
The TRUTH about BMI (Body Mass Index)

Welcome to the Fitness Adaptive live podcast! Today’s topic is BMI or body mass index. BMI is a common body composition measurement that is misunderstood and usually inaccurate. I came across BMI when I first started studying fitness. I started reading about it and thought, “This isn’t a very accurate form of measuring body composition.”
What is BMI?
BMI is measured by taking your total body weight, and dividing it by your body height. In the metric scale, take your weight in kilograms and divide it by your height in meters squared. In the imperial system, take your weight in pounds, multiply it by 703, and divide it by your height in inches squared. These equations take your total weight divided by your height. The resulting number is classified in a weight status, to determine if you are maintaining a healthy weight. (See the chart below for BMI classifications for weight.) Generally, these BMI equations are completely inaccurate for determining obesity levels.
Why is BMI inaccurate?
When someone is overweight or obese, it simply means that they have excess body fat. (Refer to the body fat percentage chart below for body fat classifications.) When someone wants to lose weight, they typically want to burn off body fat. BMI takes your total weight divided by your total height to determine obesity level. The problem is that BMI only takes into account your total body weight. It doesn’t take into account how much of that weight comes from body fat and how much of that weight comes from lean muscle mass.
Muscle weighs more than fat, so using your weight and height to determine if your weight is healthy can give flawed results. If you are someone with a lot of muscle mass, your BMI may be higher, implying an unhealthy, high weight. If you are someone with very little muscle mass, your BMI may be lower, while your body fat percentage is actually high.
Here are a couple examples.
Client A: 20 year old male, 6’3, 140lbs, 25% body fat
Client B: 30 year old male, 5’8, 200lbs, 8% body fat.
If we calculate BMI for these clients we get different story:
Client A: 140/752x703=140/5625×703 = 17.49
Client B: 200/682x703=200/4624×703 = 30.4
On the BMI scale, client A is underweight with a 17.49 BMI, however, on the body fat percent scale, 25% body fat is classified as obese, so he is obese on the body fat scale, but underweight on the BMI scale. On the BMI scale, client B is obese with a 30.4 BMI, however, on the body fat percent scale, 8% is classified as very lean, so he is obese on the BMI scale, but very lean on the body fat percent scale. As you can see from these two examples, BMI is not a reliable form of measurement.
The fact is, whether you are overweight or underweight, is determined by your body fat percentage, not your total weight. When people want to lose weight, they typically want to lose body fat. When people want to gain weight, they typically want to put on muscle. BMI is not a method of determining whether you are overweight or underweight because obesity is classified by body fat percentage, not weight. BMI doesn’t take into account how much of your weight is muscle mass, and how much of your weight is body fat.
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BMI Classification BMI(kg/m2)(lb/in2 x703) Obesity Class
Underweight |
<18.5 |
|
Normal |
18.5-24.9 |
|
Overweight |
25.0-29.9 |
|
Obese |
30.0-34.9 |
1 |
35.0-39.9 |
2 |
|
Extreme Obesity |
>40.0 |
3 |
Body Fat Percent Scale(Male)
Age
Very Lean |
<5(not recommended) |
3-7 |
4-10 |
5-13 |
8-16 |
11-17 |
12-18 |
Lean(Low) |
5-10 |
8-10 |
11-13 |
15-17 |
17-19 |
19-21 |
19-20 |
Leaner Than Average |
– |
11-12 |
14-16 |
18-20 |
20-22 |
22-23 |
21-22 |
Average(Mid) |
11-25 |
13-15 |
17-19 |
21-22 |
23-24 |
24-25 |
23-24 |
Fatter Than Average |
– |
16-18 |
20-22 |
23-25 |
25-27 |
26-27 |
25-26 |
Fat(Upper) |
26-31 |
19-21 |
23-26 |
26-28 |
28-30 |
28-29 |
27-29 |
Obese |
>31 |
23-35 |
27-38 |
29-39 |
31-40 |
31-40 |
30-39 |
Male Rating 6-17 18-25 26-35 36-45 46-55 56-65 66+
Body Fat Percent Scale(Female)
Age
Female Rating 6-17 18-25 26-35 36-45 46-55 56-65 66+
Very Lean |
<12(not recommended) |
9-17 |
7-16 |
9-18 |
12-21 |
12-22 |
11-20 |
Lean(Low) |
12-15 |
18-19 |
18-20 |
19-22 |
23-25 |
24-26 |
22-25 |
Leaner Than Average |
– |
20-21 |
21-22 |
23-25 |
26-28 |
27-29 |
26-28 |
Average(Mid) |
16-30 |
22-23 |
23-25 |
26-28 |
29-30 |
30-32 |
29-31 |
Fatter Than Average |
– |
24-26 |
26-28 |
29-31 |
31-33 |
33-35 |
32-34 |
Fat(Upper) |
31-36 |
27-30 |
29-32 |
32-35 |
34-37 |
36-38 |
35-37 |
Obese |
>36 |
32-43 |
34-46 |
37-47 |
39-50 |
39-49 |
38-45 |
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