Monday, July 30, 2012

How Much Weight Do You Need To Lose, Really?

Most inexperienced “dieters” drastically underestimate how much weight they have to lose to look “lean”.  As a quick example, let’s say you’re a big guy weighing in at 250 lbs.  You look in the mirror and guess that you need to lose 25 lbs or so in order to see your abs.

If you’re a little more meticulous and can do some basic math you might estimate that you’re about 20% body fat now, and you want to go down to 10% body fat.  If you weigh 250 lbs at 20% body fat, then that means you have about 200 lbs of lean body mass.  If you kept all that lean body mass as you dieted down, losing only pure fat in the process, then at 225 lbs you’d be 11.1% body fat.  Under these ideal conditions, you’d actually have to diet down to 222.2 lbs to be at 10% body fat (again, assuming you held on to the full 200 lbs of lean body mass that you have now) but still, seemingly not a bad guess.  The math is as follows:

Current LBM = Current Weight x (1 – Body Fat Percentage/100)

Final Weight = Current LBM / (1 – Desired Body Fat Percentage/100)

The problem is, almost nobody can hold on to all their lean body mass (LBM) as they lose weight.  No matter how good your training and nutrition is, as you lose weight you’re going to lose LBM too.  In fact, the number that shows up time and time again with natural bodybuilders dieting down from the 15-18% to the 5-9% body fat range is that for every three pounds of fat they lose, they lose about one pound of LBM with it.  In other words, their weight loss will be comprised of about 75% fat and 25% lean body mass (a 3:1 ratio of fat to LBM loss).  Some very gifted bodybuilders may do a little better than that, but a 25% LBM loss is a pretty good rule of thumb.

People under different circumstances, though, may lose more or less LBM than that.  For instance, a person checking in at 25% body fat and dieting down to 15% body fat may lose only about 15% LBM and 85% fat.  Generally, the more fat you're carrying the greater percentage of fat you'll lose as you lose weight.  Likewise, the leaner you are the more LBM you're going to lose as you diet.  A competitive bodybuilder might lose up to 50% LBM in his last bid to get down to extremely lean levels in the 4-5% body fat range.  Rank beginners might actually be able to gain a little muscle as they drop body fat... as long as their diets aren't too severe, last too long or they're not super-lean to start.  Past the beginners stages though, that's highly unlikely to happen.  Generally speaking, the bigger your calorie deficit is, the faster you lose weight, and the more total weight you lose, the more lean body mass you'll lose.

Notice that I keep referring to LBM and not just muscle.  Why is that?  The simple fact is that lean body mass is composed of much more than just skeletal muscle.  Lean body mass means your skeleton, skin, organs, contents of your stomach, glycogen in the muscles and yes, your skeletal muscles.  Even if you did manage to hold on to 100% of your skeletal muscle mass as you lost weight you’d still lose lean body mass because of these other factors… which is why even the best natural bodybuilder will lose LBM as he/she diets down.

So let's say you're about 18% body fat now, have some experience lifting weights, and would like to get down to the 10% body fat range.  How do you more accurately estimate how much weight you need to lose in order to reach a certain percentage of body fat given the nearly inevitable loss of LBM?  The equation is as follows.

Final Weight = (Current LBM – 0.25 x Current Body Weight) / ( [1 - Desired %bf/100] – 0.25 )

This is assuming the 3:1 fat-to-LBM ratio (25% LBM loss) that most likely applies if you aren't dieting and training too severely and have decent genetics for getting lean.  As an example of the equation in use, let’s say we have a bodybuilder who weighs 190 lbs at 15% body fat.  He wants to reach 10% body fat so he’ll look lean, defined and get some abs.  As his current lean body mass is 161.5 lbs he thinks he’ll have to diet down to 179.4 lbs to be 10% body fat.  That is, using the mistaken assumption that he won’t lose any LBM as he loses weight…

Current LBM = 190 x (1 - 15/100) = 161.5 lbs

Final Weight = 161.5 / (1 - 10/100) = 179.4 lbs

Unfortunately, that’s extremely unlikely to be the case.  In reality, his final weight will need to be more like

Final Weight = (161.5 – 0.25 x 190) / ( [1 - 10/100] – 0.25 ) = 175.4 lbs

That’s four pounds lighter than he thought he would have to be.  Furthermore, if you add in the effects of fluid and stomach contents loss when dieting he’d probably have to go down to a morning weight (i.e. empty stomach) of closer to 170 lbs to be a “true” 175 lbs when he starts eating more normally again (at which point his weight will quickly shoot up a few pounds due to fluid and glycogen replenishment).

If you're above the 18-20% body fat mark and only intend on dropping down to about 15% body fat, then you can optimistically try substituting 0.15 for the 0.25 factors in the above equation.  The equation becomes,

Final Weight = (Current LBM – 0.15 x Current Body Weight) / ( [1 - Desired %bf/100] – 0.15 )

But bear in mind that this more optimistic rate of LBM loss applies to people starting off fatter than a typical off-season natural bodybuilder and not dieting down to very lean levels or dropping weight too quickly (say roughly 1 pound a week or less).  So don’t think that as an experienced trainee you can go all the way from 15% down to 6% body fat and lose only 15% LBM in the process.  Unless you are very gifted and/or have reliable past experience or other special circumstances to indicate otherwise (i.e. coming back after a training layoff or start taking steroids), that isn't likely - stick to the 3:1 "rule".

Conversely, many less gifted bodybuilders/athletes will lose more than 25% lean body mass when dieting.  This may be due to a multitude of factors from over-dieting to overtraining to genetics, but 36% is an average number based on less gifted "clients" I've monitored over the years... you might even say that this 36% rate is more typical of the average person than the 25% rate that shows up often in competitive bodybuilders.  I myself tend to be in the 25% to 36% range depending on how quickly I lose weight.

If a person crash dieted and did no weight training or exercise whatsoever during the diet, up to 50% of the weight they lose may be lean body mass.  This has been observed repeatedly in research environments.

The take home here is that, no matter how hard you may try, you're going to lose some lean body mass during your diet - unless you are a rank beginner to weight training and just losing a few pounds through a mild diet and exercise program.  Similarly, if you're returning to intensive weight training after a layoff you may be able to regain some lean body mass that you previously had (i.e. "muscle memory") as you lose some fat, but that situation won't last long and the diet would have to be mild.  Otherwise, you're going to lose somewhere in the vicinity of 15% to 50% lean body mass as you lose weight, depending on how much weight you lose and how fast you lose it (i.e. how large your calorie deficit is).

An online calculator that does these calculations is here:  Final Body Weight Calculator