January 31, 2023
Measuring Body Fat & Metabolism with DEXA

Measuring Body Fat & Metabolism with DEXA

Hi my name is Albert and I am a Carpenter Tallaght, I went down to InMed today to visit my mate, Mike Turner, the exercise scientist, where he had a few really cool things to show me. For those who don’t know, I caught up with Mike a few weeks ago, for a podcast I did on Metabolic fitness testing.

Basically, there are very few accurate ways we can measure our metabolic rate and body fat percentage. InMed have a new portable calorimetry machine that measures metabolic rate and they also have the state-of-the-art DEXA.

Metabolic testing is done through what we call indirect calorimetry, which is essentially done by fasting for 12 hrs or preferably overnight. You then breathe into a tube for ten minutes. The tube is hooked up to the calorimetry machine, where it measures the ratio of oxygen usage in comparison to carbon dioxide expelled. The machine then calculates a respiratory quotient.

A respiratory quotient (RQ) is defined by the ratio of carbon dioxide production and oxygen consumption; this is how scientists measure metabolic rate. The amount of oxygen we use, compared to carbon dioxide expelled, is influenced by the types of food we eat, physical activity levels, chronic disease and genetic variability.

The RQ ranges from 1.0 to 0.7. At 1.0, this represents pure carb burning, 0.7 is the expected value for pure fat burning. If someone is following a low carb diet or losing weight, thus using fats as fuel, they would have a lower RQ. Your RQ will be closer to 1.0 when you eat too much or if you are exercising.

You can see this type of testing is so useful for someone like me, who has to program food for individuals and athletes. In the spirit of using myself as a guinea pig, I got hooked up to the machine and had my metabolism checked.

Now, it wasn’t exactly accurate because I didn’t fast and I had just rode 80km a few hours prior to the test, so the numbers may be slightly elevated. But these are my values:

RQ: 0.9

Resting energy expenditure: 2376kcal

 Measuring Body Fat & Metabolism with DEXA

I was much more on the carb-burning side of things but that’s because of the exercise I did that morning. Knowing that your metabolism is in running order is a good way to start any new health program. It also gives a dietitian more guidance on what to do with you. This is only half of the picture, though. Knowing where you’re starting from, in terms of body composition, is also useful.

Body composition scans can show the distribution of fat, muscle, and bone. It’s important to know for so many reasons, for example:

  • An athlete who wants to measure if their training program is effective in putting on muscle mass
  • A person with pre-diabetes wants to know if the fat around their organs has decreased
  • If an adult with osteoporosis needs to know if their bone density has improved.

I think the metabolic testing machine and body composition testing go really well together. Mike kindly showed me the new DEXA machine. As we discussed in the last podcast, it’s a 4 compartment model, which is more accurate than the old 2 compartment model DEXA scans.

For those who don’t know what I am talking about, a DEXA machine is the machine that hospitals and clinics use to measure bone density. If you have ever had your bone density checked, you’ve probably had this done before.

Traditionally, it was only used for this purpose; however, now it’s also commonly used for fitness enthusiasts to measure muscle mass and body fat. It is the most accurate way of measuring both, hands down, over other methods such as skin folds, girth measurements or your humble bathroom scales.

It works by using two very low-dose x-ray beams that pass different energy levels through the tissues of the body. Muscle, fat and bone have differing densities and therefore absorb the different levels of energy at different rates. It’s through this difference that the DEXA can calculate the mass of each different tissue type.

It’s also useful to show you where you’re holding fat mass. Fat around the organs (visceral fat), we know, is a good indicator for chronic disease risk, whereas subcutaneous fat (fat just under the skin) is less dangerous to our health.

It was very interesting to see how I faired. Previously, about 10years ago, I had a DEXA scan at Saint Vincent’s Hospital as part of a study I was involved with. They were looking for changes in muscle mass over the course of a steroid intervention. To cut a long story short I was a guinea pig, for an experiment I volunteered in, for the Garven Institute. The Garvan Institute was looking for ways to test for growth hormone doping in sports. Half of us where given growth hormone injections for 18weeks, and the other half where given a placebo.

I didn’t know which one I had, and still don’t know. But my suspicion was I got the growth hormone because I was lifting weights I had never lifted before. I never felt tired from training and because of this, I started training weights twice a day. I also gained 4kg, two of which was fluid and the other 2kg turned out to be muscle. The way they determined my change in muscle mass was through the DEXA scan. I was highly disappointed at the time, actually, because it said my body fat was 35%. For a person that trains all the time it seemed very high to me. This time around, though, I was much more relieved. These are my results:

Body fat = 26%

Lean muscle= 69.6% (Kicking ass here!)

Bone= 4.5%

Fat mass kg = 18.5kg

What was really interesting to note, using the old method of scanning the two-compartment model, my body fat came up at 15.6%. I found this crazy: the degree of inaccuracy of the old way of testing is huge.

Let’s put it more clearly: if a female was 10% body, fat they would be anorexic. This is the stage weight of some female body building competitors. Clearly, you can’t see the veins on my abs or ab definition for that matter. I’m a long way off 15%. More importantly, in terms of bone density, it looks like I might be curbing my predetermined genetically susceptibility for osteoporosis. Both my mother and grandmother had or have osteoporosis. It’s actually a huge problem for a large percentage of the population, as over 1 million people are affected with osteoporosis in Australia.

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