Lean Body Mass

"It's simple.  if it jiggles, it's fat" ~ Arnold Schwarzenegger

I've used this quote in the past, but it's relevant when discussing Lean Body Mass or LBM as I'll refer to it from here on in.  The below post is long winded, but I promise it's chalked full of useful information.

What is LBM?

"That part of the body including all its components except neutral storage lipid; in essence, the fat-free mass of the body." ~ Dorland's Medical Dictionary for Health Consumers

Or more simply put, LBM is everything else in your body other than fat. Skin, blood, bones, muscle, organs, these are all things that are part of your LBM.

Why is knowing your LBM important?

1. Knowing your current LBM can help determine your goal weight, and in my opinion it's far more accurate than using the Body Mass Index (BMI), which I loath and discussed numerous times, specifically in a past post "BMI can BMA" (Bite My Ass). 
2. Knowing your LBM and keeping track of it while your losing weight can help ensure your weight loss is from unwanted fat and not from muscle.

Both of these points are discussed in more detail later on.

How do you calculate your LBM?

It's actually not very hard, but you do need to know your body fat % first.  As stated in the past, I use a Tanita scale and have found it to be very accurate and reliable.   To calculate your LBM use the following formula:

LBM = Your Body Weight – (Your Body Weight x Your Current Body Fat Percentage)

Here's how my current LBM was calculated:
LBM = 233.4 - (233.4 x 33.8%)
LBM = 154.5 lbs.

So if it was possible to strip every ounce of fat of my body right now, I would still weigh 154.5 lbs.

Determining your goal weight:

To do this you first need to know what a healthy body fat range is and then take into account how trim you want to be, or more importantly how hard you want/need to work to maintain that percentage of body fat.

This chart found here is based on data collected from a study called "Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index" conducted by The American Journal of Clinical Nutrition (Am J Clin Nutr 2000;72:694–701).

According to this chart the "Acceptable Range" is between 10 - 20%.

Obviously the lower your body fat, the more work you need to put in to maintain it.  So with my LBM of 154.5 and my goal of 15% body fat the following formula is used to determine my goal weight:

Goal Weight = Current LBM / (1 - Goal Body Fat %) 

Here's what my goal weight works out to:
Goal Weight = 154.5 / (1 - 15%)
Goal Weight = 154.5 / .85
Goal Weight = 181.76 lbs.
Goal Weight = 182 lbs. 

Remember my mentioned disdain of BMI,  well according to the World Health Organizations (WHO) BMI chart found here on Wikipedia, someone of my height (5' 10") would need to weigh between between 130 - 170 lbs to be in the "Normal Range".  Based on my current LBM, 170 lbs would put me just under 5% body fat, which is a typical fat percentage of a fitness cover model... and that's just to get into the very edge of "normal".

The main reason the BMI fails to be accurate is because it never takes body fat into consideration which is even stated in the conclusion of the above listed study:

The developed provisional equations and tables in this report can fill an information gap because no comparable percentage body fat ranges exist for review for evaluation of potentially misclassified subjects referred for body-composition analysis.

Finally the scientific community agrees with something I've been saying since 2006.

Ensuring weight loss is from unwanted fat and not muscle:

Weight loss is great, but only when it's in the form of fat.  Our muscles keep us strong and they burn more calories than fat.  Changes in your LBM indicates one of the following 3 things:

1. LBM increases = muscle gain
2. LBM stays the same = no muscle loss or gain
3. LBM decreases = loss of muscle

Basically if you lose weight and your LBM is the same or higher it's safe to assume that your weight loss was from fat without sacrificing muscle. 

Where does water fall into the equation between LBM and Fat?

This was something I found myself asking as well. Initially I assumed that water would be part of the LBM, but after some digging I realized my error... it belongs to both, but primarily LBM.

Up to 60% of the human body is water, the brain is composed of 70% water, and the lungs are nearly 90% water. Lean muscle tissue contains about 75% water by weight, as is the brain; body fat contains 10% water and bone has 22% water. About 83% of our blood is water. - USGS 

With fat only containing 10% water, the remainder of the water is spread throughout the body which comprises all of our LBM.  It's safe to say that if your hydration level is off during your weigh-in that your LBM number will effected more than your body fat percentage will.   Lets face it our hydration level can change significantly from week to week (especially for women), and will effect  both weight and body fat readings, which are both used in our LBM calculations.

One thing to note is that muscle it not something we lose or gain quickly.  It's pretty safe to assume that if your LBM is varying slightly week to week it's more often than not due to variations in hydration, where as downward or upward trends of your LBM over a longer period of time probably indicate that the loss or gain is due to muscle.  

Additional Reading:

I happened to stumble upon a great article here from www.builtlean.com that helped me get a better sense of body fat percentages and what different percentages would look like on my body.  Hope you find it as helpful as I did.