Calculate Your Basic Metabolic Rate Based on Lean Body Mass

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Your body is constantly using energy to perform and function on a daily basis, even while you’re sleeping. Your resting metabolic rate (also called RMR) is the rate that your body burns energy when you’re at complete rest.

Your RMR is part of your total daily energy expenditure (TDEE) or the total number of calories you burn each day. If you are looking to drop body fat, for example, knowing your RMR is going to be essential to finding out how many calories you should be eating on a daily basis. Knowing how much you should eat to simply maintain weight is critical in determining how many calories you need to eat in order to drop body fat.

Cool, now that we understand that, we need to keep in mind that every single person on this planet has a varying resting metabolic rate. Your RMR is an estimate of how many calories you'd burn if you were to do absolutely nothing but rest for 24 hours

Key factors that influence your Resting Metabolic Rate:

Calories burned via activity/exercise.
Digestion (thermic effect of food).
Calories burned via non-exercise activity.

Now that we understand what RMR is and the variables that influence it everyday, it’s time to calculate your lean body mass. Get your calipers ready!

HOW TO CALCULATE YOU YOUR RMR

The first thing to do is take your overall body weight and subtract your body fat from that number. To obtain your body fat, I recommend using calipers as shown in this video here.

WHY DO WE NEED TO MEASURE BODY FAT?

It’s important to understand that muscle requires more energy to sustain than body fat which means two people who weigh the same via scale weight may have two very different RMR’s, depending on their body fat percentages. The scale does not pick up specific body fat which means a person with more muscle will have a higher RMR compared to someone who weighs the same but has more body fat. Reason for this is simply because the more muscle a person has, the more energy they burn while at rest.

HOW TO CALCULATE YOUR LEAN BODY MASS

Now that you know your estimated body fat percentage, let’s take that number and figure out your LBM (lean body mass).

Now, let’s say your 140 lbs and have a body fat of 28%. To figure out your lean body mass, (LBM), calculate

(1 - .YOUR BF PERCENTAGE) x YOUR WEIGHT = LBM

Which would look like: 1 -.28 x 140 = 100

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Let’s say hypothetically these are your numbers, This would mean your LBM is 100 lbs. Now we can plug this number into something called the Katch Mcardle formula. The Katch-McArdle formula is a tool that allows you to calculate your basic metabolic rate (BMR), while taking your lean body mass into account .

THE KATCH MCARDLE FORMULA IS AS FOLLOWS:

BMR= 370 + (9.82 X LEAN BODY MASS)

That would look like this: 370 + (9.82 x 100) = 1352

Now, depending on your activity level, the final number to multiply your BMR with varies depending on how active you are. The variation can be anywhere from 1.25-1.9.

Sedentary = BMR X 1.2 (little or no exercise, desk job)
Lightly Active = BMR X 1.375 (light exercise/sports 1-3 days/wk)
Moderately Active = BMR X 1.55 (moderate exercise/sports 3-5 days/wk)
Very Active = BMR X 1.725 (hard exercise/sports 6-7 days/wk)
Extremely Active = BMR X 1.9 (hard daily exercise/sports & physical job or 2 X day training, full time training, etc.)

For most of my clients who have office jobs and are stay at home moms, I use 1.3-1.5 but consider your own daily energy expenditure.

Let’s use 1.3 for hypothetical reasons. Here’s what that would look like:

1352 x 1.3 = 1757 is the number of calories you need just to maintain your weight on a daily basis.

From here, you would need to subtract anywhere from 300-400 calories from your RMR depending on how much body fat you wanted to lose and how quickly you wanted to do so. I recommend starting out with a smaller deficit and staying there for a couple weeks before you drop yourself lower. Start with a smaller deficit to make your life easier, then adjust as you go.