Body Mass Index (BMI) is a term most people have heard of but few understand. In a nutshell, BMI takes into account your weight in comparison to your height, and is represented by a number. That number is then used to determine whether or not your weight falls within a healthy range on a chart. While this is a very simple process, BMI fails to account for some very important factors. Due to the following limitations, it would be wise not to put too much stock in your BMI.
1. BMI is a screening tool
The simplicity of the BMI makes it an easy and accessible screening tool for physicians. With a simple calculation, they can determine if you fall into the underweight, normal, overweight or obese range. Depending on what category you’re in, your doctor can then decide if your weight may be putting you at risk for certain health problems such as cancer, heart disease or diabetes. The issue here is that it’s only a screening tool—just because your BMI labels you as overweight does not mean that you will develop a disease, and classification in the “normal” range does not mean you’re not at risk for health problems.
2. BMI doesn’t detect all cases of excess fat
The term “skinny fat” describes another issue the BMI does not detect. Even though you may be a normal weight according to the BMI chart, you may still have an unhealthy amount of body fat. This is particularly true for older individuals who have less than ideal amounts of muscle and reduced bone mineral density. Because the weights of these fat-free tissues are lower, the overall bodyweight will also be lower, resulting in a lower BMI number. Don’t let this fool you into a false sense of security—excess fat is the danger, not the number on the scale.
3. BMI doesn’t take muscular builds into account
Individuals with muscular physiques have the opposite problem with the BMI chart; it’s constantly implying that these people are overweight when, in fact, they are not. An athlete or bodybuilder may weigh more than what’s deemed normal by the BMI classification. However, when you take into account that the majority of the weight is lean muscle and dense bones, you’re looking at a pretty healthy individual with low risk for disease.
4. BMI doesn’t recognize activity levels
When it comes to health, your level of activity is one of the most crucial determining factors. A study published in the American Journal of Clinical Nutrition found physical inactivity to be responsible for twice as many deaths as obesity. This means that even if you’re labeled overweight or obese by your BMI, if you regularly engage in physical activity then you have a smaller risk of health problems than a sedentary person of normal weight. If you’re currently inactive and overweight, don’t sweat it—the same researchers mentioned that taking a 20-minute brisk walk every day could reduce your risk by 16-30%!
5. BMI doesn’t consider body type
The fact that you have excess body fat isn’t quite as important as where you store it—this is yet another factor BMI can’t accommodate. A pear-shaped body with fat storage around the hips and thighs is less dangerous than an apple-shaped body with fat around the middle. Abdominal (or visceral) fat is known to increase your risk of cardiovascular disease as well as different types of cancers. If you tend to pack extra weight around your middle, increasing your activity level would definitely be a good idea—even if your BMI scores you in the normal weight range.
Ulf Ekelund , et al. “Physical Activity and All-cause Mortality across Levels of Overall and Abdominal Adiposity in European Men and Women: The European Prospective Investigation into Cancer and Nutrition Study (EPIC)”. American Journal of Clinical Nutrition. January 14, 2015.