The amount of calories we burn is typically what most people think of when they are talking about their ‘metabolism’ or ‘metabolic rate’.
Your metabolism is actually an entire network of biochemical processes that sustain life! Most biochemical reactions require energy.
To provide energy for these reactions, other reactions enable us to harness energy by breaking down carbohydrates, proteins, fats and alcohol.
The Difference Between Calories and Energy
Calories measure how much energy is contained in the macronutrients that make up food – but the body doesn’t technically “use” calories.
This would be like saying the body uses “litres” when talking about using water. Litres are the way we measure water, but water is the thing we use.
A molecule called ATP is the thing the body actually uses for energy. In creating ATP, carbon dioxide, water, and heat are produced as by-products (this becomes important later as this is how we can measure your metabolism).
The amount of calories you need each day is determined by the summation of the following 4 elements:
- The amount of planned physical activity you do (i.e. exercise).
- The amount of unplanned physical activity you do (ie moving around, fidgeting etc).
- The thermic effect of food. Consuming, digesting and absorbing food is not a ‘passive’ process. It requires some calories, although we get more back than we expend so there is still a net gain of calories.
- And most significantly: to satisfy the requirements of your resting metabolic rate (RMR). This is what is tested when you go for the type of metabolic testing that this post is about.
You can’t really “break” your metabolism, but certain aspects of each of these 4 contributors to the metabolism and the calories they require can adjust up or down based on what you are doing.
Estimating Your Resting Metabolic Rate
Your RMR is the number of calories that your body uses to sustain basic health and physiological function while at rest. RMR is also often referred to as basal metabolic rate (BMR) or resting energy expenditure (REE). These terms are used somewhat interchangeably.
When you aren’t actively “doing” anything, it still costs energy just to ‘be alive’.
Even doing things like sleeping or laying on the couch watching Netflix, your body needs calories. These go towards maintaining your core temperature, brain function, and organs like the heart, liver, kidneys, and lungs all still require energy!
RMR is influenced by age, height, weight, gender, muscle and fat mass, and a few other variables. In the majority of people, your RMR accounts for over 50% of your daily calorie needs (with the possible exception of very physically active people).
There are predictive methods to estimate your ‘metabolism’ (RMR and TDEE) such as the one used in the Ideal Nutrition Calorie Calculator.
Reasons Why You Could (Or Should) Test Metabolic Rate:
1) If you are having an unexpected response to eating the number of calories from a predictive equation i.e. your metabolism is “broken”.
If you’ve been eating at a calorie target based on a calculator and aren’t getting the results you expected, you could use IC testing to measure the exact amount your BMR is burning. You can find out if there is a discrepancy in the calculated number of calories you are eating and the measured amount that you need.
For example, if a calorie calculator predicts you need to eat 1800 calories per day to lose 0.5kg per week, and you’re doing this but not losing any weight.*
*I would only use this reasoning after eliminating other reasons why you may not be losing weight. In my experience, it is more common that extra calories are unintentionally being introduced into the diet than the predictive equation is wrong.
This might also be of relevance if someone feels like they have been legitimately dieting strictly for a very long time, but are now no longer losing weight.
2) If you’re outside of the general population or feel that calculators may not be able to correctly estimate your requirements.
As mentioned above, obese, anorexic, diabetic (type II), hospitalised, or critically ill people’s requirements are not well estimated by prediction equations. These could be worth testing with IC.
When working with clients with PCOS, we sometimes test metabolic rate, as there is evidence that women with a PCOS diagnosis can experience reductions in basal metabolic rate from 14-40%. This can be pretty significant and worth taking into account when planning a diet, and won’t be captured by calorie calculators.
3) In monitoring individual response as part of a longer-term test, re-test strategy.
As mentioned, the metabolism is adaptable. The number of calories required may increase or decrease over time based on the conditions placed upon the body (ie energy deficit or energy surplus).
With calorie restriction, we would expect some decrease of metabolic rate over time. IC testing could be used to ensure that the drop hasn’t been too significant or to help dictate strategy moving forward.
Longer-term diets have more potential to affect RMR.
