BMI is short for Body Mass Index and is used as an estimate of body fatness. Specifically, it represents a numerical relationship between your weight and height. The index was originally intended as a population/epidemiological tool to identify weight-related problems in adults, but evolved as an indicator of health risk in the medical field. This relationship between height and weight is based on the 19th century (yes, as in the 1800’s!) work of Belgian astronomer, mathematician, and statistician, Lambert Adolphe Jacques Quetelet. His work, I might add, was not the diligent assessment of all people, of all backgrounds, but instead isolated to a non-representative, homogenous population. But according to the population he assessed, Quetelet found that weight varied more closely to height squared than height alone, hence the equation derived below.
BMI = weight (kg)/ height2 (m2)
BMI = weight (lbs)/height2 (in2) x 703
Being on either end of the BMI spectrum puts you at greater risk for health problems.
<18.5 = underweight
18.5-24.9 = normal weight
25-29.9 = overweight
According to the CDC, the BMI scale is a “fairly” reliable indicator of health. The use of the term “fairly” is with the understanding of the inherent limitations of the index. This estimate of body fatness doesn’t consider genetic variations in fat distribution, and likewise fails to distinguish between fat mass and lean mass. For this reason, the scale is particularly inaccurate for those with a more muscular build given that muscle is more dense than fat. To put in another way, a pound of muscle will take up less space than a pound of fat, therefore a muscular individual may have a BMI that places them in the overweight category when in fact they are not. (According to the BMI scale, I am one step away from being overweight, when in fact I am not.) Interestingly, one can be more muscular than they think, even though it may not be visually apparent. Likewise, the scale doesn’t accurately reflect body fat in older populations and other individuals who have lost muscle.
Studies show that an individual’s waist-to-hip ratio and waist size are better indicators of health. Women should aim for a waist size of less than 35 inches and men should aim for no more than 40 inches. This goes very well with my philosophy even as an African American youngster–“everything can be large on me except my waist!”
Although BMI is a rough indicator of your health status, the bottom line is to think more about the size of your waist more so than your BMI or your weight. It’s not necessarily fat or weight that is the problem, but in fact where the fat is distributed throughout your body (more on that later).