The TRUTH about BMI (Body Mass Index)

by | Nov 15, 2021 | Physical Training, Tips & Tricks

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. 

Start your fitness journey!

If you are interested in losing weight, looking better, feeling better, and living healthier, check out Fitness Adaptive. I have a weight loss course that specifically goes over how to lose weight, how to track your progress, weight loss myths, and how to calculate nutritional needs for weight loss. If you have any questions or if you’re ready to take your abilities to the next level, contact me. Test my methods and prove its ability to produce results. Check out my website for information on courses, training and more fitness tips. Contact me and let’s go over your goals. This is my invitation to you, to take the next step and get the results that YOU WANT!

 

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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