There are three small words that can a have massive impact in indicating disease for a person. Those three words are body mass index or BMI. What is body mass index (BMI)? According to the National Heart, Blood, and Lung Institute (NHBLI), BMI is a measure of body fat in proportion to your height and weight. It is also an indicator of overweight and obesity. And most important of all, it measures your risk for disease.
How do you calculate it? Calculating BMI is simple: it is defined as the weight of a person divided by their square height. The Center for Disease Control and Prevention does a great job of categorizing the data by height and weight as shown in the table:
Well, how do you know where you stand? As seen, a healthy weight is indicated as the color green. An overweight is indicated as the color yellow. And an obese weight is indicated as red. For example, if a male is 5′ 7″, and he weighs 160 lbs., his BMI would be 27, which is considered overweight.
Why is BMI so important to know? It is a hot topic during table talk with friends on a Friday night because you are usually trying to compare with each other. Or you have discussed it once with a primary care doctor when considering weight loss. BMI is important because the NHBLI states that an overweight or obese person is at high risk for cardiovascular disease and diabetes. An overweight or obese person may exacerbate their pre-existing health risk with these secondary factors:
- High blood pressure (hypertension)
- High LDL cholesterol (“bad” cholesterol)
- Low HDL cholesterol (“good” cholesterol)
- High triglycerides
- High blood glucose (sugar)
- Family history of premature heart disease
- Physical inactivity
- Cigarette smoking
Yes, we’ve heard that BMI is not a great indicator of measuring your body fat or risk because we have the John Cena, or the Reggie Bush builds who are pounds of muscle. However, there are two concepts that one should consider when it comes to interpreting BMI: validity and reliability.
If you have not taken a research methods class, here is a quick overview of what validity and reliability is. Validity is when an apparatus is measuring what it is intended to measure. Reliability is when an apparatus does not produce the same results. For example, a weight scale is valid in that it is to measure body weight. However, it may not be reliable if it is not consistent in measuring your body weight.
So, now that we have established that, we can better interpret BMI. We have the bodybuilders who pack on pounds of muscle, and based on the BMI scale, are overweight or obese even though they may have low fat percentage. What is the point here? BMI is not reliable in measuring body fat, but it is valid in measuring risk.
As addressed earlier, a person with a BMI of greater than 25 is considered overweight. Now, let’s fiddle with an idea. Let’s say there are two people who are both 5’4″ and weigh 140 lbs respectively. According to the BMI scale, they both have a BMI of 24. Now, let’s say one of them has more lean muscle compared to the other who has more body fat, yet they still weight the same. If you were to look at the numbers, you would not be able to indicate that one has a higher body fat.
Aside from BMI, the NHLBI does recommend measuring waist circumference, which is a good indicator of diabetes and heart disease. The normal ranges are less than 35 inches for women or less than 40 inches for men. Another cheap and easy way of measuring body fat is pinching your waist by using a body fat capilar. The study Accuracy of subcutaneous fat measurement: comparison of skinfold capilars, ultrasound, and computed tomography published in the Journal of the American Dietetic Association in 1994 where researchers found that using skinfold calipers is more accurate in measuring body fat than ultrasound or tomography (using images of cross sections in the body).
What’s the take away here? BMI is not the end-all-be-all in measuring your health risk. It can only help you. If you have any discrepancies with your BMI, consult with your primary care doctor and get a clinical measurement instead. And of course to lower your BMI, exercising and having a balanced diet with low sugar is definitely the key. But again, you should talk to your doctor before you plan on lowering your BMI or losing weight. Physicians do not do heart testing before going on an exercise/diet regime for no reason. This is why, and now you know!