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Wednesday, 4 December 2019

Accurate body fat measurement and revised target weight

Having compared my plot of 21-day moving average body fat % readings (currently ~20%) vs BMI (currently about 32) using the daily data from my bathroom scales, it seemed that either my scales were very inaccurate OR my body fat wasn't as high as is 'normal' for someone of my weight/BMI. Indeed the 'normal' range of body fat for someone my height and weight is around 25%-30%, which is why according to my BMI I'm still in the 'obese' range.

Before I continued dieting my way all the way down into the 'healthy' weight range (56.6 kg -76.2 kg) for my height (1.75 cm), based on BMI (which had led me to have an initial 'goal' of getting my weight down to 74-76 kg), I decided I'd better get an accurate reading of what my body fat percentage actually is!

The 'gold standard' for determining body fat percentage accurately is hydrostatic weighing, where you are submersed completely underwater (and breath out as much as possible) in a special tank while they weigh you. As the density of fat mass (fat) and lean mass (muscle, bones, blood etc.) is well known, once your displacement (volume) and weight are known, your body fat percentage can be quite accurately calculated. A more recent version of this displacement method is the 'body pod' which uses Air Displacement Plethysmograph (ADP) to determine body composition (ratio of fatty mass to lean mass) in a similar way. But both these methods are a relatively inconvenient (especially as there isn't one close to where I live or work).

So a more modern alternative that provides comparable (or better) accuracy is the DEXA scan (Dual-energy X-ray absorptiometry). It uses low-intensity X-rays at two different energy levels that are absorbed differently by different tissues. DEXA scans can provide bone density and body fat measurements.

I went and got a DEXA scan done at lunchtime today, which took about 15 minutes (plus a 15 minutes discussion of the results) and cost me $110. A second scan normally costs $90, so I decided to buy a two scan 'package' for $160 as I will want to do a follow up scan in about 4-6 months to check on progress as I get close to my 'ideal' weight. Getting a package deal meant that this second scan will effectively cost $50 rather than $90.

Anyhow, my result showed a couple of interesting things:

* my weight at the time of the scan was 99.1 kg. This is a bit higher than my morning weigh-in of 95.7 kg, but that difference is probably partly due to having eaten breakfast and drunk a fair amount of fluids during the morning.

* my body fat percentage is currently exactly 20.0%. This suggests that my bathroom scales (on average) give a quite accurate reading, although the precision (day to day fluctuations) mean the precision is only 2-3% using my bathroom scales. But that is good enough for me to use the 21-day moving averages of body fat and weight to track my progress. It also shows that I am relatively muscular, with my fat mass index being 6.48 (normal range is 3-6, and obese is 9-12). So I'm actually (now that I've lost some weight) just 'overweight' rather than 'obese'.

* unfortunately the scan also showed that my visceral fat (Android) is 2.086 kg, whereas my goal should be to get this down to <= 1.000 kg. Fortunately losing weight will reduce my total body fat, and will reduce my visceral fat to a more health level.

* the results also estimated my basal metabolic caloric requirement at 1953 kcals/day, which is close to what I had thought.

The consultant advised that my rate of weight loss (to reduce fat without losing excessive amounts of muscle) should not be more than about 1kg per week (I've been averaging around 0.7 kg per week over the past 20 or so weeks).

Going to the gym 2-3x per week while dieting should result in losing equal amounts of fat:muscle, so if I got down to my original 'target' of 75 kg I'd end up with a body fat percentage of around 10% which would be too lean, and I'd lose excessive amounts of muscle. A better goal is apparently to get down to around 80kg over the next 24 weeks, which should result with a very good body fat of around 12.8%. I'll check on how things are progressing in 4-5 months time (when I'm getting close to my new 'target' of 80kg).

One interesting aside is that the scan results quite clearly show that I'm right handed, and have slightly more/better muscle mass in my right arm. Both arms came in at just over 1.0 kg of fat, but my right arm has a lean mass of 4.4 kg whereas my left arm has a lean mass of only 3.9 kg. (Left arm 20.3% fat, Right arm 19.0% fat).
My left and right Rib measurements were symmetrical (23.4% fat) as were my left and right Leg measurements (19.2% fat). I'm carrying most of my fat around my stomach, and some on my arms and thighs. So I should also look a lot more 'athletic' when I get down to 80kg and a body fat of under 13%.

The scan clearly shows that inside my current fat body is a slim person waiting to escape!

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