T Nation

DEXA Scan Questions


#1

Last week I got a shock. Volunteered for a study done by the Exercise Sciences Dept at the local Univ. Got a free DEXA scan. Surprise, surprise, as Gomer Pyle says.

My result, besides above avg bone density, was a fat percentage of 30+%. Scan was done by a student, then the PhD came in to explain the results. I was stunned and mute as he politely explained that I was past the overweight category and was borderline obese! Went in with the self image of an older athlete and weight lifter and came out confused.

Problem is, no way am I carrying around 60 lbs of fat! I mean, jeepers, that would be 29 quarts -- 7 gallons -- of fat!?

Pinching an inch on the lower abdomen does not equate to a 30% body fat percentage. Does it?

Questions are:

  1. Does the DEXA software subtract out the weight of the bones and give only a fat/soft tissue ratio, which would mean significantly less fat than 60 lbs?

  2. Are calibration errors possible? Likely for a shiny new state of the art exercise lab at a major univ?

  3. Operator error? I do know one hand was off the scan field.

  4. Is muscle tissue in those of us 60+ less dense? In endurance athletes?

Note. I am not massive but built more like a swimmer or triathlete, tho at 38" around the waist (45" chest, 42" hips), not slim. I mostly train low-weight high-rep resistance + cycling and hiking. Height 5'10", Wt. 204 lbs. BMI (for what that's worth) 29.


#2

Delt,
Is your gut HARD? Just cause your lacking "soft" typical fat, doesn't mean (and please, no offense and im not implying) your free from visceral or "hard" fat.
I'm sure your familiar with visceral fat, but incase your not, its a type of body fat that exists in the abdomen and surrounds the internal organs. Everyone has some, especially those who are sedentary, chronically stressed, or maintain unhealthy diets. A different type of fat ? subcutaneous fat ? which builds up under the skin, has less of a negative impact on health and is easier to lose than visceral fat.

Im guessing with your larger 38 inch waist, you have a decent amount of visceral fat....Maybe you could go back to university and ask for some clear direction on this?


#3

Like any measuring stick, DEXA scanning is not perfect. That said, I ran your numbers through a BF% calculator using the height, weight and waist numbers you provided above. (I don't know your age, so I went with 61.) The calculator returned an estimate of 27%--not far from the DEXA estimate. This provides converging evidence that your BF% is in fact in the neighborhood of 30.

Here is a link to the calculator:

http://www.healthyforms.com/helpful-tools/body-fat-percentage.php


#4

Thanks, guys. I do appreciate the replies and encourage more.

The fat percentage based on height, weight, sex and age is BMI, the too often used Body Mass Index. Using the BMI formula, an elite bodybuilder at 6% body fat would be classified as obese. If a subject is more than a little muscular, the BMI is way off. This is why BMI should not be used for anyone who is not sedentary and even then with a grain of salt.

Last year I was 18 lbs lighter than now, and by caliper measurement figured body fat was around 15%. My BMI was about 26-27.

Subject 17 is correct up to a point. No question I have visceral fat and that is has increased. This of course is a real health problem, since visceral fat (unless it is the type known as "brown fat") is correlated with all kinds of health problems.

But I probably have no more than a quart or two of visceral fat. At about 2 pounds per quart of fat (.92 kg/liter), that's only 4 lbs. 3 quarts, that's 6 lbs. 3 quarts ought to make quite a bulge in the middle.

Many men my age have a gut that hangs over the belt even if they are not technically obese; others have a pot. I don't; I have a gut convexity under the muscle. So where is the rest that 60 lbs of fat?

I was invited to try the DEXA scan to give the grad student practice in running it, though in reality the PhD was being nice. I don't see any of them as expert in this area and if the PhD had been a medical doctor or expert on body type wouldn't he have had questions about the accuracy of the scan?

Thanks again. I'll try to post a photo.


#5

20%, sure. Surely not 30.

Cat is obese. I am merely overweight.

Granting that, what was the problem with that DEXA scan?


