Despite the widespread use of the BMI method of body measurement, there is a growing consensus that there may be better approaches than the one-size-fits-all approach.
The Body Mass Index (BMI) is a mathematical formula that divides a person’s weight by their height to determine their body weight category:
- Very severely underweight
- Severely underweight
- Underweight
- Normal (healthy weight)
- Overweight
- Obese Class I (Moderately obese)
- Obese Class II (Severely obese)
- Obese Class Ill (Very severely obese)
Besides identifying one of these categories, a high BMI can also indicate high body fat, which can be used to screen for unhealthy weight levels that could pose health risks. It is not an accurate diagnostic tool for body fatness or overall health, even though it is often used as one.
The BMI is not a perfect measurement, as most fitness-focused people know. When it counts, the BMI measurement may overestimate or underestimate a person’s body fat most of the time – and when it does, it does.
Experts make the same comment every few months: “BMI is flawed.”. As soon as the news hits the headlines, everyone agrees, then everything quiets down.
Those with BMIs between 18.5 and 25 are considered average weight, while those between 25 and 30 are considered overweight. Obesity is classified as a BMI of 30 or higher.
Professor Mitchell Lazar, MD, PhD, Director of the Institute of Diabetes, Obesity, and Metabolism, and Professor Rexford Ahima, MD, PhD, Professor of Medicine and Director of the Obesity Unit at the Institute for Diabetes, Obesity and Metabolism, discuss in Science the challenges health professionals face when studying obesity-related mortality risks and health [1].
Where does BMI go wrong?
People typically slam BMI for not distinguishing between body fat and muscle mass, which is crucial since muscle weighs more than fat. Former Olympians Michael Phelps and Usain Bolt, who narrowly missed the ‘overweight’ category in BMI’s Blind Spots, are good examples of this misinterpretation [2]. NFL quarterback Tom Brady has an obese BMI, according to his BMI.
In addition, basketball player Lebron James and NHL right winger Phil Kessel have BMIs between 25-29.9, which is considered overweight. How does this weird error happen?
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Imagine a 6-foot-2, 203-pound sedentary person. A BMI of 27 would be appropriate for them. Now, consider a sprinter who is also six feet tall but weighs 211 pounds and their BMI is 28 – a glob of muscle weighs about 18 percent more than the exact size glob of fat.
Consequently, sprinters are more overweight than sedentary individuals based on their BMI. But as mentioned earlier, a glob of muscle weighs about 18 percent more than the exact size glob of fat, so this is not an accurate statement.
BMI also fails in this area. Additionally, BMI is not reliable in elderly adults, who typically have lost some muscle and bone mass [3].
A person who has a normal BMI could actually be overweight, despite having a normal BMI. It’s also important to note that BMI calculations are primarily based on Caucasian body types, so they may not be appropriate for people of other races [4].
Asians appear to have four percent more body fat than white Europeans with the same BMI. Especially South Asians suffer from abdominal obesity, which can throw off the BMI calculation.
How does BMI relate to health?
When a person’s BMI indicates that they are overweight or obese, they are generally considered unhealthy, while people with a normal BMI are generally considered healthy. However, a 2016 study suggested that 75 million Americans were wrong about this [5].
There are 54 million overweight or obese Americans, according to researchers. Further investigation revealed that other cardiometabolic measures (blood pressure, blood sugar, cholesterol and the like) were normal.
According to those other cardiometabolic measures, 21 million people were deemed “normal” regarding their BMI but unhealthy later on. To determine this, the researchers analyzed data from the US National Health and Nutrition Examination Survey.
They analyzed the link between BMI and other standard health markers (like blood pressure, blood sugar and cholesterol) and found that BMI incorrectly pegged people’s health at both ends of the weight scale [6].
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Other scientists have suggested that although some people may appear overweight but healthy, the extra weight still puts them at higher risk of certain diseases as they age. So there is some dissension in the ranks.
BMI is still promoted by authorities
In accordance to the Centers for Disease Control and Prevention (CDC), “BMI is a fairly reliable indicator of body fatness.” [7] Per the NIH (National Institutes of Health), “To determine whether your weight is healthy for your height, you should calculate your body mass index.” [8] Do they need to revise their statements? [9]
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[1] http://www.sciencemag.org/content/341/6148/856.summary
[2] http://blog.bodylabs.com/2015/01/29/visualizing-bmis-blind-spots/
[3] https://www.scientificamerican.com/article/is-bmi-an-accurate-way-to-measure-body-fat/#:~:text=Even%20though%20it%20is%20often,far%20from%20a%20perfect%20measurement.
[4] https://www.hsph.harvard.edu/obesity-prevention-source/ethnic-differences-in-bmi-and-disease-risk/
[5] https://newsroom.ucla.edu/releases/dont-use-body-mass-index-to-determine-whether-people-are-healthy-ucla-led-study-says
[6] http://www.cdc.gov/nchs/nhanes.htm
[7] http://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/
[8] http://www.nlm.nih.gov/medlineplus/ency/article/007196.htm
[9] https://www.medicalnewstoday.com/articles/265215#An-example-of-the-biggest-flaw-in-using-BMI.
