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BMI not ideal in assigning 'perfect' weight


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POSTED: Sunday, April 12, 2009

For many years, weight-for-height tables that the military developed during World War II by studying GIs showed a range of healthy weights for a given height for each gender.

Almost no one fit the model of the tables.

The group of GIs was not representative, living under different conditions and on a different diet from the typical American. As more data was collected, it became clear that the range of body types in the population made the weight-for-height tables questionable.

In the 1980s the Body Mass Index (BMI) replaced those tables. It is a number that represents kilograms per square meter by dividing weight by the second power of height.

BMI is more reliable than the tables it replaced, but it does have limitations.

Normally scaling a three-dimensional object is a third-power relationship. A brass cube twice the size of one that weighs 3 pounds would weigh 24 pounds, eight times as much.

People are not scaled that way. Taller people tend to have a slimmer build relative to their height than shorter people. The power that scales people lies between 2 and 3, but it varies among individuals.

A power of 2.6 yields the best fit for children age 2 to 19, but the second power is used for everyone by convention and for simplicity since raising a number to a noninteger power is not straightforward math.

The exact scaling is different for different ages and body types. Not all human bodies are made of the same proportions of fat, muscle, bone, sinew, organs and water.

The BMI is not as accurate in adults who gradually lose muscle and bone mass and could still be overweight although their BMI falls within a normal range.

BMI numbers might have different meanings for different ethnic groups. For example, one group might be at risk for health problems at a lower BMI than another.

Different individuals might be genetically predisposed to different healthy body fat amounts, and individuals who have extremely low body fat or who are extremely tall will fall outside the “;healthy”; range on the BMI chart.

A muscular basketball player who is 6 feet 9 inches tall and weighs 275 pounds has a BMI over 29 and would be considered obese if going strictly by the numbers.

The National Institutes of Health recommends that doctors assess healthy weights based on BMI, waist circumference and, most important, other common risk factors such as blood pressure, cholesterol, blood sugar and smoking.

BMI should be just one of many gauges used to assess weight problems. There is no easy way to determine an individual's “;perfect”; weight, if it is possible at all.

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Richard Brill is a professor of science at Honolulu Community College. E-mail questions and comments to .(JavaScript must be enabled to view this email address).