#6

Right from wikipedia herself on DEXA scanning: However, it has been suggested that, while very accurately measuring minerals and lean soft tissue (LST), DXA may provide skewed results as a result of its method of indirectly calculating fat mass by subtracting it from the LST and/or body cell mass (BCM) that DXA actually measures.

Having your hand out of the scanning area should not make too much of a difference. I think it comes down to statistics where lean and fat fall in a range of densities and there's likely to be overlap. Then we look at calibration of the machine as readings from one machine will differ slightly from another and in your case it could be way off, but I'm not an radio-physicist so I can't say one way on the other.

I agree you look closer to the 20-25% marker.


#7

CI, I read that and didn't understand it. Is that calc DEXA makes a matter of software? If DEXA doesn't measure fat directly then a calculation must be made from data in the scan. Is it that the software creates a guesstimate that is sometimes off?

This is important because the DEXA scan is touted as the bee's knees, the gold standard of body fat measurement. Do we need repeated scans including when we are in, for example, different states of hydration to achieve accuracy?

Personal note. I fasted for close to 12 hours before the scan, with maybe 8 ounces of water. Does fasting/hydration affect scan results?

Maybe I've been seeing too many superman movies, but I thought the muscular density from ultra high rep training would show up in some way in the scan. Obviously it didn't.

Scientific note. We are mostly water. "Hard" muscles are mostly water. Fat is a little less dense than water (about .92) and muscle tissue a little more dense (about 1.06), so the fat/muscle scan has to differentiate tissues that deviate from water by only 6-8%. But as CI points out, the DEXA scan infers fat proportion instead of measuring it directly.


#8

Hey Delta.I a 54yr old ex-competitive mr.Canadas in 1978.I had a Dexascan done about 6-month's ago and my f"in fat was 26%.. Like yourself I was to say the least a little shocked.
I am 6-2 and weight 230 and thought no more than 15-18% What the fuck??? Are these test's accurate,or or we fat bastard's. I don't believe this was an accurate assessment. There is this other contraption called: The Bod Pod,maybe try thid thing out.see ya john


#9

Hey, Johnny-K! Well, I dropped 5-6 lbs and can now see my abs shadowed above the navel under the subcutaneous fat. Does that put me at 25%? :slight_smile: Or 15%? I lean toward 18%.

I think there was definitely something wrong with the DEXA scan. Question is, what?

Mentioned above that fat has a density of about .92, water at 1.00, and muscle at around 1.06. Could it be that old ... ahem ... mature guys have muscle density lower than 1.06? Or fat density below .92? Have there been studies on DEXA scan accuracy in older subjects?

Actually, I'd think that older bodybuilders would have leaner muscle tissue. "Muscle maturity." More dense, not less?


#10

I wonder if muscle displaying a higher degree of hypertrophy is less dense. It should still be denser than water but if a major part of the hypertrophy is water absorption in the muscle then wouldn't the density be closer to water? That might throw off the test, especially if using creatine.


#11

I had a Dexa scan last year as part of an obesity research project, my BMI was high (I powerlift) but they struggled to get a biopsy of fat off my stomach. Took three bio needles before they had enough. Also had a MRI as well and a very detailed pulse blood pressure.

Have not had full report yet, but my Dexa was 30% like yours. I have more muscle but say you are leaner. My electro scales had me at 20% +/- 2% so I was totally shocked. I did some reading..

everyone is shocked at how high their fat is on a Dexa, everyone. No matter what other gauge you use a DEXA will have you fatter, some by as much as 10%.

I intend to get a Dexa every few years, maybe 3 maybe 5, like blood pressure and use it as a comparative measure, but don't compare your Dexa with someone else's calliper,

oh and pulse blood pressure was taken continuously over 10 minutes, low pulse of 49 high in the high 60s, 90% of all readings 51-57

blood pressure 90% of all readings 131-137 over 71-77,

59 and never do cardio, but do high volume lifting,

take care guys