.5 to 20 as being the lowest weight, the most frequently stated being 19. A BMI nearing 15 is usually used as an indicator for starvation and the health risks involved, with a BMI <17.5 being one of the DSM criteria for the diagnosis of anorexia nervosa.
BMI-for-age BMI is used differently for children. It is calculated the same way as for adults, but then compared to typical values for other children of the same age. Instead of set thresholds for underweight and overweight, then, the BMI percentile allows comparison with children of the same gender and age.[3] A BMI that is less than the 5th percentile is considered underweight and above the 95th percentile is considered overweight. Children with a BMI between the 85th and 95th percentile are considered to be at risk of becoming overweight.
Recent studies in England have indicated that females between the ages 12 and 16 have a higher BMI than males by 1.0 kg/m² on average.[4]
International variations These recommended distinctions along the linear scale may vary from time to time and country to country, making global, longitudinal surveys problematic. In 1998, the U.S. National Institutes of Health brought U.S. definitions into line with World Health Organization guidelines, lowering the normal/overweight cut-off from BMI 27.8 to BMI 25. This had the effect of redefining approximately 30 million Americans, previously "technically healthy" to "technically overweight". It also recommends lowering the normal/overweight threshold for South East Asian body types to around BMI 23, and expects further revisions to emerge from clinical studies of different body types.
In Singapore, the BMI cut-off figures were revised in 2005 with an emphasis on health risks instead of weight. Adults whose BMI is between 18.5 and 22.9 have a low risk of developing heart disease and other health problems such as diabetes. Those with a BMI between 23 and 27.4 are at moderate risk while those with a BMI of 27.5 and above are at high risk of heart disease and other health problems.Singapore BMI Cut-offs.
Applications
Statistical device The Body Mass Index is generally used as a means of correlation between groups related by general mass and can serve as a basic means of estimating adiposity. However, the duality of the Body Mass Index is that, whilst easy-to-use as a general calculation, it is limited in how accurate and pertinent the data obtained from it can be. Generally, the Index is suitable for recognising trends within sedentary or overweight individuals because there is a smaller margin for errors.[5]
This general correlation is particularly useful for consensus data regarding obesity or various other conditions because it can be used to build a semi-accurate representation from which a solution can be stipulated, or the RDA for a group can be calculated. Similarly, this is becoming more and more pertinent to the growth of children, due to the majority of their exercise habits.[6]
The growth of children is usually documented against a BMI-measured growth chart. Obesity trends can be calculated from the difference between the child's BMI and the BMI on the chart. However, this method again falls prey to the obstacle of body composition: many children who are generally born, or grow as an endomorph, would be classed as obese despite body composition. Clinical professionals should take into account the child's body composition and defer to an appropriate technique such as densiometry.
Clinical practice BMI can be calculated quickly and without expensive equipment. However, BMI categories do not take into account factors such as frame size and muscularity.[7] The categories also fail to account for varying proportions of muscle, fat, bone & cartilage, and water weight.
Despite this, BMI categories are generally regarded as a satisfactory tool for measuring whether sedentary individuals are "underweight," "overweight" or "obese." It has been used by the WHO as the standard for recording obesity statistics since the early 1980s. In the United States, BMI is also used as a measure of underweight, owing to advocacy on behalf of those suffering with eating disorders, such as anorexia nervosa and bulimia nervosa.
Individuals who are not sedentary - especially athletes - as well as children, the elderly, the infirm, and individuals who are naturally endomorphic or ectomorphic (i.e., people who don't have a medium frame) are ill-fitted to assessment using the BMI. Or to state the problem more accurately, the BMI measurements at which these people may be underweight, overweight or obese are different from sedentary mesomorphs whose ages are between about 20 and 70.
In athletes, the problem is that muscle is denser than fat. Some professional athletes are "overweight" or "obese" according to their BMI - unless the number at which they are considered "overweight" or "obese" is adjusted upward. In children and the elderly, differences in bone density and, thus, in the proportion of bone to total weight can mean the number at which these people are considered underweight should be adjusted downward.
Methods for actually measuring body fat percentage are preferable to BMI for measuring healthy body size, but are often expensive or inaccessible to most people.
