Do you have your BMI checked at the doctor’s office at your annual physical exam? Have you ever thought about how much it really matters?
I had a client who was constantly told by her doctor to lose weight since her BMI was in the overweight range, but she didn’t feel like she needed to lose weight. She had excellent blood results, was in shape, felt energetic and had good muscle definition. Despite being in nearly perfect health and not having that much excess fat, she was still categorized as an overweight person, just because her BMI was high. It got me thinking…do we really need to rely on the BMI as a marker for good health? Does it really paint an accurate picture of your body fat?
What is the BMI?
The body mass index (BMI) is a measure of body fat based on height and weight . It is basically a simple calculation of your weight in kg divided by your height in meters squared (if you’re using the metric system). It has been used as a marker for obesity for decades and is something most physicians will calculate at your annual physical.
BMI = WEIGHT/ (HEIGHT)2
Based on your BMI, you can be classified into several categories:
- Underweight – BMI below 18.5
- Normal weight – BMI between 18.5 and 24.9
- Overweight – BMI between 25 and 29
- Obese – BMI above 30
Why is it used?
BMI is a quick and easy way to estimate body fat and is very inexpensive compared to other, more accurate ways of determining body fat. For example, using a DEXA scanner, which is a more reliable way of calculating body fat, can cost more than $100 per person. The BMI doesn’t cost anything as long as you have access to a weighing machine and a measuring tape! So the BMI became the “it” tool to assess obesity. Its simplicity was what made it popular, but simplicity sometimes comes with flaws.
What are the flaws?
BMI has been used for many decades and most health professionals never questioned it as it seems to be a reasonable estimate of body fat. However, now there is more research on the topic and it is very clear that the Body Mass Index is NOT an accurate way to assess body fat. It is just an rough indicator. In fact a study was published in 2005  that came up with the following conclusion:
BMI is a poor indicator of percentage fat and the commonly used cut-off values were not sensitive enough to detect cases of childhood obesity.
Not only can it NOT detect obesity with accuracy, it is inherently flawed due to the issues below.
1. Body composition – muscle/fat distribution
BMI doesn’t differentiate between bones, muscles and fat and doesn’t take into account where your weight is coming from. That is why it is possible for a person with heavier bones and a muscular physique to have a normal body fat percentage, but still be considered overweight according to BMI. Many athletes tend to be classified as overweight according to BMI for precisely this reason. They are heavier, but not due to fat – it’s due to more muscle than the average person. Having more muscle is not equal to being overweight due to fat.
Isn’t it odd that the BMI ranges are exactly the same for men and women? Does that make sense? For example, let’s say there is a man who is 160 pounds and 5 foot 6. His BMI would be 25.8 and that would put him in the overweight category. Let’s say there is also a woman who is 160 pounds and 5 foot 6 – she would have the same BMI. They would both be classified as overweight, but it’s likely that the man is heavier since his body composition is different from the woman. Men tend to have heavier bones and carry more muscle than women, and the BMI calculation does not take that into account at all.
Did you know that BMI ranges that are considered normal and healthy for Caucasians should not be the same as the ranges for Asians/South Asians? Many studies show that Asians and South Asians have a higher risk of obesity related diseases and lower BMIs [5,6]. If you’re of Asian or South Asian descent, your risk of diabetes and heart disease starts at a BMI of just 22 .
The BMI ranges are the same for all adults, but does it make sense for the ranges to be the same for someone in their 20s vs. someone in their 70s? As as age, we tend to lose muscle mass and put on weight in the form of fat.
For example, let’s say there is a 20 year old woman at a BMI of 24 (the higher end of normal) with a normal body fat percentage. Let’s say that number is 30% which is at the higher end of acceptable. Now let’s assume her weight does not change over the years and she still has a BMI of 24 at age 70. I highly doubt she will have the same body fat percentage she had when she was in her twenties. Most likely she will lose a lot of her muscle mass and gain fat, so she is probably going to have a higher body fat percentage. Unfortunately, the BMI won’t tell her that. In her case, the BMI remains the same despite her changing body composition. What was considered a healthy BMI at age 20 shouldn’t really be considered healthy at age 70.
5. Body shape
Ever thought about how some people tend to deposit fat around their bellies while others deposit fat around their thighs? What does this mean from a health perspective?
Well, people who tend to be apple shaped (more fat around the tummy), tend to be a a higher risk of disease than those who are pear-shaped (more body fat around the hips and thighs). Sadly, the BMI does not take the body shape in account at all. So we could have two people with exactly the same BMI, but one has more fat around the stomach than the other. Who is at a higher risk of disease? The apple-shaped person is a higher risk of diabetes compared to the pear , but their BMI won’t reflect that.
What have we learned so far? The BMI isn’t really a good measure of body fat and has many flaws, but I haven’t even touched upon it’s worst flaw.
The worst flaw is that it can classify a person with a high body fat percentage as healthy. You could walk out of your doctor’s office thinking you’re at a healthy weight and not even realize that you’re still considered “fat”. Which leads me to the next question…
Can someone with a healthy BMI have the same health problems as an obese person?
Yes! And there is actually a term for people who are at a healthy weight, but are actually obese/overweight in terms of their body fat. It’s called being “skinny fat”. These people look like they are at a normal and healthy weight, but they have a high body fat percentage. Thin on the outside, fat on the inside (TOFI) is another term used for this situation.
TOFI people have a healthy BMI, they are at a higher risk of diseases such as heart disease, diabetes, etc . The problem is most of these people don’t even know that they have a problem because their doctors would likely just screen their BMI. Dr. Mark Hyman believes that being skinny fat or TOFI is actually worse than being overweight or obese. At least overweight and obese people are aware that they have an issue and they have a chance to work on it, but skinny fat individuals have no idea that they have a health problem .
The best way to figure out if you are TOFI or not is to have your body fat properly assessed, using better tools than the BMI.
What measurements should we use instead?
Get your body fat percentage tested professionally! That is the only way you’ll know for sure if your body fat percentage is normal or not. There are a few ways you can test your body fat % and each method has some pros and cons (which I won’t get into), but any of the following methods is a reasonable way to figure out how “fat” you are.
- DEXA scan – this is by far one of the most accurate methods. It costs about $100 dollars. It will give you a great assessment your body fat percentage and fat distribution. The fat distribution is important since it will tell you if your fat is the dangerous visceral fat around your organs that can lead to diabetes and heart disease.
- Hydrostatic weighing – this was the gold standard before the DEXA scan came along. Your body is weighed underwater for this test. This test is good for determining your body fat %, but can’t help you figure out your body fat distribution.
There are other methods of calculating body fat % such as the caliper method and bioelectrical impedance, but they can be wildly inaccurate, so don’t rely on them completely. There are also calculator tools used by the US Navy and YMCA, but I don’t know how accurate they are, so I can’t recommend them. Do your research and find out what works for you and your budget.
Note: If you’re on a budget, or can’t find a place to get a DEXA scan or hydrostatic weighing session, then you can use your waist measurement as an indicator of how much fat you’re storing around your internal organs. Your waist circumference is the distance around your natural waist (just above the navel). According to the American Heart Association, if your BMI is above 25, your goal for waist circumference should be less than 40 inches if you’re a man and less than 35 inches if you’re a woman .
Go on and get measured!
At the end of the day, your BMI is just a number. It is a tool that is a very, very rough gauge of your overall health, but doesn’t give you the full picture. You can have a healthy BMI and still have some of the health problems that obese people experience. Get your body fat percentage tested properly and use that as a measure for obesity.
What do you think? Scroll below and leave us a comment!