Thursday, October 18, 2012

BMI vs BF%

Body Mass Index vs Body Fat Percentage

According to BMI                                  Weight ranges for Someone with a height of 5'7"
15 and Below  - Very Underweight     96-
16 to 18.4        - Underweight              102 to 117 
18.5 to 24.9     - Normal                       118 to159
25 to 29.9        - Overweight                160 to 191
30 to 34.9        - Obese                        191 to 223
35 and Above - Very Obese                224+     

According to BF% it varies by Age and Sex and Chart

This chart plots BMI vs Body Fat Percent of Men from data collected in 1994  National Health and Nutrition Examination Survey by the National Center for Health Statistics . 

(I added in some my my stats in large colored blobs to show how I fit into the BMI spectrum)

Even at my peak shape back in May (the yellowish blob) I am considered overweight (BMI greater than 25) even though by Body Fat Percent was 12.9. Now I know I was in bad shape in the brownish blob. That was from November 2008 when I was approx 233 pounds and 31% body fat. 

You must take into account that BMI was invented in 1832 in Belgium. It was never meant to be used for medical purposes. Adolphe Quetelet was an astronomer, mathmatician, statistician, and sociologist. Using the new science of probability and statistics (mainly used in astromony at the time) he attempted to apply it to social science he called "social physics." Using his concept he set out to describe the "average man" with data he collected. 

A study in June, 2008 by Romero-Corral et al. examined 13,601 subjects from the United States' Third National Health and Nutrition Examination Survey (NHANES III) and found that BMI-defined obesity was present in 21% of men and 31% of women. 

Using body fat percentages (BF%), however, BF%-defined obesity was found in 50% of men and 62% of women.

 While BMI-defined obesity showed high specificity (95% of men and 99% of women presenting BMI-defined obesity also presented BF%-defined obesity), BMI showed poor sensitivity (BMI only identified 36% of the men and 49% of the women who presented BF%-defined obesity).

The medical establishment has acknowledged major shortcomings of BMI. Because the BMI formula depends only upon weight and height, its assumptions about the distribution between lean mass and adipose tissue are inexact.
BMI generally overestimates adiposity on those with more lean body mass and underestimates excess adiposity on those with less lean body mass.

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