Body Composition Analysis and Predictions

I am restarting a prior discussion (See [Percent Composition of Weight Loss: Fat vs. Muscle vs. Other Lost]), having improved my analysis of body composition.

Background: Back in June 2018 I was asking for help identifying “other” weight loss. The problem was that, given an impedance scale reporting weight, %fat, %muscle, and %water, where %fat + %muscle + %water > 100%, I chose to ignore the scale’s %water and instead used the formula weight = fat + muscle + other, where other included skin, bones, organs, etc. But, since my calculations showed only about 2/3 of my weight loss was fat, and about 1/3 was “other,” then clearly a loss of “other” couldn’t actually be a combination of loss of skin, bones, organs, etc.

Fast forward to today: Having regained weight back up to 174.8, I restarted my diet and exercise on Jan 1, and today Jan 25 I have gotten back down to 157.8. Now I’m redoing the prior analysis previously discussed in the above link. This time, instead of ignoring the problem that calculated fat + muscle + water >> weight, I focused on the before vs. after delta results of the calculations for fat, muscle, and water pounds.

With some surprise that I had missed such an easy resolution, it turned out that calculated water loss is almost exactly equal to calculated “other” loss; i.e., the difference is well within any expected rounding errors given the limitations of the impedance scale technology. This might be a coincidence, but is more likely to be a calibration / validation constraint within the scale firmware so that any total of fat, muscle, and water gain/loss will be equal to weight gain/loss within the rounding error.

So, with apologies for single decimal rounding errors, here is a snapshot of the revised spreadsheet calculations for current data:

While actual numbers still depend on the accuracy of the impedance scale technology (currently a Weight Gurus 0375) that only uses foot pads, I expect there would only be a few percentage points difference if I invested in an upgrade to a scale that uses feet and hands together like some of the Omron and InBody models.

The bigger picture going forward: Now having an analysis I can trust, regardless of the exact numbers, I have a methodology for predicting future progress, answering my favorite two questions regarding my future weight target (134.0 pounds for a 21 BMI):

Q. "Given that my %fat is pretty poor at 22.2% today, but getting better since Jan 1, what %fat can I hope to get down to at my target weight (assuming no change in my diet or exercise program)?”
A. %fat at 134.0 = (35.0 – 63.8% * (157.8 - 134.0)) / 134.0 = 14.8%


Q. “Given that my %muscle is pretty poor at 28% today, but getting better since Jan 1, what %muscle can I hope to get up to at my target weight (assuming no change in my diet or exercise program)?”
A. %muscle at 134.0 = (44.2 - 5.3% * (157.8 - 134.0)) / 134.0 = 32.0%

Both of those results make a lot of sense in the context of extended research on healthy target fat and muscle, so I am feeling quite good about these estimates.

And, if I put substantially more effort into avoiding the 5.3% muscle loss per pound of weight loss – for example gaining 5.3% instead of losing 5.3% – my %muscle might get up to a better number like 34%, and my %fat might get down to a better number like 13%.


1 Like

So, three days later, I simplified most of the analysis by just calculating fat pounds, muscle pounds, and waist inches lost per weight pound lost. From there I could forecast realistic fat%, muscle%, and waist inches at my next goal and my target long term goal. This resulted in a big improvement to the Fitness Dashboard I’ve been using for many years:

P.S. Of course there is a lot of behind-the-scenes logic going on in the above image, which is a spreadsheet I designed many years ago and have been tweaking ever since. On the left is a logarithmic scale from my starting weight 174.8 on the current diet and exercise plan started this Jan 1 (but which scales correctly regardless of starting weight). From there, the values on left and right are conditional formatting color-coded, with the right side projecting forward based on Long Target Rate down to the Long Target BMI.

Do you have any seasonal fluctuations in body weight and composition that might interfere with your linear extrapolations?

Rethinking my prior reply, I have completely replaced it here:

Historically (comparing calculations from 2018 to now) changes in percentage of weight pounds loss that is fat loss vs muscle loss vs water loss has been statistically insignificant, but only because over the years there has been little change in my diet and exercise choices to achieve weight loss from season to season; i.e. counting calories and walking several miles daily. So, if there is a seasonal effect I am not aware of it.

However, keeping in mind that reversing muscle loss might be possible with heavy use of my Total Gym for strength training, if I ever began to succeed in that effort, daily recalculation of the predicted fat% and muscle% at my target BMI would slowly shift in that favorable direction.

It won’t be much of a shift because I lost less than 1 pound muscle with 16 pounds weight loss. If I regain 2 pounds muscle (losing the same 2 pounds fat), the difference is only 1.5% less fat and 1.5% more muscle at 134 pounds weight.

1 Like