Problems The units associated with Body Mass Index are problematic. Body Mass Index is expressed in "kilograms per meter squared" or "pounds per inch squared", depending on which Quetelet equation is used. Technically, this looks like a calculation of pressure, which is confusing. However, what BMI actually calculates is area-density (mass per unit area) as distinct from pressure (force per unit area) or normal volumetric density (mass per unit volume). Mathematically, BMI models the human body as a simple cylinder (or - equivalently - a set of similar cylinders) with constant uniform density, and whose radius varies with the square-root of height, so that the circular cross-sectional area varies directly in proportion to height. Measured against this model, a person who is narrow for their height would be expected to weigh less than the norm and hence should have a BMI lower than the norm; likewise, a person who is wide for their height should have a BMI higher than the norm. But because real people are not made from cylinders of constant uniform density, but rather of different tissues in varying proportions, actual BMI calculations can give perplexing results. Two people of different build may have the same height: but one who is lean and well-muscled may weigh the same as the other who is toneless and flabby; yet both have the same calculated BMI. Any physician could assess at a glance which of these bodies is likely to be in better condition. BMI values alone can't tell the whole story.
A simple modification of the BMI system, called "BMI Prime", was proposed to eliminate the use of these confusing units. By definition, BMI Prime is the ratio of a person's Actual Weight (or Mass) to his/her Upper Weight (or Mass) Limit, calculated at a BMI of 24.9. Since BMI Prime is a ratio, it is expressed as a pure, dimensionless number. Another advantage of the "BMI Prime" system is that it enables people, at a glance, to determine what percentage they deviate from their Upper Weight (Mass) Limit. For further details: Quetelet's Equation, Upper Weight Limits, and BMI Prime
As a general rule, developed muscle is denser than fat and the BMI does not account for this. Therefore a person with more muscle mass, such as a body-builder, will seem to be overweight. Likewise it could be stipulated that some long-distance or endurance athletes would be classified as ectomorphic (underweight, anorexic or starvation), this type of athlete tends to have low body fat and well developed slow twitch muscle, which does not contribute greatly to muscle mass. These individuals could be widely regarded as the perfect composite for their particular sports. Due to these limitations, body composition for athletes would not be calculated using the body mass index, and instead the body fat would be determined by such techniques as skinfold measurements or underwater weighing. In parallel to this, sportsmen or women from sports such as Rugby and American Football, where size and muscle are often of importance, could be listed as overweight, due to a large amount of muscle. At the extreme, many bodybuilders would be classified as morbidly obese, such as Markus Rühl who has a BMI of 46.
Another issue is that competitive athletes often know very accurately what their actual height and weight are, while the general public has tendencies toward over-estimating their height, and under-estimating their weight. The BMI standards, as a public health tool, take this tendency into account. This can lead to athletes having a higher reported BMI than a lay person of the same height and weight.[citation needed]
Mayo Clinic researchers say the BMI doesn't accurately predict risk of cardiovascular death because it doesn't distinguish between muscle and fat. They say a better measure may be your Waist-hip ratio.
In an analysis led by Lopez-Jiminez of 40 studies involving 250,000 people, heart patients with normal BMIs were at higher risk of death from cardiovascular disease than people whose BMIs put them in the "obese" range. [citation needed]
In fact, the ones in the study who had the highest death rates were people who weighed the least; in other words, they had the lowest BMIs.[citation needed]
The implications of this finding can be confounded by the fact that many chronic diseases, such as diabetes, cause weight loss before the eventual death. In light of this, higher death rates among thinner people would be the expected result.
Related topics @ Wikipedia
Wikisource has original text related to this article: Body mass indexBMI Chart (height 150 to 174 cm) Body fat percentage Muscle Muscle mass Skeletal muscle Basal metabolic rate
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Web Resources for Body Mass Index
Body Mass Index - Body Mass Index (BMI) is a number calculated from a person’s weight and height. BMI provides a reliable indicator of body fatness for most people and is used to screen for weight categories that may lead to health problems.
Body mass index From Wikipedia - Calculation, usage, accuracy, bmi categories, thresholds, bmi-for-age, international variations, applications, statistical device, clinical practice, limitations and shortcomings.
Bmi calculator and body weight comparison
Body Mass Index - BMI uses a mathematical formula that takes into account both a person's height and weight. BMI equals a person's weight in kilograms divided by height in meters squared
Body Mass Index (BMI) Nomogram - section for more information about weight classification and answers to questions about weight and health
Is your weight affecting your health? - Find out if your Body Mass Index puts you at risk
Body mass index - The healthy weight range is based on a measurement known as the body mass index (BMI). This can be determined if you know your weight and your height
Body Mass Index and Weight Loss Information
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