Why Does the Same BMI Look Different from One Person to Another?

measuring BMI

What does a 5”1’ 160 lbs woman have in common with a 220 lbs 6’ man? They both have a BMI of 30 and would be considered obese. BMI or body mass index is a simple screening that’s designed to help determine if you are at risk for obesity related conditions. All you need is your height and weight, plus a chart or app to get the results. In my office, the electronic medical record automatically calculates the BMI for all of my patients.

As with all screening tests, your results have to be interpreted. The BMI correlates with the amount of body fat present for most people, but the calculation can be falsely high for those with increased lean muscle mass like athletes for example. If a BMI’s results are questionable there are several direct measurements of body fat that can be done. These include skin fold measurements, bioelectrical impedance testing, densitometry (underwater weighing) and dual energy X-ray absorption testing (DXA).

A typical BMI reading chartNormal BMI readings are 18.5 to 25. Twenty-five to 29.9 is considered overweight and a reading of 30 and over is obese. Health risks increase at a BMI of 25 and the National Institute of Health reports that people with a BMI of 30 or more are at 50-100% increased risk of early death when compared to normal weight individuals. Those with BMIs lower than 18.5 are also at increased health risk.

There are a host of health issues related to an elevated BMI including heart disease, hypertension, stroke, high cholesterol and diabetes. Some cancers such as colon, breast, endometrium and gallbladder occur more frequently in obese people. Osteoarthritis of the knees, hips and spine are more common in overweight people, along with sleep apnea. Researchers have also found an increased risk of depression, anxiety and other mental illness in those who are obese.

While the evidence is clear there are many serious health risks that go along with being overweight or obese, the good news is these risks will significantly reduce as your BMI decreases with ongoing weight loss.

measuring tapeI recommend that each person know their BMI. If it’s in the healthy range, ask yourself how much of your weight is fat vs. muscle. To learn this, you can have an advanced body composition exam done at Roper St. Francis that can show you your body’s distribution of fat mass, lean muscle tissue and bone mass all in one safe, fast procedure. Healthy fat percentages are <20% for men and <30% for women. It is this fat/muscle ratio that’s important. You can be in the healthy BMI range, but not have a healthy ratio.

If your BMI is elevated or your fat/muscle ratios is off, you need to work on a plan to not only lose weight, but also increase muscle mass. It’s important to both eat well and to exercise. There are many programs, such as Weight Watchers, that can help with the weight loss portion. But, to build muscle you have to exercise. Aim for walking at least 15 minutes and doing resistance training for 30 minutes 3-5 times a week. Resistance training can include activities such as yoga, Pilates or weight lifting.

In addition to the above suggestions, those with extremely high BMIs should consider attending a bariatric surgery seminar to learn about the surgical and nonsurgical weight loss options available that could help.

The good news is that you can avoid the increases in diseases and early death by changing your lifestyle and losing weight. The first step on this journey is finding out your risk with the simple and fast BMI test.

Dr. Valerie Scott


Written by Dr. Valerie Scott, who is the Roper St. Francis wellness ambassador physician champion. Dr. Scott is board certified in family medicine with Roper St. Francis Physician Partners.