There are no strict guidelines on this, but in practice, we could do something along the lines of testing your BMR at the start of a diet phase, then every 3-4 months. If the results of IC show more than a 20% reduction in BMR we might look to spend some time on maintenance calories. This could allow the metabolism to ‘speed back up’ before further dieting.
This can be really difficult to try to account for using a standard calorie calculator.
Sometimes seeing the specific numbers on paper help people comprehend and accept a certain calorie target and whether or not it is being achieved.
Lots of people undereat or undershoot their calories. This ranges from the average person aiming for a bit of fat loss, to athletes trying to perform at high levels. Seeing these specific numbers on the page can sometimes be useful to reinforce their actual needs and encourage them to increase calories to a more suitable number.
Not to sugar-coat it, but a lot of people also blame failed weight-loss interventions on “slow” metabolisms. You can test your metabolism to see if it is in fact not “slow” or not. If it is not, the test results might help open you up to the possibility that there is something in your control that you may not be addressing.
This could be useful in changing the mindset from ‘I can’t help it, I’ve got a slow metabolism’ and opening you up to other factors that are preventing weight loss.
4) (And my personal reason) Out of sheer curiosity about what your metabolic rate is and to use it as a learning experience.
Currently, I am in a phase trying to gain weight and lean mass so am in a small calorie surplus. Just as the metabolic rate can decrease when in a calorie deficit, it can increase when in a surplus.
Here are my results for an example of how you might see this as a learning experience!
From the last test you can see my REE is 104% of it’s predicted value. Meaning I had a small increase (4%) in my resting metabolic rate. Even though it seems pretty minimal this equates to about 100 calories above what I’d predict I’ll need to maintain my rate of weight gain.
That means I need to add an extra ~100cals in on top of my current surplus to continue at the rate I am aiming for. Without this data it would have been pretty hard to quantify this amount.
What Is Calorimetry and Indirect Calorimetry and How Do They Work?
Earlier I mentioned that carbon dioxide, heat, and water were by-products of metabolism.
It is, therefore, possible to measure the changes of heat and/or gasses in a known quantity of air, and calculate how much energy was burnt during this time. This is the basis of indirect calorimetry.
Direct calorimetry involves placing a person in a calorimeter and measuring the heat given off by their body mass. It is very expensive and impractical, and due to its limited clinical application, it is rarely used outside of specific scientific studies.
Direct calorimetry is similar to how calories in food are calculated, where foods are placed inside a BOM calorimeter and literally burnt to measure the energy given off as heat.
Neither of these options is practical or useful in practice – especially the total incineration option…Enter indirect calorimetry.
Indirect calorimetry (IC) is considered to be the gold standard of measurement techniques for determining your RMR. In a nutshell, it works by measuring pulmonary gas exchange – that is, the composition of the air you breathe in vs the air you breathe out.
From this, the amount of energy expended can be calculated.
What Happens During the Test?
There are a few different types of IC machines, the most common being either a hood or mouthpiece set up to measure oxygen and carbon dioxide. Both are equally valid forms of measurement when performed by a skilled practitioner.
During the test, you lay in a reclined position for around 15 to 20 minutes. When booking the test they will advise the conditions leading up to it that need to be followed (ie fasted, no alcohol or caffeine, no exercise within a certain period of time etc).
This is done to control for things that could have a temporary effect on your metabolic rate and to standardise the test so that each measurement is done under conditions as similar as possible.
During the testing window, a period of time where your oxygen consumption is relatively stable is selected to perform analysis on.
We know that one litre of oxygen consumed generates 3.94 calories, and 1 litre of carbon dioxide produced generates 1.11 calories.
Using the measurements of how much oxygen is consumed, and how much carbon dioxide is produced obtained during the testing the Weir equation can be used to calculate daily calorie requirements.
The skill and experience of the practitioner also matter when performing almost any test. IC testing is no different.
IC machines need to be correctly calibrated for use, the testing conditions need to be specific and consistent each time. This includes consistency in things like:
- the reclined position the person is in
- room temperature
- fasted patient
- no exercise or stimulants prior to testing
- the operator’s interpretation of the period of time during the test that is most suitable for technical analysis
If any of these things are done incorrectly or inconsistently, the accuracy of the results can be compromised.
