WHO (1995) states that ‘BMI’ provides a simple numeric
measure of a person’s thickness
or thinness, allowing health
professionals to discuss overweight and underweight problems more objectively
with their patients. However, BMI has become controversial because many people,
including physicians, have come to rely on its apparent numerical authority for
medical diagnosis, but that was never the BMI’s purpose; it is meant to be used
as a simple means of classifying sedentary
measure of a person’s thickness
or thinness, allowing health
professionals to discuss overweight and underweight problems more objectively
with their patients. However, BMI has become controversial because many people,
including physicians, have come to rely on its apparent numerical authority for
medical diagnosis, but that was never the BMI’s purpose; it is meant to be used
as a simple means of classifying sedentary
(physically inactive) individuals,
or rather, populations, with an average body composition. For these
individuals, the current value settings are as follows: a BMI of 18.5 to 25 may
indicate optimal weight, a BMI lower than 18.5 suggest the person is
underweight, a number above 25 may indicate the person is overweight, a number
above 30 suggests the person is obese.
For a given height, BMI is proportional to mass.
However, for a given mass, BMI is inversely proportional to the square of the
height. So, if all body dimensions double and mass scales naturally with the
cube of the height, then BMI doubles instead of remaining the same. This result
in taller people having a reported BMI that is uncharacteristically high
compared to their actual body fat levels. In comparison, the Ponderal index is
based on this natural scaling of mass with the third power of the height.
However, many taller people are not just “scaled up” short people,
but tend to have narrower frames in proportion to their height. Nick Korevaar
(a mathematics lecturer from the University of Utah) suggests that instead of
squaring the body height (an exponent of 2, as the BMI does) or cubing the body
height (an exponent of 3, as the Ponderal index does), it would be more
appropriate to use an exponent of between 2.3 and 2.7 (as originally noted by
Quetelet). (MacKay, 2010).
However, for a given mass, BMI is inversely proportional to the square of the
height. So, if all body dimensions double and mass scales naturally with the
cube of the height, then BMI doubles instead of remaining the same. This result
in taller people having a reported BMI that is uncharacteristically high
compared to their actual body fat levels. In comparison, the Ponderal index is
based on this natural scaling of mass with the third power of the height.
However, many taller people are not just “scaled up” short people,
but tend to have narrower frames in proportion to their height. Nick Korevaar
(a mathematics lecturer from the University of Utah) suggests that instead of
squaring the body height (an exponent of 2, as the BMI does) or cubing the body
height (an exponent of 3, as the Ponderal index does), it would be more
appropriate to use an exponent of between 2.3 and 2.7 (as originally noted by
Quetelet). (MacKay, 2010).
References
MacKay, N.J. (2010). “Scaling of human body mass with height: The
body mass index revisited”. Journal of Biomechanics 43 (4):
764–6
body mass index revisited”. Journal of Biomechanics 43 (4):
764–6
WHO, (1995). “Physical Status: The
Use and Interpretation of Anthropometry”. WHO Technical Report Series
(Geneva, Switzerland: World Health Organization) Vol. 854: 9. 1995.
Use and Interpretation of Anthropometry”. WHO Technical Report Series
(Geneva, Switzerland: World Health Organization) Vol. 854: 9. 1995.