If you wear a fitness tracker, there’s a decent chance it will tell you how many calories you have burned that day. Some of these will be active calories – the energy you have expended as a result of activity. The rest are the calories you burn simply to stay alive.
That remainder is your resting metabolic rate (RMR) and your tracker will be estimating it based on things like your age, weight and sex. However, like many estimates, that approach is not without its problems. To find out why it’s worth getting an accurate measure of your RMR and how to get one, we spoke to Dr Garry Palmer, sports physiologist and founder of Stanley Sports, the UK distributor for the VO2 Master portable VO2 max and RMR analyser.
What is your resting metabolic rate and why does it matter?
Resting metabolic rate is the energy that the body needs to survive. If you assess your resting metabolic rate properly then you can get your energy requirements right – calculating the calories that you need for training, or the calories you need if you want to get into deficit for weight loss. You can offset the likelihood of illness and injury by making sure that you’re putting the right amount of fuel in the tank.
Can you use online calculators to get your resting metabolic rate?
These population-wide values are a starting point and they’re better than nothing, but if you’re finding that it isn’t giving you what you need, or you’re starting to become a lot more serious about training or weight loss, then you need to drill down and get the numbers that are right for you.
If you stuck 100 people in the room and you used a standard formula that uses age, height, weight and gender of the individual, you could predict the average resting metabolic rate of that room, actually very accurately. But you get huge variations, so the calorie target for individuals based on the standard formula can be completely out.
How can you get a better test?
You need to go to somewhere where they can actually assess it, whether it’s a university or a personal trainer who has a device to measure it. There are lots of practitioners out there that will be able to offer a resting metabolic rate test.
What does a resting metabolic rate test measure?
It’s measuring the volume and composition of each breath and how much oxygen you’re taking out. From that, you can calculate how much energy is being used for you at that particular time, when at rest.
What kind of factors can affect your metabolism and resting metabolic rate?
One of the biggest things is muscle mass and activity. You can have two people of exactly the same height and weight but if they’ve got different muscle mass, the person with far more muscle mass is going to need far more energy to keep themselves ticking over.
If you’re trying to lose weight, why is it important to maintain muscle and your resting metabolic rate?
I often find that people who are trying to achieve weight loss will cut their calories down too far, and so the body thinks it’s almost gone into a starvation situation and slows its metabolic rate down. That will mean you’ll start to eat into your muscle mass and the body will hang on to the fat mass, because that’s what’s going to provide it with low-intensity energy for a long period of time.
So if you estimated your resting metabolic rate wrong, you could end up massively under-eating. You’ll find that in the first couple of weeks, you might lose a lot of weight, because you’re losing carbohydrate, losing body water, and starting to strip down muscle mass, but you’ll find that your body fat isn’t shifting.
Home Resting Metabolic Rate Test
I used the VO2 Master home breath analyser to test my resting metabolic rate and it’s a very simple test to take. You simply strap on the mask, set up the test in the partner app and chill out for 10-15 minutes while it measures your breath.
However, choosing the right time to do the test is less straightforward, because doing it at the wrong time can change your results considerably. I tested myself in the afternoon after a hard run in the morning, and a big meal with coffee at lunch, and got a considerably higher RMR result than when I did it first thing in the morning immediately after waking up. Neither is the perfect time to do the test.
“What I would suggest to people is they will do it relatively early in the morning,” says Palmer. Leave a little time after waking up, but take the test before you have breakfast. “You don’t want to be falling asleep, and you don’t want to be doing it straight after a big meal or a coffee.”
Exercising before the test, or even doing a hard session the day before, can also affect your numbers, but that can still help you gauge your calorie intake during an intense training plan like one you might do before a marathon. This is exactly what I’m doing, and using the VO2 Master to check my RMR (at the right time, on the third occasion) gave me a higher daily calorie intake than I got from online calculators, which means I can eat more. It’s always worth getting accurate info on your body, but that’s especially the case when it means you can eat more to maintain weight.
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Nick Harris-Fry is a journalist who has been covering health and fitness since 2015. Nick is an avid runner, covering 70-110km a week, which gives him ample opportunity to test a wide range of running shoes and running gear. He is also the chief tester for fitness trackers and running watches, treadmills and exercise bikes, and workout headphones.