Is BMI Really An Accurate Measure Of Our Health
Body Mass Index (BMI) is a mathematical formula, first devised in the 1830’s, that divides a person's weight by the square of their height to arrive at a number that falls into one of these bodyweight categories:
Very severely underweight
Normal (healthy weight)
Obese class 1 (moderately obese)
Obese class 2 (severely obese)
Obese class 3 (very severely obese)
Interesting bit of trivia – the most obese classification has only been added in the last couple decades. Prior to that, there was not a need for the additional, more obese, category.
Beyond assigning one of these categories, a high BMI can be also an indicator of high body fat and therefore can be used to screen for certain weight levels that could lead to health problems. Even though it is often used as one, it is not a true diagnostic of body fatness or of an individual’s overall health.
As most of us fitness-focused folks have likely heard, BMI is far from a perfect measurement. Much of the time, and often when it really counts, the BMI measurement may actually overestimate or even underestimate a person’s body fat.
A common example that is often brought up when discussing accuracy of BMI is that it doesn't distinguish between body fat and muscle mass. This is important because a hunk of muscle weighs more than the same size hunk of fat.
Some good examples of this misinterpretation are former Olympians Usain Bolt and Michael Phelps. Both of these highly trained and very fit athletes just narrowly miss the 'overweight' camp. Then there is star NFL quarterback Tom Brady whose BMI categorizes him as obese. Also, basketball player Lebron James and NHL right winger, Phil Kessel, both have a BMI of 27.5 and would fall into the overweight category.
To state it simply – BMI exaggerates thinness in short people and fatness in tall people. BMI divides the weight by too much in short people and too little in tall individuals. This results in tall people believing they are fatter than they really are, and short people thinking they are thinner.
The obvious question becomes – if BMI is inaccurate, what should we use to measure relative fitness in ourselves? The answer is the ratio of our waist size compared to our height, both measured in inches.
Keeping our waist circumference to less than half our height can help increase life expectancy for every person in the world. Thus, a 6ft-tall man should have a waist circumference of 36 inches or less, while a 5ft 4in woman’s waist should not exceed 32 inches.
The waist-to-height ratio can be considered a more accurate screening tool than BMI. BMI does not take into account the distribution of fat around the body. Abdominal fat affects organs like the kidney, liver, and heart more severely than fat around the bottom or hips. Waist circumference gives an indication of abdominal fat levels.
This should not make the reader feel that BMI is of no use. It is still a good general indicator of overall weight parameters. However, keep our waist circumference to less half our height is an easier measurement to hold on to than BMI.