Why it's useful to calculate BMI
- Here's what your particular index predicts
WHAT IS BMI?
Body Mass Index (BMI) is a measure of body weight relative to height. To calculate BMI is simple. The index is useful to gauge whether or not one is maintaining a healthy weight based on the weight-height ratio, and acts as a predictor of the risk of disease and death. The BMI index is applied to both men and women but does have certain limitations. (These are addressed below.)
HOW TO CALCULATE BMI
The calculation was created by Belgian Mathematician Adolphe Quetelet during the 1840’s, and proposes an ‘index’ by taking an individual’s body weight and dividing this value by the person’s height-squared (kg/ m2).
For example, an adult weighing 76kg and whose height is 1.77m will have a BMI of 24.3, as per the following calculation:
BMI = 76 kg ÷ (1.77 m)2 = 24.3
Alternatively, you can use the tool below to calculate BMI. It will show your BMI value and corresponding weight status category. (Note, the scores are applicable to adults of 20 years or more.)
Your BMI is:
Based on BMI Calculator
The Body Mass Index can also be determined by means of a chart, like the one based on the World Health Organization (WHO) data below. It shows how to calculate BMI as a function of weight on the X (or horizontal) axis and height on the Y (or vertical) axis. The contour lines indicate the different BMI values and the colors show the respective weight status categories.
According to the National Heart, Lung, and Blood Institute (NHLBI) guidelines, assessment of "overweight" involves using three key measures. One would calculate BMI, waist circumference and consider risk factors for diseases and conditions associated with obesity.
Whereas the BMI is a measure of weight relative to height, waist circumference measures abdominal fat (risks increase in men with waist measurements above 40 inches, and in women whose waist measurements exceed 35 inches). Combining these with information about one’s additional risk factors yields the inherent risk for developing obesity-associated diseases like high blood pressure, high cholesterol, heart disease, stroke, cancer and/or death.
WHAT IS THE BODY MASS INDEX USED FOR?
Although the formula to calculate BMI has met with controversy and criticism, it is widely used to classify conditions of underweight, normal, overweight and obesity in adults and as a predictor of the risk of ill-health and/or mortality.
BMI continues to be broadly recognized as a body fat assessment method, most likely due to its ease-of-use. However, rather than a consistent accurate measure, it provides more a rough means of estimating adiposity (or ‘fatness’) and identifying trends of sedentary or overweight individuals (these groups represent narrower margins for error).
The BMI is decidedly less accurate when applied to athletes, who invariably weigh more due to increased levels of muscle mass. The BMI formula merely takes body weight into account and does not discern between fat and muscle.
WHO has used the BMI as a standard means for observing obesity statistics since the 1980s. In the US, it is deemed a satisfactory tool in for measuring inactive individuals, despite its limitations in defining body fatness.
The medical world in particular, continues to calculate BMI as an aid to assessing the risk to health of increasing body size. Furthermore, most private health insurers in the US utilize BMI to determine cut-off points for determining coverage or premiums.
WHAT ARE THE APPLICAPBILITY AND LIMITATIONS OF BMI?
According to WHO, BMI values are neither subject to age nor gender. However, they may not relate equally to levels of fatness in different populations (certain racial groups in Africa and Asia) as a result of differing body-proportions. Also, the BMI index may not accurately reflect the fatness levels of people with builds falling far outside the average.
In terms of body type and levels of muscularity (in this case we refer to somatotypes), Mesomorphs usually have medium builds, while Ectomorphs and Endomorphs would be the lighter and heavier diversions from the norm.
As mentioned, individuals who are active, especially sportsmen and sportswomen, will tend to have BMI levels of "overweight" or even "obese", due to their higher levels of muscle mass. BMI categories also fail to account for varying proportions of fat, bone, cartilage, water weight, and more. Sometimes, adjusted versions of the index are used to cater for professional athletes. Note that muscle is denser than fat and takes up less volume.
Although to calculate BMI maybe useful for most men and women, it may overestimate body fat in athletes or in those with higher muscularity; on the othe hand, it may underestimate body fat in older persons or those who have lost muscle mass.
Body Mass Index for Children and Teenagers
According to the Centers for Disease Control and Prevention (CDC), "overweight" is defined differently for children and teenagers than for adults. Due to growth and differing maturation rates, a modified BMI compares heights and weights against growth charts which also consider gender. This index is known as the BMI-for-age percentile (for children 2-19 years of age), and reflects BMI as compared with boys and girls of the same age.
HOW TO MEASURE BODY FAT MORE ACCURATELY THAN BMI
Body composition alternatives do exist. These are of particular importance to professional athletes and bodybuilders, who require higher accuracy from such body composition tests.
Among several methods, two examples are the commonly used skin fold test method and the more accurate, but also more costly Hydrostatic or underwater weighing. The skin fold test measures the thickness of subcutaneous fat in multiple places on the body, while underwater weighing measures mass density based on the Archimedes’ principle. Additional body composition testing include the Body volume index, Waist-hip ratio and Sagittal Abdominal Diameter (SAD).
HOW VALUABLE IS BMI?
The medical establishment generally acknowledges some failings of BMI as it depends solely on weight and height. It also does not consider the distribution of muscle and bone mass, and therefore tends to overestimate adiposity in those with higher lean body mass like athletes, while underestimating adiposity in those with less lean body mass, like the aged.
At best, the BMI is a general guideline and does not match other more thorough body composition tests. Even if it is a good predictor of health if your height and weight fall within average statistical values, it should be used only as an indicator. It should not be seen to replace the professional opinion of a medical practitioner.
Nonetheless, BMI is a simple and useful means for an individual to know whether his or her health is at stake.
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