24 Jun Registered Nutritionist Discusses the BMI Roulette
A person’s BMI – body mass index – is determined by taking his weight (in kilograms) divided by the square of height in meters. (It sounds a bit like one of those math questions: If you take a person’s height, square it, and divide it into her weight in kilos, how many cupcakes will you have left in Singapore?) This test, though, gives a nutritionist, dietitian, personal trainer or physician a number that is virtually meaningless.
Did you read that right?
The ever-important BMI that we’re supposed to use to measure the level of fat in our bodies doesn’t, in fact, do so. Not necessarily. The BMI was invented by a Belgian mathematician, Lambert Adolphe Jacques Quetlet, to aid the government in assessing the obesity rate of the general population in the early 19th century. It was a mathematical shortcut that, he assessed, should never be used to determine an individual’s level of obesity. (Keith Devlin, Top Ten Reasons Why the BMI is Bogus, npr.org).
Though it can be used as a screening tool, our individual BMIs can’t be used to determine (just the number alone) anything concrete about a person. The BMI doesn’t take into account that muscle and bone weigh more than fat. Muscular individuals and individuals with high bone density will, invariably, score a high BMI. Moreover, measuring a person’s BMI doesn’t take into account waist size – the greatest factor that determines whether an individual is obese or not.
The CDC says that “A high BMI can be an indicator of high body fatness and having a low BMI can be an indicator of having too low body fatness. BMI can be used as a screening tool but is not diagnostic of the body fatness or health of an individual.” (cdc.org) It’s almost contradictory, explicitly stating the BMI is not diagnostic of a person’s body fat or level of health. The high numbers could be indicators, but aren’t necessarily so.
So, why after all these years (over 200 more or less) do we still hold onto this vaguely inaccurate diagnostic tool? It’s frustrating because even on government health websites, like the NIH, state that “the most common way to find out whether you’re overweight or obese is to figure out your body mass index (BMI). BMI is an estimate of body fat, and it’s a good gauge of your risk for diseases that occur with more body fat.” (nhlbi.nih.gov)
A registered dietitian, nutritionist, and/or personal trainer might measure a client’s BMI but will never use that measure as the sole indicator of a client’s level of body fat or health. More accurate ways to measure a person’s level of fat and health include:
Body Composition Testing: This is a huge part of any fitness goals process. Lange calipers are used to measure body fat as well as bioelectrical impedance analysis to measure total body water, fat-free mass and fat mass. Learn more>
Waist to Hip Ratio: Abdominal obesity is an indicator of obesity and increases disease risk. “Long-term follow-up studies showed that so-called “abdominal obesity” was strongly associated with an increased risk of type 2 diabetes, cardiovascular disease and death, even after controlling for body mass index (BMI).” (Waist Size Matters, hsph.harvard.edu)
Fitness Assessment: Cardio, muscular and flexibility as well as posture muscle imbalances or weaknesses. A personal trainer needs to measure these things in her clients before starting on any program. Learn more>
Though the BMI might be an indicator of being overweight or obese, we can’t, as we’ve discussed before, reduce our health to a number. A more complete diagnoses must be made, using other tools, to determine our level of health. Don’t play numbers with your health. Get the whole picture.