Is BMI the same as BMR?

Are BMI and BMR
interchangeable terms?

In a recent conversation with an M.D., when giving an opinion he incorrectly used the term BMI instead of BMR. I interjected “BMR?”, and he frowned & shook his head sideways indicating “no”. 

I continued with “BMR, not BMI,.. you know .. Basil Metabolic Rate, … from formulas like Harris-Benedict, or Mifflin? The calories burned without activity, like when you are in a coma.”  He responded on a different track, saying people used a lot of energy even in a coma, and never made any correction, or acknowledgment that he even understood his error.

So I thought, Is there some basic information about BMI & BMR that I missed? Are they, in some way, interchangeably connected?  Further research verified my original understanding, which is spelled-out below.

BMI or BMR?

First, although the abbreviations BMI and BMR sound similar, they stand for two separate things.

Your BMI, or body mass index, is a number calculated from your height and weight which is then used to assess your body composition.

Your BMR, or basal metabolic rate, is the number of calories you burn when your body is at rest. 

These two terms are independent of each other, but your BMI may indirectly affect your BMR.

Body Mass Index (BMI)

Your BMI is an indirect measure of your body composition -- or how much body fat you have. Although BMI doesn’t measure body fat directly, it uses your weight and height to determine whether you’re classified as underweight, normal weight, overweight or obese. This measurement correlates moderately well with other measurements of body fat such as skinfold measurements and underwater weighing.

BMI is based on your height & your weight.   There are many online calculators that will easily run BMI equations and provide your individual numbers.   Here’s a link to one that I commonly use.

The equation looks like this: BMI = (weight / height x height) x 703.

A BMI below 18.5 indicates that you’re underweight; a BMI between 18.5 and 24.9 classifies you as a normal weight; a BMI between 25 and 29.9 puts you in the overweight category; a BMI of 30 or above classifies you as obese.

Limitations of BMI

Because it's not a direct measurement, BMI is only used as a screening tool and is not considered a diagnostic test. There are also some limitations to this measurement.

Because BMI uses only height and weight, it doesn’t account for people who may be of below-average height but above-average muscle mass, like bodybuilders. If a man has a lot of muscle, which is denser than fat, his BMI may categorize him as overweight, when his weight is actually healthy.

BMI also doesn’t adjust for age or gender. Women naturally have more body fat than men, and older people tend to have more body fat than younger people. So a sedentary older woman with a normal-to-overweght BMI, who has low muscle mass might be mistakenly considered to be "overly" fat.

Basal Metabolic Rate

Your BMR is the number of calories your body burns at rest to do basic functions like breathing, digesting, keeping your heart beating and all the other physiological tasks that keep you alive.

Your BMR is partly determined by genetics, but other factors, like your body composition and activity level, may also have an effect on your BMR.

There are equations used to determine your BMR, and they differ based on whether you're a man or a woman.  The two most widely formulas to determine BMR are the newer and more accurate Mifflin St JEOR equation, and the old pre-1920s Harris-Benedict equation. The formulas use height, weight, and age.   

While these equations can be a good starting place, it is important to understand that these numbers are based on AVERAGES, and it is common for someone’s ACTUAL personal number to be 15% lower (or higher). There are also Outliers, who have individual numbers which can be VERY far away from the Average stated number.  There are many online calculators that will easily run these equations and provide your individual Number Average. Here’s a link to one I commonly use.

The Mifflin equation (1990) looks like this: 10 × (weight in kg) + (6.25 × height in cm) - (5 × age in years) - 161.  The Harris-Benedict equation (1918) looks like this:  655 + (4.35 x weight in pounds) + (4.7 x height in inches) – (4.7 x age in years).

How BMI and BMR are Connected

Several factors affect your basal metabolic rate -- and body fat composition is one of them. Those with more muscle mass tend to burn more calories at rest because muscle tissue requires more calories to maintain than fat tissue.

BMI and BMR are not directly related. However, if you have a high BMI because you have a high body fat percentage, your BMR might be lower than the number given. If you have a high BMI because you have a large amount of muscle mass, your BMR might be higher than the number given. Additionally, if you are very active, but overweight, your BMR might be higher than the number given.

BMI and BMR are Guidelines that allow nutrition and medical professionals to make educated determinations about your body composition and calorie burn, but every person should still be viewed individually.