How much should you weigh?
What is a realistic goal for weight loss?
The first question gets complicated (and was discussed in a previous article). We will look at answers for the second question in this article.
This is the second of three articles dealing with body weight, body composition, and weight loss.
The first article discussed “body composition” concepts and different methods to estimate body fat percentage. This article assumes that you understand the difference between “percentage body fat” and “lean weight.” An estimate of body fat percentage is used as the starting point.
HOW MUCH TO LOSE?
The nutritional experts suggest setting you initial target for weight loss at 5 to 10 percent of your current weight (1).
Even if you want to lose a larger percentage of your current weight, you should think in smaller steps. This should be “common sense” – but you need to lose 5% before you lose 10%, and you will lose 10% before you lose 20%. Set attainable goals and then reap the psychological high from attaining them and use it to keep going until you are at your “healthy” weight.
Setting smaller goals also ensures that you are losing the “fat” weight you want to lose and not just dehydrating yourself or consuming lean muscle (both are bad and are a major reason that most people end up worse off after going on the latest fad diet).
THE MATH
In order to make all of the 5th grade math teachers out there feel appreciated – I’ll point out that the math is straightforward and can easily be done by hand or with a calculator.
For the rest of us who are a little “math lazy,” I’ll also give you the spreadsheet formulas and make an Excel/Open Office download available (http://www.fulleffort.com/)
EQUATIONS
First we need to find out how much body weight is fat:
#1. fat weight = (fat % /100) x body weight
Then we can find lean weight:
#2. lean weight = body weight – fat weight
Finally we calculate desired weight:
#3. desired weight = lean weight / [1 – (desired body fat % / 100)]
EXAMPLES
A couple of completely made up examples:
1. A 300-pound man with 40% body fat wants to get down to 35% body fat.
Step 1 – determine fat weight
Fat weight = (40 / 100) x 300
Fat weight = 0.40 x 300
Fat weight = 120 pounds
Step 2 – find lean weight
Lean weight = 300 – 120
Lean weight = 180 pounds
Step 3 – calculate desired weight
Desired weight = 180 / [1 – (35 / 100)]
Desired weight = 180 / [1 – 0.35]
Desired weight = 180 / 0.65
Desired weight = 277 pounds
Losing 23 pounds would greatly improve our hypothetical man’s health. This weight loss would amount to almost 8% of his current weight and is not only realistic but should be attainable.
2. A 150 pound woman with 35% body fat want to get down to 30% body fat.
Step 1 – determine fat weight
Fat weight = (35 / 100) x 150
Fat weight = 0.35 x 150
Fat weight = 52.5 pounds
Step 2 – find lean weight
Lean weight = 150 – 52.5
Lean weight = 97.5 pounds
Step 3 – calculate desired weight
Desired weight = 97.5 / [1 – (30 / 100)]
Desired weight = 97.5 / [1 – 0.30]
Desired weight = 97.5 / 0.70
Desired weight = 140 pounds
This 10-pound weight loss would amount to almost 7% of her current weight and would be a great accomplishment.
SPREADSHEET FORMULAS
This section amounts to a basic introduction to spreadsheet formula creation. If you’re comfortable with spreadsheets you can skip this…
I like to start my spreadsheets a column over and a few rows down – feel free to use whatever cell you want (and obviously you’ll have to change the cell references).
Enter the following text in B6 through B10:
Body Fat %
Body Weight
Fat Weight
Lean Weight
Desired Weight
In D6 put Desired Body Fat %
In C8 enter the following (the stuff between the quotation marks) “=(C6/100)*C7”
The “=” sign tells the spreadsheet that we are entering a formula. Cell C6 will contain the current body fat % and C7 will contain the current body weight – therefore cell C8 performs our “Step 1” calculation.
In C9 enter “=C7 – C8” – just the stuff between the quotation marks (this is our Step 2 calculation)
In cell C10 enter “=C9 / (1-(E6/100))” – Cell E6 contains the desired body fat %, this performs the Step 3 calculation (and you did remember to just enter the stuff between the quotation marks, right?)
Now you can play with the numbers. Enter data into Cells C6, C7, and E6 and the spreadsheet will automatically calculate the “formula” cells.
CONCLUSION
That is all there is to calculating desired body weight using body fat percentage.
The third article in this series will offer practical suggestions on how to safely and effectively lose weight.
REFERENCE:
1. Clark, Nancy, Nancy Clark’s Sports Nutrition Guidebook, 2003, Human Kinetics, 195
