ENERGY EXPENDITURE 1.1: How much energy do you use during training?
“When you are considering caloric burn, is it better to do 7 minutes of HIT (high intensity training) or 40 minutes of zone 1 (light,easy) training?”. This is where we left off on the last Energy Expenditure Week article. We left of with a question that is similar to so many questions that both my students and some clients ask. Questions about this just keep coming my way through social media, in the class room and in random conversations, so I figured it is time to explain these metabolic concepts to you.
I also get asked about those tables you frequently see in magazines or on the internet, telling you how many calories you are burning during 30 minutes of jogging, swimming, dancing or other? Or they tell you how long it with take you to burn of that bun you just ate or that latte you just had? Those tables probably do not apply to you AT ALL, because you might be burning much more or much less depending on multiple factors that affect your metabolism.
A lot of people are struggling with the understanding of the relationships between training intensity, training duration and oxygen uptake between when it comes to energy expenditure a.k.a how many calories you use for a specific task. Energy expenditure can be defined in different ways, but we’ll use the one that refers to the sum of calories a person utilizes for X task. Have I lost you already? Don’t worry, just keep reading and you’ll get there.
If you read this article from start to finish I am sure you will have a much better understanding of how oxygen demand is linked with metabolism. FYI: the angle here is aerobic exercise and movement tasks, so please don’t go all “what about sprinting or power lifts” on me. We’ll get to that and all in due time in another blog post.
In the last Energy Expenditure-post we established what type of factors may affect your energy expenditure. Today you’ll learn how much energy you may be using per time.
Can you breathe oxygen? We have so much in common!
How much energy your body is using when you are resting, jumping or running is dependent on a lot of factors, but oxygen uptake (known as VO2) is often used to quantify the energy cost of an activity.
Your maximal oxygen uptake (VO2max) represents the highest rate at which the body can take up, transport and utilize oxygen during maximal exertion. For example when running at your absolutely maximal capacity.VO2max is a measure for health, predictor for performance, a tool for training intensity and it is usually measured in a physiology lab by a method called indirect calorimetry.
VO2max is expressed by ml/kilo body weight/min, which means that the results will depend your bodyweight among other things. Your VO2 liter uptake is expressed as L/min and is not dependent on body weight. In some sports body weight will be of significance and sometimes a predictor for performance. Examples of this are running, cycling, cross-country skiing and high jump. In some other sports bodyweight is less of a concern but the oxygen liter uptake is very interesting, such sports are kayaking, rowing and swimming.
Your maximal oxygen uptake is movement specific, which means that your VO2max in running is not representative for cycling, or any other activity, but it gives me a pretty good idea of what you are capable of. Unless you are a very good cyclist, for example, you will most likely have your highest VO2max in running. This is due to the fact that ever since you were a sweet little baby and started your journey in movement, your leg muscles have been trained to and specialized in taking up and utilizing oxygen in movement patterns like gait or running. Even if you don’t see yourself as a runner per se and per now, these are our most natural movement patterns of transportation and the once we are born to be best at doing.
Let’s get some oxygen-perspective
What are common VO2max values? First let’s have a look at some uncommon values just to get some perspective:
For several years Bjørn Dæhlie has the “world record” of 96 ml/kg/min, which has been broken and unofficial measurements of a Norwegian cross-country skier has been as high as 98! On the other hand a COPD (Nor: KOLS) patient can be maxing out his or her maximal oxygen uptake at 20 ml/kg/min.
Average VO2max values for the Norwegian population regardless of age is:
Men: 39.5 (SD 9.8) ml/kg/min
Women: 32.2 (SD 7.8) ml/kg/min
More perspective to come: If your VO2max is around the Norwegian average, what does that mean?…Well, let’s just say that the average here is not that great from a both a health perspective and most certainly a performance perspective. It means that, from a health perspective, you would greatly benefit from some cardiovascular activity a.k.a typical out-of-breath training.
It does not get better with age because there is also a powerful age related trend for both men and women where VO2max falls with an average of 3.2 for men and 4.3 ml/kg/min per decade. The difference between the sexes is 20-30% on average.
These data are collected from the stunning work of Elisabeth Edvardsen, where 407 men and 370 women participated in a national survey from 2009-2010 about the physical form of the Norwegian population (1).
What does VO2 have to do with energy expenditure anyway?
How much energy your body is using when you are running or cycling is dependent on the absolute training load. An easy example of this is when you are running with 5 % incline on 10 km/h, this load represents a certain energy cost, meaning oxygen cost that can be measured.
Training intensity in endurance training can can be expressed as % of maximal heart rate (HRmax), % of VO2max or by blood lactate measurements, for example. Neither blood lactate or relative or absolute heart rate will tell us anything about caloric cost of the training you are doing, but knowing the oxygen consumption will help a lot!
