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When you get your annual physical, does your doctor discuss your BMI? Many do. Depending on where you fall on the BMI chart, this discussion can be pleasant (“Keep up the good work!”), or it could fall more into the category of a stern warning (“Make changes or else!”).
The problem is that not everyone in the health field buys into using a BMI calculator. This has created confusion about whether it is effective or if another technique should be used instead.
In this article, we discuss what BMI is, as well as its pros and cons. We also do a deep dive into what the research says. All of this together will give us a more educated answer to the question: Is BMI really a good measure of your health?
BMI stands for body mass index. It is calculated using a person’s height and weight. This provides a number that proponents say can be used to determine a person’s risk factor for chronic disease. As such, some medical and fitness professionals rely on it as an indicator of overall health.
The idea behind using this calculation dates back to 1832. It was founded by Adolphe Quetelet, a Belgian mathematician who liked to apply calculus to human physiology. So, it was originally known as the Quetelet Index. It wasn’t until 1972 that Ancel Keys, an American physiologist, renamed it the Body Mass Index.
Though many online calculators exist, you can also figure out your BMI by hand. Simply divide your weight in kilograms by your height in meters squared. If you prefer to use pounds, you divide your weight in pounds by your height in inches, then multiply that number by 703. This provides your BMI measurement.
A standard BMI chart is as follows:
18.5 or less – indicates that you are underweight
18.5 to 24.9 – indicates that you are at a normal weight or healthy weight
25.0 to 29.9 – indicates that you are overweight
30.0 to 34.9 – indicates that you are obese (obesity class I)
35.0 to 39.9 – indicates that you are obese (obesity class II)
40.0 or higher – indicates extreme obesity(obesity class III)
According to this chart, a healthy BMI is somewhere between 18.5 and 24.9. If you have a higher BMI, you are either classified as overweight or obese. If your BMI result is lower, you are considered underweight. But what does this number actually mean?
BMI proponents say that a person’s weight is directly related to their health. More specifically, as your waist circumference increases, so too does your risk of weight related diseases.
This belief is rooted deeply in science. As the Centers for Disease Control and Prevention indicates, obesity has been connected with a variety of chronic health conditions. These include high blood pressure, high cholesterol, type 2 diabetes, and different types of cancer. Being obese also increases your risk of early death.
So, if you have a normal BMI, you’re less likely to develop one of these health issues. Conversely, if you have a high BMI, your health risk is higher too. Makes sense, right? On the surface, yes it does. But proponents stress that a person’s weight does not tell the whole story.
Imagine that you’re a doctor and you look at your patient’s chart before you enter the exam room. You note that their height is 5’7 and they weigh 170 pounds. This places them in the overweight BMI range. You prepare your speech in your head, yet when you open the door, you see a person who is incredibly fit. In fact, they appear to have a lot of lean body mass and very little fat. Herein lies the problem with BMI.
Those who oppose using a BMI calculation say that, even though this formula is said to assess a person’s body mass index, it really doesn’t. The only thing it assesses is body weight to height. It doesn’t take into consideration how much of that weight is fat mass and how much is muscle mass.
This means that anyone who has spent a decent amount of time building muscle will likely be erroneously placed in the overweight or obese categories. This is even though their body fat percentage may be relatively low. In cases such as this, BMI isn’t an accurate predictor of disease because it has nothing to do with body fat.
Both sides make good arguments. A person’s health risk is elevated if they carry excess weight. Yet, since BMI only looks at total weight and not body fat percentage, it doesn’t tell the whole story. What does research have to say about the accuracy of BMI for determining whether a person has a greater risk of disease? Let’s look at a few studies involving heart disease and diabetes.
A 2000 study followed 5,209 people for 24 years. Each participant’s weight and waist circumference were measured multiple times to determine their BMI. After analyzing the data, researchers concluded that a higher BMI was associated with a greater risk of coronary heart disease and death. A 2015 study added that this increased health risk exists regardless of a person’s sex.
Studies such as these support the use of BMI to measure a person’s risk of heart disease. Yet, other pieces of research suggest that it isn’t quite that simple.
One was published in 2013 and involved data collected from 97 studies. It noted that, while a high BMI is generally associated with an elevated risk of heart disease, half of this risk is likely due to having high blood pressure, cholesterol, and blood sugar. This suggests that, when it comes to heart disease, the focus should not be so much on lowering BMI as on reducing these other health issues.
Research has also looked at whether a link exists between BMI and diabetes. Many studies have found that there is a connection. This includes a 2007 study of 780 people. It found that while underweight 18-year-old males have a 7.6% chance of developing diabetes later in life, that risk rose to 70.3% if they were “very obese.” The same was true for 18-year-old females, increasing from 12.2% to 74.4%.
Again, other pieces of research have suggested that the connection is much more complex. An example of this is a 2014 study. While it did note that BMI has a “dramatic influence” on insulin sensitivity and resistance, it isn’t the only factor. In fact, a person’s level of the serum amino acid glycine had the strongest correction with their glucose disposal rate, “regardless of BMI.”
From a scientific point of view, BMI is connected to overall health, at least in part. However, it is just one piece of the puzzle. All of the other pieces must also be considered for a more complete picture.
Plus, most of the studies involving BMI place a heavy emphasis on obesity and disease. This doesn’t address the fact that this method doesn’t specifically measure body fat. It also measures lean body mass. This creates issues for people who have a higher body weight due to having more muscle versus carrying excess weight in the form of fat.
Since obesity appears to be one of the biggest puzzle pieces when determining disease risk, using methods that determine body fat percentage seems to be a better approach. This is especially true when working with clients who have above-average amounts of muscle. And several options exist.
A few ways to determine a client’s amount of body fat include:
Dual energy x-ray absorptiometry (DEXA). This x-ray helps differentiate a person’s fat from lean tissue. Prices range from $20 to $150 or more per scan. You can find a location near you using the DEXA Scan online search.
Underwater weighing. This method is also referred to as hydrostatic weighing. It involves taking a person’s weight then weighing them again while underwater. The more their weight differs, the greater their percentage of fat. This method is considered accurate and usually not very expensive.
Skinfold thickness. Approximately half of a person’s fat is directly under the skin. Therefore, measuring skinfolds can provide a fairly accurate estimate of their fat percentage. This is also a great way to measure a client’s progress without a scale. You can buy skinfold calipers at both retail and sporting goods stores. They are also available online using sites like Amazon and eBay. This is one method that you can easily use as a personal trainer or other fitness professional.
Learn additional ways to help clients achieve a healthy weight by earning your Weight Management Specialist certification. This self-paced online course covers the ins and outs of weight loss, weight gain, and long-term healthy weight maintenance.
As an ISSA Weight Management Specialist, you will be able to coach your clients with empathy and develop the skills to support them through their transformation.
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