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February 22, 2010, 12:00AM PT in Exam Preparation Blog  |  1 Comments

Desired Body Weight: How to Get There From Here

***8/26/14: Please refer to Math as a Fitness Professional and Nutrition Math for more up-to-date information on this topic.


On a scale

“I just want to lose a little weight.”

“Well, actually, I want to get down to 23% body fat.”

“ But how much will I weigh at 23% fat? Can I get there by the end of the month?”

Personal trainers may be familiar with this sequence of statements. Losing weight is one of the leading reasons why clients head for the gym and the wisdom of their trainer. In the ACE 4th edition Personal Trainer manual you can find information about the different ways to assess a client’s body composition as well as what it takes to lose those stubborn pounds.

In preparing for your exam, one number you want to be familiar with is 3500. That is, 1 pound of fat is equal to 3500 calories. Also, a safe rate of weight loss is 1 – 2 lbs per week.  That’s it, regardless of what Biggest Loser says. As we talk about maintaining or adjusting a client’s body weight, those are two statements you want to memorize and tuck back into your brain for future reference.

Despite all the ads, bottles, pills, and formulas, weight loss can be defined by a simple statement: Calories in (the food you eat) < Calories out (what you burn through activity and metabolism) = weight loss

Yep, it really is that simple. Another equation to tuck back into your brain, and one we’ll discuss in a future blog post.

When you have a scenario where a client wishes to lose weight, how you answer the question depends on the information provided.  For example…

Sarah Jayne weighs 182. She wants to lose 20 lbs in time for her class reunion in 2 months. Can she do this safely?

If she loses 2lbs per week, then in 8 weeks she will lose 16lbs. Nope, not enough weight to meet her goal.

If she loses 3lbs per week, then in 8 weeks she will lose 24lbs. Yep, she can meet her goal.

But…what if Sarah Jayne comes to you with a different goal? She’s heard that 20% body fat is the place to be for women of her age. If she’s 28% body fat now, how much weight does she have to lose to get down to 20%? How long will it take?

Here is where we use the Desired Body Weight equation of

 DBW = LBW / (1 – DBF%)  where LBW is Lean Body Weight and DBF is Desired Percent Fat

For Sarah Jayne you need to perform several steps:

1: 100% - 28% fat = 72%lean (this is Sarah Jayne’s percent lean weight)

2: 72% lean x 182 lbs of body weight) = 131.04lbs of lean body weight

3: Desired Body Weight = 131lbs lean body weight/ (1-20%)

3a: DBW = 131lbs lean body weight/ .8 (this .8 is the desired lean she wants to be, 80% lean and 20% fat)

4: DBW = 163.75lbs (this is what Sarah Jayne would weigh at 20% body fat)

So part 2 of the question says, how much weight does she need to lose to get to this point of 164 lbs and 20% body fat?

We take her original weight of 182lbs and subtract her final weight of 164 lbs.

182lbs – 164lbs = 18lbs

How long will it take her to lose these 18 lbs safely? 6 – 9 weeks.

When taking the exam, and especially when doing math on the exam, remember that some questions are easy while others are multi part. When you come across one of those multi part questions just take a deep breath and break it down into its parts. Then tackle each part in order to get the best answer.

Questions? Just contact one of our Education Consultants at 1-888-825-3636 x782.

By April Merritt
April Merritt holds a bachelor’s degree in Exercise Science, a master’s degree in Health Promotion, and several ACE certifications including Personal Trainer, Group Fitness Instructor and Health Coach.

April Merritt holds a bachelor’s degree in Exercise Science, a master’s degree in Health Promotion, and several ACE certifications including Personal Trainer and Health Coach.

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