It’s official – I have a slow metabolism

I've been talking about my own weight loss struggle, and the journey I'm on to find out exactly what’s going on (and what I can do about it). You can catch up on the first three posts here, here and here. This week, it's all about metabolism.

Last week I told you about my body scan, and how it calculated my resting (basal) metabolic rate. Your resting metabolic rate is basically how much energy your body needs just to keep ticking over, with no physical activity (for brain activity, breathing, digestion etc).

Unscientific BMR

You can work out your resting metabolic rate yourself, using a very crude formula along the lines of:

Women: BMR = 655 + ( 4.35 x weight in pounds ) + ( 4.7 x height in inches ) – ( 4.7 x age in years )
Men: BMR = 66 + ( 6.23 x weight in pounds ) + ( 12.7 x height in inches ) – ( 6.8 x age in year )

Here is the metric version:
Women: BMR = 655 + ( 9.6 x weight in kilos ) + ( 1.8 x height in cm ) – ( 4.7 x age in years )
Men: BMR = 66 + ( 13.7 x weight in kilos ) + ( 5 x height in cm ) – ( 6.8 x age in years )

Using this formula, I get a BMR of 1608 calories a day. Of course, this formula uses your total body mass, so if you had identical twins of the same height, age and weight, they would get the exact same result for their BMR – even if one had 8% body fat and the other 40% (and this just happened to work out to the same overall weight).

Slightly more scientific BMR

The body scan I had calculated my resting metabolic rate as 1639 kcal a day. This would have used a similar calculation, but it would have taken into account my exact lean muscle mass and my exact fat mass (as they require different amounts of energy to maintain).

As soon as I saw this figure it caught my attention. During my months of experimenting with different ways to lose weight, I’d spent a significant period eating far less calories than this – whilst sprinting and swimming. How can I have been expending so many more calories than I was in taking – and not burning off any fat to make up the deficit? It just doesn’t make sense?

What if my metabolism is lower than the average used in these calculations? What if my body uses far less energy than an average person each day? Perhaps my metabolism is a lot lower than the calculated 1639 a day?

I had to find a way to calculate my exact resting metabolic rate, to understand how much of an impact this was having on my difficulty to burn off fat.

Scientific BMR

I found out about the most accurate way to have your basal metabolic rate tested. Apparently there are special chambers which are completely sealed. You are weighed and all of your biometrics are taken before you enter the chamber, which is then sealed. Everything that goes into and out of the chamber is weighed and measured. You effectively live in this chamber for 24 hours and go about a normal(ish) day, resting, sleeping, eating etc, By calculating your weight, the amount of gas inhaled and exhaled, what you eat (and what leaves your body), they are able to calculate exactly how much energy your body uses in a day, at rest (whilst you’re asleep) and when awake and going about a typical day.

I had to spend 24 hours in one of these chambers.

Unfortunately my extensive googling revealed there doesn’t seem to be a single one of these chambers in the whole of Australia. The chambers that I thought I may be able to convince to let me test my BMR all seem to be in North America – which unfortunately isn’t realistic at the moment. So I had to find the next best alternative…

Scientific-enough BMR

I found out about a metabolic testing option called VO2 (as in volume of oxygen) testing, that is something I can realistically have tested – in my own city. VO2 testing seems to be geared around athletes wanting to find out how efficiently their oxygen use is (and ultimately make this process more efficient and improve their athlete performance).

The VO2 test measures both your resting (basal) metabolic test and you exercise metabolic rate. In my quest to find out more about my metabolism, it was the resting metabolic rate that I was particularly interested in, but in the interest of comparison had the exercise rate tested too.

For the resting test, you need to be as un-awake as possible, so they like to run this test early in the morning. I usually have a long commute, so I stayed locally the night before to avoid any extra stress from the commute. They told me not to listen to music or check emails before, so I did as I was told and got to the studio at 6am as un-awake as I could manage.

As soon as I arrived I was given my mask which was hooked up with some tubes, connected to the machine that was to analyse my breath. I also put on a heart monitor that was connected to the machine and laptop. They lowered the lights and I laid down for about 15 minutes, while the test got underway. Trying to breath normally – when you know it is being analysed – was surprisingly hard, but once all the data had been collected the machine beeped to indicate the test had concluded. By measuring my oxygen consumption and carbon dioxide production during this period of rest, an accurate daily caloric expenditure is calculated. Exactly the information I was looking for!

The exercise assessment happened straight after. For this I wore the same mask (not a good look!) and moved into the gym, where I chose to do the assessment on the treadmill (the other option was the bike). The laptop was hooked up to the machine and told the trainer when to increase the intensity of the treadmill (he increased the speed and gradient), depending on my heart rate. By the end of the assessment it got really difficult, but this is how it has to be to ensure the test captures all of the required data. This test is supposed to measure VO2 utilization (amount of oxygen you're able to use during exercise), heart rate response, the precise number of calories your body burns during exercise, and whether fats or carbohydrates (sugars) are being used as the primary energy source. The assessment also maps your appropriate heart rate training zones. Interesting stuff!

The Results

So the whole purpose for this was to find out my actual resting metabolic rate. Was the formula calculated value of 1639 right? Well it turns out it wasn’t even close… The VO2 tests calculate my resting metabolic rate as 1316 calories a day. That’s over 300 calories less than where the formula put me! Given that it wasn’t a true resting assessment as I was well and truely awake, perhaps that means the calculate rate was still too high? Either way, 300 calories is a huge variance – that’s a meal!

On the exercise side (which I'm less interested in, but will be a useful comparison point) it looks like my primary energy source is fat, not carbohydrates (which given my paleo diet isn't really a surprise). But if my exercise is efficient at burning fat – then why is regular HIIT not burning my fat stores?

A long way from average

Seeing the concrete proof that I do in fact have a low metabolism raises so many more questions than it answers. Why is my BMR so much lower than average? Has it always been this way? What came first, the slow metabolism or the weight gain? Do all of my slim friends have higher than average metabolisms? Do all overweight people have slow metabolisms? And of course the big question – (how) can I change my metabolism?

What I really don’t understand is how I sustained daily exercise on top of eating quite a bit below this number of calories each day. Where did the surplus come from, as it clearly didn’t come from burning fat stores?

There has to be more to it…

Next week I’ll be sharing more of what I’ve found out on this journey into weight loss