Body Mass Index (BMI)
BMI, formerly called the Quetelet index, is a measure for indicating
nutritional status in adults. It is defined as a person’s weight in
kilograms divided by the square of the person’s height in metres
(kg/m2). For example, an adult who weighs 70 kg and whose height is
1.75 m will have a BMI of 22.9.
70 (kg)/1.752 (m2) = 22.9 BMI
The BMI ranges are based on the effect excessive body fat
has on disease and death and are reasonably well related to adiposity.
BMI was developed as a risk indicator of disease; as BMI increases, so
does the risk for some diseases. Some common conditions related to
overweight and obesity include: premature death, cardiovascular
diseases, high blood pressure, osteoarthritis, some cancers and
diabetes. BMI is also recommended for use in children and adolescents.
In children, BMI is calculated as for adults and then compared with
z-scores or percentiles. During childhood and adolescence the ratio
between weight and height varies with sex and age, so the cut-off
values that determine the nutritional status of those aged 0–19 years
are gender- and age-specific. For
BMI calculations on
children and baby girl and boy charts, please refer to the
percentile and
standard children growth website. The cut-off points of the 2006
BMI-for-age reference for children aged 0–5 years for the diagnosis of
overweight and obesity were set as the 97th and the 99th percentile,
respectively. For those aged 5–19 years, overweight is defined as a
BMI-for-age value over +1 SD and obesity as a BMI-for-age value over
+2 SD.
History
BMI is very easy to measure and calculate and is therefore the most
commonly used tool to correlate risk of health problems with the
weight at population level. It was developed by Adolphe Quetelet
during the 19th century. During the 1970s and based especially on the
data and report from the Seven Countries study, researchers noticed
that BMI appeared to be a good proxy for adiposity and overweight
related problems. Like any other measure it is not perfect because it
is only dependant on height and weight and it does not take into
consideration different levels of adiposity based on age, physical
activity levels and sex. For this reason it is expected that it
overestimates adiposity in some cases and underestimates it in others.
Other measures, such as waist circumference (WC), can complement BMI
estimates. Association between WC and health risks is not an easy task
and should be done scientifically using proper techniques.