This series on obesity continues with the discussion of body mass index (BMI), and whether BMI is over-valued when assessing the health of individuals.
BMI is a medical calculation that is almost 200 years old. Developed in 1832 by a Belgian mathematician, Lambert Adolphe Jacques Quetelet, the BMI was designed to quickly estimate the degree of overweight and obesity in a defined population that would help governments decide the allocation of health and financial resources. Quetelet believed the BMI was useful only in providing a snapshot of a population’s overall health – his intent was not to measure the health of single individuals.
BMI is derived from your height and weight, calculated by dividing weight (in pounds) by height (in inches squared) and multiplying by 703. If you’re curious, you can quickly learn your own BMI here. Once calculated, the BMI is compared to the BMI scale, which is broken down into five categories:
- Underweight: BMI below 18.5
- Normal: BMI ranging between 18.5 and 24.9
- Overweight: BMI between 25 and 29.9
- Obese: BMI of 30 or higher
- Severe Obesity: BMI of 40 or higher
Is BMI Outdated?
Critics believe BMI is being overused and not very useful when analyzing the health of individuals. In fact, BMI is considered too simplistic, if not misleading. The critics make a number of points, including the following factors:
- Fat versus Muscle – Some people have high BMIs, but not much body fat. One prime example is an athlete, such as a football player. This athlete can have high muscle tissue (and low body fat) which pushes up his weight – and his BMI number.
- Activity Level – An individual who is very inactive, has lots of body fat and low levels of muscle and bone, may have a BMI in the normal range. But this individual is often elderly, in poor shape and sometimes sick. However, this individual with a ‘normal’ BMI has risks similar to people who carry a high amount of body fat and have a high BMI. These underlying risks are not reflected in the BMI score.
- Body Type – The location of your fat can make a big difference to your health. The belly fat (apple shape) type is generally at higher health risk, and the chance of heart disease and type 2 diabetes increases. This fat settles around the waist instead of the hips. Likewise, fat that appears on the hips and thighs (pear shape) is not considered to be as harmful. BMI does not indicate the location of fat and, therefore, does not reflect those risks.
- Age – As one ages, they may carry a little more weight, which may actually be beneficial. One possible reason is that as we age, a little extra weight can serve as reserves to draw on when fighting an illness. Because of this, an ideal BMI score may change as one ages.
- Ethnicity – The BMI can be different based on ethnic groups. For example, people from India face higher health risks at relatively lower BMIs. As mentioned earlier, the standard definition of overweight is a BMI of 25 – 29.9. However, people of Indian descent may begin to develop a risk of diabetes when the BMI is 21 or 22. Asian-Americans tend to develop health risks, such as diabetes, at lower BMIs than whites. In other words, a healthy BMI for Asians ranges from 18.5 to 23.9, which is one point lower than the standard range. Asians are considered obese at 27 or higher. Compared to whites with the same weight and BMI, African-Americans may have less fat around their organs and more muscle mass. This means that African-Americans, despite having a high BMI, may have fewer health risks than whites.
These five factors give credence to the argument that BMI is both outdated and inaccurate when assessing the health of individuals.
So why continue to use BMI?
The counter-argument to continue using BMI can be equally persuasive.
Despite the concern of accurately identifying whether a person is healthy, most studies do link low and high BMI scores with a person’s risk of chronic disease and premature death. As an example, a 2017 study of 103,218 deaths found that people who had a BMI of 30 or greater (“obese”) had 1.5 to 2.7 times greater risk of death after a 30-year follow-up.
A 2014 study that included 16,868 people showed that “obese” BMI individuals had a 20 percent increased risk of death from all causes and heart disease when compared to those in the “normal” BMI range. From this same study, researchers found that people in the “underweight” category and the “severely obese” or “extremely obese” categories, died an average of 6.7 years and 3.7 years earlier, respectively, than those in the “normal” BMI category. Adults with a BMI greater than or equal to 40 are considered severely obese.
The data on BMI greater than 30 is quite substantial as it relates to the increased risk of chronic health issues such as heart disease, type 2 diabetes, breathing difficulties, kidney disease, non-alcoholic fatty liver disease, and mobility issues. (See references below)
Despite the criticism of BMI, one can safely assume that BMI may be more useful at predicting future health – rather than diagnosing the current health.
Standard BMI definitions of overweight and obesity were based on white populations, which differs from other races and ethnic groups. Because of this, BMI should not be the only diagnostic tool when estimating a person’s risk of developing various chronic diseases. Many alternative measurement assessment tools exist to determine obesity and related health issues. The Harvard T.H. Chan School of Public Health has listed such measurements here.
Though BMI oversimplifies the health of individuals, research suggests it can serve as an important barometer to estimate a person’s risk of chronic disease, risk of early death and metabolic syndrome.
It’s important to know your BMI, but it is equally important to recognize its limitations.
This post is the second in a five-part series. You can read “Obesity Requires Employer Attention and Focus – (Part 1)” here.
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- Morbidity and mortality associated with obesity
- Association of Body Mass Index (BMI) With Lifetime Risk of Cardiovascular Disease and Compression of Morbidity
- Body mass index and risk of cardiovascular disease, cancer and all-cause mortality
- Relation between BMI and diabetes mellitus and its complications among US older adults
- Body Mass Index and mortality in patients with and without diabetes mellitus
- BMI and its effects on liver fat content in overweight and obese young adults by proton magnetic resonance spectroscopy technique
- BMI and Risk of Nonalcoholic Fatty Liver Disease: Two Electronic Health Record Prospective Studies