Should I Get the Test Done?
The tests are probably most useful in evaluating the specific calories you require to suit your BMR in cases of a depressed metabolism. That is, you can literally see how much/if your metabolism has slowed.
This is a great piece of data for us to have. However, most dietitians who are experienced in working with these types of patients would do the same thing to treat the issue regardless of if we had the specific numbers or not.
For example – it is not uncommon for clients to start working with us who have been eating consistently in a significant calorie deficit and below their predicted needs. Sometimes they’ve been doing so for an extended time, but are no longer losing weight.
In such cases, we would likely:
- Go over a full patient history
- Explore what an appropriate calorie deficit would be for weight loss
- Eliminate other factors that may be interrupting weight loss.
This could be things like incorrect calorie counting, unconscious snacking, not correctly measuring portion sizes/food weights, overeating on weekends, additional calorie intake via alcohol, and other lifestyle factors.
If all of these things were on point, we could make the logical deduction that there has been some degree of reduction in metabolic rate.
From there it doesn’t really matter if we know this effect is happening in a general sense, or the exact amount from a test: the treatment is the same.
The only solution for increasing your metabolic rate is to increase calories up to where you’re predicted maintenance calories are.
The testing can also be relatively expensive, often somewhere between $90 to $150, so it may not fit everyone’s budget.
It is a legitimate testing method, but it is not absolutely necessary for us to address the issue of a depressed metabolism. If there are budget or logistical constraints that are stopping you from getting tested, there are ways around having to test!
How to “Fix” Your Slow Metabolism
Over time the metabolism slows down if you’re in a calorie deficit. Your BMR and the functions it requires calories to perform can start to ‘switch off’ or down-regulate some of these processes.
It’s like when your phone gets to under 10% battery and goes into power-saving mode. Your screen brightness dims, your GPS switches off, apps that constantly update get paused…there isn’t enough energy to keep the phone running with all of these things turned on, so they get switched off.
Then you plug the phone into a charger and EVERYTHING comes back to life. It suddenly has plenty of power (energy) to satisfy all of the things it can do. Brightness shoots up, 20 different DMs and notifications pop up, and the weather and location data is updated. All those non-critical things that got turned down to keep the system on come back to life.
This is kind of what happens when you move back to maintenance calories after being in an extended deficit. But just like recharging your phone back to 100% capacity, your metabolism doesn’t immediately jump back to 100% either.
Depending on the person, just like depending on the specific phone and size of the battery, it will different amounts of time to fully charge.
This is where IC testing can be really useful. You can re-test your metabolism which was previously depressed, after a period of time on maintenance calories to determine if you’ve ‘fixed’ the problem.
This is just like if you take the phone off the charger before the battery is 100%. If you try to jump back into a calorie deficit too soon, you run the risk of your metabolism not having fully bounced back.
In my experience, it really is highly variable in how long it takes someone to recover from metabolic slowdown. A repeat test is a great idea here and can stop you prematurely trying to diet again.
IC testing is a valid way to give a precise measurement of your BMR and can indicate if your metabolism has slowed down or sped up beyond its predicted normal level.
It can be very useful from a diagnostic and educational standpoint, and often something we recommend clients in practice get.
However, it is not critical in treating or managing a “slow” metabolism. It is simply a useful tool to provide some extra data throughout this dietary process.
One last note to consider with IC testing: it doesn’t provide your total daily energy expenditure.
It provides your BMR. TDEE is calculated using BMR but this is only one part of the 4 components that make up your total calorie requirements.
Unless you are in a coma and not moving at all, there is still an element of estimation that goes into calculating your calorie needs. This is based on how much exercise and general movement you do.
Aiming to find out your exact daily calorie target is futile, and IC won’t really help you achieve this as it will fluctuate each day based on factors other than your BMR.
If you can afford it or if your dietitian thinks it will help IC could be worth doing. Alternatively, if you’re simply interested in exploring the results, or confirming that you’ve successfully addressed a ‘slow’ metabolism IC testing is a great tool!