Let's Talk BMI
If you’ve been to a doctor before, it’s likely you’ve had your BMI measured. Despite knowing its limitations, this is a measure physicians use because it remains the standard of care, and is still considered important according to the guidelines given to medical professionals. However, our persistent need to focus on BMI means that we are failing patients with obesity and are often causing more harm than good.
When was BMI developed?
In the early 19th century, a Belgian mathematician and statistician named Adolphe Quetelet was tasked with investigating the correlation between weight, height and mortality in Belgium. He developed the concept of the "Quetelet Index" as a means to assess the degree of obesity in populations.
Quetelet's Index, also known as the Quetelet Ratio, was calculated by dividing an individual's weight in kilograms by the square of their height in meters.
After these studies were done, it melted into the history books until the concept was reinvigorated by Ansel Keys in 1972. Ansel Keys was an American physiologist and nutritionist who played a significant role in promoting the use of the BMI in assessing body weight.
In the 1940s, Keys initiated the landmark "Seven Countries Study," which aimed to investigate the relationship between diet, lifestyle, and cardiovascular disease. As part of this study, Keys recognized the need for a practical and standardized method to assess and classify body weight. He further developed and popularized the concept of BMI.
Since this study was published by Keys, this is what the medical profession has continued to use to categorize the health of individuals. This means that in reality, we are still being compared to the average white man in Belgium in the 1830s, which is completely mind boggling to me.
Why do we still use it?
As each year goes by, we learn more and more about the human body, physiology, and what happens when we gain or lose weight. So why are we still using this measure now that we have more information that essentially refutes its helpfulness?
- Medicine is very slow to change. We need to see not just one study, but multiple studies done by different people in different parts of the world that all come up with the same conclusions before we will change our practice around a topic.
- The standard of care for doctors is to measure BMI. For example:
The US Preventive Services Taskforce has recommendations that explicitly state that doctors should screen everyone with BMI, and for anyone who has a BMI of 30 or higher, weight loss and behavioral interventions should be offered. Therefore, doctors are given the message that recording, monitoring, and tracking BMI is required.
Criticisms of BMI
Your BMI doesn’t actually accurately represent what it is intended for. We currently use it as a way to estimate body fat percentage. There are multiple ways to measure body fat percentage - some are simple and inexpensive, but not accurate, like skinfold measurements. Other more accurate methods are more expensive or time consuming, and not easily done in a clinic setting. BMI has therefore been adopted as an easily accessible, no-cost surrogate measure for body fat percentage.
BMI is simply a measurement of your weight in kg divided by your height in meters squared. Those are the only two factors taken into consideration: height and weight.
The crux of the problem is that it is only looking at your height and weight, and not other things that have a huge effect on both of those, including age, race, ethnicity, gender, genetics, and body composition. It certainly does not take other important factors that define health into consideration such as blood pressure, physical activity levels, and cholesterol.
Why do we care about body fat percentage?
Adipose tissue is the medical term for fat. It is important not because of aesthetics, but because it is a metabolically active organ. It produces hormones that affect most of the other body systems.
It’s crucial that we look at your adipose in the same way we look at your kidneys, your heart, and your liver. It’s an organ whose function really affects how the rest of your body functions. With more adiposity, some hormones are produced more, and others less. Unfortunately medical science has not advanced enough yet to measure these hormones in a clinical setting. So we use this measurement of body fat percentage to give us an idea of what's going on.
An example of how easily BMI can miscalculate us, is with the actor Tom Cruise, who is classified as obese. Why? Because he's a short guy, and he’s got muscle mass. With a lower height and a higher weight due to the muscle mass, the division comes out with a higher number.
The CDC reported that the US obesity prevalence was 41.9% in 2017 – March 2020 (meaning a BMI of 30 or higher) and around one in three people are overweight. That totals roughly 70% of the American population that is defined as overweight or obese by BMI.
But how many of those people are actually unhealthy in terms of their metabolic function?
We have a way to measure this - metabolic syndrome.
To be diagnosed with metabolic syndrome, you need at least 3 of the 5 following things:
- Elevated triglyceride levels
- Low LDL cholesterol
- Central adiposity (measured with weight circumference - greater than 40 inches in men or 35 inches in women)
- Elevated fasting glucose
This shows clearly that obesity is not causing high blood pressure, cholesterol problems, or diabetes. It is a symptom of the underlying metabolic dysfunction that causes all of these things. Obesity is the symptom, it is not the cause.
This is particularly frustrating because if you are diagnosed with prediabetes or diabetes, there is a good chance your doctor might tell you that you can help your diabetes by losing some weight. This sends the message that it’s the weight that is causing the diabetes, which is absolutely not true. It is the metabolic dysfunction underlying everything that causes both weight gain and diabetes.
When you treat the actual cause of diabetes with correct nutrition, exercise, and sometimes medication, weight loss can happen, but that's because you’re treating the underlying cause.
If you categorize people as “healthy” or “unhealthy” based on this list to define metabolic syndrome, one third of people who are classified as obese by BMI are actually metabolically healthy.
An even bigger problem is that about the same number of people who are in the “normal” BMI category are metabolically unhealthy. If we are only offering investigations and treatments to people with a BMI of 30+, 30% of the population may have risk factors such as high blood pressure, but we don't know it because we're not checking because of their "healthy" BMI.
Some research suggests that people who are at a normal weight in terms of BMI, and who are not metabolically healthy, actually have up to a three times higher risk of cardiovascular disease and death. I strongly suspect that not getting access to medical treatment early enough could play some part in these findings.
Secondary harms of BMI
If you have obesity as a diagnosis, it can make insurance premiums much higher for you. This is based on the false premise that you are at a higher risk for all of the diseases that result in death if you have a higher BMI. This is a very clear weight bias and weight discrimination, all based on BMI.
So what is the alternative?
An alternative measure to BMI is what I like to call your ”ideal weight”, or your “happy weight”.
This is a weight that combines several things:
- A weight that you’re happy with (in terms of your body rather than a number on a scale). You feel good, vibrant, strong and you like the way you look.
- You are metabolically healthy.
- You have a lifestyle that you love, are comfortable with, and can sustain long term.
If you are metabolically healthy, you feel good at the weight you’re at, and you like the lifestyle that you’re currently living, then that is my definition of health. I don’t think there’s any need to change anything, even if it means your BMI is in the overweight or obese category.
While the BMI remains widely used due to its simplicity, ease of calculation, and value as a population-level screening tool for overweight and obesity, we have seen that it also has many limitations. It is important to note that the BMI should be interpreted cautiously and in conjunction with other health assessments for individual evaluation.