The Body Mass Index (BMI) or Quetelet’s index is a value derived from the mass (weight) and height of an individual. The BMI is defined as the mass of the body, to which the division by the square of the height of the body is applied, and is universally expressed in units of kg / m2, as a result of the mass in kilograms and the height in meters .
BMI can also be determined using a table or graph that shows BMI as a function of mass and height using contour lines or colors for different BMI categories, and which can use other units of measurement (converted to metric units for calculation ).
BMI – What is it
BMI is an attempt to quantify the amount of tissue mass (muscle, fat, and bone) in an individual, and then categorize that person as underweight, normal weight, overweight, or obese based on that value. That categorization is the subject of some debate about where the dividing lines between categories should be on the BMI scale.
Commonly accepted BMI ranges are underweight: less than 18.5 kg / m2, normal weight: 18.5 to 25, overweight: 25 to 30, obese: more than 30. People of Asian descent have different associations between BMI, percentage of fat body and health risks than those of European descent, with a higher risk of type 2 diabetes and cardiovascular disease in BMI lower than the WHO cut-off point for overweight, 25 kg / m2, although the limit for the observed risk varies between different Asian populations.
BMI – History
Obesity and BMI
The basis of the IMC was devised by Adolphe Quetelet, Belgian astronomer, mathematician, statistician, and sociologist, from 1830 to 1850, during which time he developed what he called “social physics.” The modern term “body mass index” (BMI) for the relationship between human body weight and square height was coined in an article published in the July 1972 issue of the Journal of Chronic Diseases by Ancel Keys and others. In this paper, Keys argued that what he called BMI was “… if not totally satisfactory, then at least as good as any other relative weight index as an indicator of relative obesity.”
Interest in an index that measures body fat came with the rise of obesity in affluent Western societies. BMI was explicitly cited by Keys as appropriate for population studies and inappropriate for individual assessment. However, due to its simplicity, it has been widely used for preliminary diagnosis. Additional metrics, such as waist circumference, may be more helpful.
BMI is universally expressed in kg / m2, as a result of mass in kilograms and height in meters. If pounds and inches are used, a conversion factor of 703 (kg / m2) / (lb / in2) should be applied. When the term BMI is used informally, the units are generally omitted.
BMI provides a simple numerical measure of a person’s thickness or thinness, allowing healthcare professionals to discuss weight issues more objectively with their patients. The BMI was designed to be used as a simple means of classifying average sedentary (physically inactive) populations, with an average body composition.
For these people, the current value recommendations are as follows: a BMI of 18.5 to 25 kg / m2 may indicate optimal weight, a BMI less than 18.5 suggests that the person is underweight, a number of 25 to 30 may indicate that the person is overweight, and a number of 30 and up suggests that the person is obese. Lean athletes often have a high muscle-to-fat ratio and therefore a BMI that is deceptively high relative to their body fat percentage.
IMC – Scalability
If all the dimensions of the body are doubled and the mass increases naturally with the cube of the height, then the BMI doubles instead of staying the same. This results in taller people who have a reported BMI that is abnormally high, compared to their actual levels of body fat. In comparison, the Ponderal index is based on the natural scale of the mass with the third power of the height.
A common use of BMI is to assess how much of an individual’s body weight is normal or desirable for a person’s height. Being overweight or underweight can, in part, be explained by body fat (adipose tissue), although other factors, such as muscle, also affect BMI significantly (see discussion below and being overweight).
BMI in children (2 to 20 years)
BMI for age percentiles for children 2 to 20 years of age.
BMI for age percentiles for girls ages 2 to 20.
BMI is used differently for children. It is calculated in the same way as for adults, but then compared to typical values for other children of the same age. Instead of comparing with the fixed thresholds for underweight and overweight, BMI is compared to the percentile for children of the same sex and age. A BMI that is less than the 5th percentile is considered underweight and above the 95th percentile is considered obese. Children with a BMI between the 85th and 95th percentile are considered overweight.
Dr. Samantha Robson ( CRN: 0510146-5) is a nutritionist and website content reviewer related to her area of expertise. With a postgraduate degree in Nutrition from The University of Arizona, she is a specialist in Sports Nutrition from Oxford University and is also a member of the International Society of Sports Nutrition.