Is the Body Mass Index (BMI) Accurate?
August 29, 2016What is the Body Mass Index (BMI) and How It Is Used?
The Body Mass Index (BMI) is a measure of body fat and general health. Height and weight are used to determine a person’s BMI.
- BMI is a proxy measure of body fat because it measures excess weight rather than excess body fat. The association between BMI and body fat can be influenced by multiple factors like age, gender, ethnicity and muscle mass.
- BMI does not distinguish between excess fat, muscle or bone mass. Muscle is four times as dense as fat. Highly trained athletes may have higher BMI as they have increased muscle mass as compared to someone with high fat and low muscle tone. As a result of this, some athletes might be placed in an obese category when they are actually in a better shape than the average person.
- BMI also does not take into consideration different types of fats and their distribution. Belly fat which is known as visceral fat is much more harmful to a person’s health than the fat under the skin. Increased visceral fat predisposes a person towards diabetes mellitus, hypertension, and hyperlipidemia. Thin people can have high visceral fat and would be inaccurately classified as healthy as they have normal BMI.
Researchers from Medical Research Council Epidemiology Unit, UK, reported in PLoS Medicine that waist circumference is strongly and independently associated with type 2 diabetes, even after accounting for BMI. Overweight people with large waists, over 102cm (40.2 inches) for men and 88cm (34.6 inches) for women, have approximately the same or higher risk of eventually developing type 2 diabetes as an obese individual.
Reasons To Continue Using The Body Mass Index - BMI
The fat that accumulates around the waist may be more hazardous for long term health than fat that accumulates around the hips and thighs. BMI is probably more misleading in women than in men. On the average, women have greater total body fat than men for the same BMI. BMI does not take into account how the weight is distributed in a person’s body.
If BMI is not accurate, then why are we still using it to evaluate obesity? BMI is a simple, noninvasive and inexpensive measurement, which can be obtained during clinic visits or at home. Since it solely relies on weight and height, no fancy equipment is needed. It is easy to routinely follow an individual’s BMI. While it does not directly measure body fat, it is more precise in approximating degree of body fatness than weight alone.
Studies also show levels of BMI to correlate with multiple weight-related health problems, morbidity, and mortality.
The BMI provides an easy way for clinicians to screen who might be at greater risk of developing weight-related health problems. BMI also provides a relatively standardized tool which has been extensively used to study various populations and their risk of various diseases.
There are several other ways of measuring body fat. Some of them are simple and more practical than others. Dual-energy X- ray absorptiometry (DEXA) scan can be used to measure total body fat. It is usually used to measure bone density but can also distinguish between bone, muscle, and fat. It can provide body composition level for each section of the body. DEXA scan cannot be used at every doctor visit as it is expensive. CT scan and MRI can also provide information regarding a person’s body makeup but again are too expensive to be used for screening of obesity.
Skin calipers can be used to measure subcutaneous fat layer in different parts of the body. Bioelectrical impedance scale can be used to measure body fat. Impedance is low in lean tissue and high in fat tissue. Both these tests are more precise, however, they require trained professionals to perform them.
Hydrostatic weighing is one of the most accurate ways of measuring body composition. It requires a person to be completely submerged in water while their weight is measured. It is obviously not practical to use it in a clinical setting.
Measuring waist and wrist circumference appear to be an easy and more practical way of measuring body fat. There is no data to suggest that it should replace BMI, however, it could be used along with BMI to screen for obesity. There are many ways of measuring body fat accurately, however, they can be expensive, interfering, not easily available and difficult to standardize across observers.
Since there is no single test that can predict if a person is healthy or is at an increased risk of health problems, BMI should continue to be used as a screening tool to identify people who are overweight and obese.
BMI is easier to measure, has a long history of use and most important, does an excellent job of predicting disease risk. Measuring BMI and tracking it over time offers a simple and reliable way for people to be screened for the risk of developing weight-related health problems.
Health care providers should use an individualized approach to assess a person’s health. Apart from BMI, they should evaluate other factors like fat distribution, muscle tone, body structure, diet, and fitness level to determine disease risk. Further studies are required for better understanding of the correlation of BMI, body fatness, fat distribution and various diseases.
 | ABOUT THE AUTHOR Dr. Shireesh Saurabh, MD, is the Medical Director for Mercy’s MBS Program. Dr. Saurabh is board certified by the American Board of Surgery. He completed a Minimally Invasive and Bariatric Surgery Fellowship at the University of Iowa Hospitals and Clinics; did his General Surgery residency at Drexel University College of Medicine in Philadelphia, and received his medical education at the All India Institute of Medical Sciences. |
