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CALCULATION OF THE INDEX The index is calculated from an individual’s weight and height as : or (a version adapted for Imperial Unit s): : 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 , 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. 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: Males
Females
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. 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 BMI - Body Mass Index: BMI for Children and Teens . 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/m&2 on average Health Survey for England: The Health of Children and Young People . 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 USES 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 Jeukendrup, A & Gleeson, M. (2005) ''Sports Nutrition'' Human Kinetics. 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. Barasi, M. E (2004) ''Human Nutrition - a health perspective'' 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 factors such as frame size and muscularity. Jeukendrup, A & Gleeson, M. (2005) ''Sports Nutrition'' Human Kinetics 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 1980 s. 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 than 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 BMI for Gold Medal Olympians - 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 ''under''weight should be adjusted downward. In all cases, methods for actually measuring Body Fat Percentage are always preferable to BMI for measuring healthy body size. Problems The first problem with the BMI is that it is not scientific but ''pseudo''-scientific. Like the old American height-weight charts, it is not a product of medical research - although both tools, paradoxically, have been used by medical professionals. Much has been learned about the human body since the BMI was first developed nearly two centuries ago, and since the American height-weight charts were invented one century ago, but neither tool has incorporated any of this information. As a general rule, developed muscle contributes more to weight than fat; and it is through this fact that the body mass index can be readily classed as widely erroneous. It could be stipulated that 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. REFERENCES Articles about Body Mass Index |