Body Fat Testing and Weight Loss Targets

by Jimmy Akin

in Diet

body fat testing

One of the most common measures of how much weight someone needs to lose is based on what’s known as their Body Mass Index (BMI).

The trouble is, BMIs were not designed for this function, and they have severe limitations in diagnosing overweight and obesity. That’s really not what they’re for.

In particular, they don’t differentiate between lean body mass and fat. People with lots of muscle and bone can thus be told they’re overweight even though they’re not carrying much fat.

If you want to determine how much fat you’re carrying, you need to use special tests–the gold standard of which is being dunked in a tank of water and seeing how much of it you displace, allowing your overall body density to be determined.

From that, your personal level of lean body weight (i.e., everything but fat) and fat can be estimated. Info on that here.

In the last few months, through Intermittent Fasting, I’ve lost around 70 lbs, and it’s time for me to start thinking about what my final goal will be.

When I first began Intermittent Fasting, I decided not to initially set a final goal for my weight loss. Instead, I would figure out my ultimate goal as I went along, based on factors like overall health and on my body composition–that is, how much fat I still had on me.

So recently I went to get dunked in a tank to get an initial read on what my body fat percentage is.

Here are the basic results:

  • Current weight: 187 lbs
  • Lean body weight: 140 lbs
  • Fat lbs: 47 lbs
  • Lean body %: 75%
  • Fat %: 25%

That 25% rating is listed as “Fair.” By comparison, the two weights I’m about to mention (below) would both be listed as “Very Poor.” For me the “Good” range would start at around 22% or 182 lbs, so that’s only five lbs away.

Incidentally, based on my present body composition, they estimated my current Resting Metabolic Rate would require me to consume 1881 calories per day. That means I’d need to eat 1881 calories just to lie in bed. Any exercise raises the number of calories beyond that.

I am exercising, and I’m not eating that many calories per day, so I’m losing weight.

As a matter of historical curiosity, I did a back-of-the-envelope calculation to see what my body fat % would have been when I was at my peak weight of around 318 lbs and what it would have been when I started Intermittent Fasting at around 256 lbs.

For purposes of the calculations, I assumed the same lean body weight of 140 lbs. It wouldn’t have been exactly that, but it would have been close to it. Here are the results:

  • Fat lbs (at 318 total lbs): 178 lbs
  • Fat % (at 318 total lbs): 56%
  • Fat lbs (at 256 total lbs): 116 lbs
  • Fat % (at 256 total lbs): 45%

So, since my peak, I’ve managed to cut my body fat % by an estimated 31%, and in the last few months by an estimated 20%.

Of more interest is where it’s going to go, because that helps me establish what my final fat loss goal will be.

Our bodies do need some fat to function properly, but there isn’t an exact number that has been established as optimal (note: optimal and average are not the same thing). For a sample of ranges, see here.

Presently, for my purposes, I’m going to assume that somewhere between 10% and 17% is what I’ll shoot for. That would give me the following parameters as a goal:

  • Total body weight (at 17% body fat): 170 lbs
  • Total body weight (at 10% body fat): 156 lbs

These numbers are also historically in line for what a man of my height (6 feet) would weigh before the obesity epidemic began to set in during the 1960s and 1970s. So no, they are not too low, except by the inflated average weights of our own day.

The numbers assume the present level of 140 lbs lean body mass. If I change that (e.g., if–after I finish losing fat–I decide to build additional muscle, which I’m inclined to do), the numbers will rise accordingly.

I would therefore need to lose between 17 and 31 lbs to be in that range.

Precisely how much I lose is something I will continue to re-evaluate as I go, based on overall health, etc.

Then I’m likely to start building muscle (and thus putting weight–of the good kind–back on).

I’m not trying to do both at once, however. My understanding, including from professional trainers, is that it’s almost impossible to do both at once (for reasons I won’t go into in this post).

What I could do is alternate periods of fat loss with periods of muscle building, but that would only slow down the process of fat loss. I’d rather get rid of the fat and then start building muscle.

Wish me luck!

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Steve August 8, 2017 at 8:17 am

What kind of intermittent fasting are you doing? I’m a prediabetic have you been tracking your blood sugars as well? If so how has that gone?

Gale August 8, 2017 at 10:24 am

Thanks for all the information you post! I am still losing too – down to 161 from about 186. Eating just one meal a day has done wonders to break my constant thinking about food and snacking mindlessly. It’s like magic really. I never thought I would be able to do it, but I haven’t had any noticeable problems, and as you say, if I do have any light-headedness, etc. I can always eat something if I need to. But taking liquids helps tremendously and most of the time, any hunger signals are turned off by drinking water or some other liquid.

Mary August 8, 2017 at 2:26 pm

Building muscle can help lose fat. First, it increases your metabolism. Second, it means you’re lugging around more weight. Notice that sprinters are a lot more muscular than marathoners — at marathon length, the value of most muscle is less than its detriment.

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