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Body Mass Index
BMI is the common acronym given to Body Mass Index, a number calculated from your weight and height that roughly correlates to the percentage of your total weight that comes from fat, as opposed to muscle, bone or organ. The higher a person's BMI, the higher the percentage of fat in their body. If your BMI is under 20, you might be underweight. Between 20 and 25, you are probably at a good healthy weight for your height. A BMI over 25 is considered overweight, and over 35 is considered obese.
Charts and calculators to determine your BMI are easily found on most health and diet sites. Of course, these calculators are rough translations of true percentages, and there are a number of factors that might influence whether or not your BMI is a true reflection of your total body fat.
For instance, muscle is denser than fat and takes up less space. Therefore a heavily muscled person might weigh more than a same sized over-weight person, or two individuals with identical BMI might have widely different percent body fat. In this case, calculating your percent body-fat might require more sophisticated equipment, such as an immersion tank. Since fat is more buoyant than muscle, two same-weight individuals will not float at the same level if they have different percentages of body fat.
Women typically carry more subcutaneous fat than men do, particularly in the breast and hips, so their percent body fat may be higher without it necessarily being reflected in their BMI or having any adverse health effects. Very low body fat, which may or may not show up in a BMI, depending on the individual's musculature, might be unhealthy as well. Your body needs some stores of fat to draw upon for energy and if fat is absent, the body will begin to consume muscle mass to keep itself going. Athletes who overtrain may find themselves losing strength rather than gaining strength, for instance.
BMI, for all its approximation, is a good tool to use in determining whether you need to lose weight, gain weight, or congratulate yourself for being just right.
Body Mass Index (BMI)
Body mass index, or BMI, is a new term to most people. However, it is the measurement of choice for many physicians and researchers studying obesity. 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. (BMI=kg/m2).
Risk of Associated Disease According to BMI and Waist Size
BMI Waist less than or equal to
40 in. (men) or
35 in. (women) Waist greater than
40 in. (men) or
35 in. (women)
18.5 or less Underweight -- N/A
18.5 - 24.9 Normal -- N/A
25.0 - 29.9 Overweight Increased High
30.0 - 34.9 Obese High Very High
35.0 - 39.9 Obese Very High Very High
40 or greater Extremely Obese Extremely High Extremely High

Determining Your Body Mass Index (BMI)
The table below has already done the math and metric conversions. To use the table, find the appropriate height in the left-hand column. Move across the row to the given weight. The number at the top of the column is the BMI for that height and weight. Or, use our BMI calculator.
(kg/m2) 19 20 21 22 23 24 25 26 27 28 29 30 35 40
(in.) Weight (lb.)
58 91 96 100 105 110 115 119 124 129 134 138 143 167 191
59 94 99 104 109 114 119 124 128 133 138 143 148 173 198
60 97 102 107 112 118 123 128 133 138 143 148 153 179 204
61 100 106 111 116 122 127 132 137 143 148 153 158 185 211
62 104 109 115 120 126 131 136 142 147 153 158 164 191 218
63 107 113 118 124 130 135 141 146 152 158 163 169 197 225
64 110 116 122 128 134 140 145 151 157 163 169 174 204 232
65 114 120 126 132 138 144 150 156 162 168 174 180 210 240
66 118 124 130 136 142 148 155 161 167 173 179 186 216 247
67 121 127 134 140 146 153 159 166 172 178 185 191 223 255
68 125 131 138 144 151 158 164 171 177 184 190 197 230 262
69 128 135 142 149 155 162 169 176 182 189 196 203 236 270
70 132 139 146 153 160 167 174 181 188 195 202 207 243 278
71 136 143 150 157 165 172 179 186 193 200 208 215 250 286
72 140 147 154 162 169 177 184 191 199 206 213 221 258 294
73 144 151 159 166 174 182 189 197 204 212 219 227 265 302
74 148 155 163 171 179 186 194 202 210 218 225 233 272 311
75 152 160 168 176 184 192 200 208 216 224 232 240 279 319
76 156 164 172 180 189 197 205 213 221 230 238 246 287 328
Body weight in pounds according to height and body mass index

The index is calculated from an individual’s weight and height as

or (a version adapted for Imperial units):

This is the body weight of the individual scaled according to the square of the height. In physiology the word “weight” means the same as "mass”. The reason height is squared rather than cubed or raised to some other power is simply that, taken over people of different height, the resulting index correlates reasonably well with degree of underweight or overweight. No law of physics or physiological growth is implied.
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.
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 using categories: eg, severe underweight, underweight, optimum weight, pre-obese (or overweight), obese, morbidly obese. The exact index values used to determine weight categories vary from authority to authority, but in general a BMI less than 18.5 is underweight and may indicate malnutrition, an eating disorder, or other health problems, while a BMI greater than 25 is overweight and above 30 is considered obese. These range boundaries apply to adults over 20 years of age.
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:
• Underweight: less than 20 (<20)
• Ideal: greater than or equal to 20 but less than 25 (≥20 but <25)
• Overweight: greater than or equal to 25 but less than 30 (≥25 but <30)
• Obese: greater than or equal to 30 (≥30)
• Starvation: less than 17 (<17)
• Underweight: less than 18.5 (<18.5)
• Ideal: greater than or equal to 18.5 but less than 25 (≥18.5 but <25)
• Overweight: greater than or equal to 25 but less than 30 (≥25 but <30)
• Obese: greater than or equal to 30 (≥30)
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 17 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.
Different ages
Body mass index calculations are not just for adults—they can also be used to identify the growing number of overweight children. BMI for children aged 2 to 20 years is calculated just as it is for adults, but it is classified differently. Instead of set thresholds for underweight and overweight, it is their BMI percentile compared with children of the same gender and age that is important [1]. A BMI that is less than the 5th percentile is considered underweight and above the 95th percentile is 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 [2].
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 WHO 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...
For Asians,the new cut-off BMI index for obesity is 27.5 compared with the traditional WHO figure of 30. An Asian adult with a BMI of 23 or greater is now considered overweight and the ideal normal range is 18.5-22.9. Singapore BMI Cut-offs
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 [3].
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. [4]
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. [5] and the categories do not distinguish what proportions of a human body's weight are muscle, fat, bone and cartilage, or 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 for sedentary mesomorphs whose ages are between about 20 and 70.
In athletes, the problem is that muscle is denser than fat. Most professional athletes are "overweight" or "obese" according to their BMI[6] - 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.
In all cases, methods for actually measuring body fat percentage are always preferable to BMI for measuring healthy body size.
As a general rule, developed muscle contributes more to weight than fat. It could be stipulated that some long-distance or endurance athletes would be classified as underweight, despite the fact that the individual could be widely regarded as the perfect composite for their particular sport. 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.
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.
Also, there is often an assumption that athletic performance equals good health. This assumption is often false. For instance, many of the players in the NFL qualify as obese by BMI standards, when they actually fall on the far end of the curve for body size, and could not easily fit into the healthy categories, despite being very physically active and having normal bodyfat percentages. This does not seem to protect them from health issues associated with obesity, such as increased risk of heart disease and diabetes. 

Source: BMI

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