Burn, baby, burn!
Oxygen uptake isn’t the only thing that has a say in your calorie burn, but in aerobic activities it says a great deal. We know that the energy equivalent for oxygen is 5 calories (20.92 Kilojoules) burned per liter oxygen used. So if we know how many liters of oxygen your body is using in a given task we can multiply this by 5 and get the approximate calorie consumption per minute in that task.
Kristin’s VO2max is 65 ml/kg/min
Her bodyweight is 68 kg
What is her oxygen uptake in liters? To figure that out we do the following calculations:
65 ml/kg/min x 68 kg = 4420 ml
4420 ml / 1000 = 4.42 L
Now we know that at the workload where Kristin reaches her VO2max she is using 4.42 L oxygen per minute.
1L » 5 cal
4.42 x 5 = 22 cal/L/min
The fact that her body is burning 22 calories per minute at VO2max applies to the work that she is doing, namely running on a given incline and at a given speed. When you are doing endurance training you are usually not working as hard as in a such a test, thus Kristin will not need as much energy as 22 calories per minute when she is training. 22 calories is the maximal amount of calories her body is able to use per minute right now.
Threshold intensity – A highly relevant training intensity
More often than not you will have taken a lactate profile test in addition to you VO2max in order to estimate your lactate threshold (LT). This is the training intensity at which there is equilibrium between production and elimination of blood lactate, and is an important predictor for distance endurance performances. LT is also a relevant training intensity, because here you will train your muscles abilities to utilize oxygen and to buffer lactic acid and other components that are creating an acidic environment in your muscles.
If we have such a test to go off on we will have simultaneous measurements of HR, VO2 and other variables on sub-maximal loads. Let’s say we estimate Kristin’s lactate threshold and that her oxygen uptake at threshold lies around about 80% of her VO2max.
That would be ca 48.7 ml/kg/min.
With lactate threshold being such a relevant and widely used intensity we will repeat the calculations, but now with the threshold values:
48.7 ml/kg/min x 68 kg = 3315 ml
3315 ml / 1000 = 3.31 L
3.31 L x 5 = 16.6 cal/min
So when Kristin is running her threshold workouts she will be burning off approximately 16.6 cal/min as long as her weight still is 68 kg and her oxygen uptake at threshold is the same and that the conditions are very similar to the ones she was tested under. Lots of influencing factors, as you can see, but now we are getting quite close to what she’ll burn during a threshold session.
If she is running on lactate threshold for 5 minutes Kristin will use 16.6 cal x 5 = 83 calories during 5 minutes
The fact is she is probably using even less calories because she it will take a little bitt of time to reach the threshold intensity and keep working that level of intensity for the remaining interval time.
Now I really hope you better understand how the oxygen cost is related to calorie expenditure! You can also see pretty clearly why general tables and lists telling you how many calories you are using for, let’s say 30 minutes of running, says nothing, or at least very little about your body’s use of energy.
How many calories you are burning is dependent on the work that you are doing and the oxygen cost of that work. Of course, individual differences in body composition, sex, training history, genetics, training status, hydration- and nutrition status, your body’s anthropometrics and biomechanical prerequisites and so on will of course also affect energy expenditure in a given task.
The body is indeed incredibly complex and the more I learn, the humbler I feel about what I already know, and whether you are educated in medicine, science or not at all: we all just know the tip of the iceberg! However, understanding how the body uses energy is fundamental and it can increase your understanding of what capacity and energy systems in ypur body you are actually training and how much energy you need when you are using it.
Take home message, even if you are already at home
- “When you are considering caloric burn, is it better to do 7 minutes of HIT (high intensity training) or 40 minutes of zone 1 (light,easy) training?”. The two don’t really compare because you are using different energy systems. However you will use more calories per time on high intensities than on a low training intensity, like 65-72% of maximal heart rate. You will most likely not use as many calories during 7 minutes HIT as in 40 minutes of light training. Really, you can only directly compare 7 minutes of HIT with 7 minutes of low intensity training. Anyway, there are LOTS of factors affecting training effects and calorie consumption/energy expenditure, thus it all depends
- The more liters of oxygen you use, the more calories you will be burning! It is beneficial for both endurance performance and isolated energy expenditure during aerobic training to have a relatively high VO2max.
- The higher the VO2max, the more bang for your buck in terms of calorie consumption you get.
- In other words: More pancakes for all!
Now, I hope you understand a little more about how oxygen demand can govern calorie consumption during aerobic training. If not: Just go a head and ask me!
1.Rapport IS-1816. Fysisk form blant voksne og eldre i Norge. resultater fra kartlegging i 2009-2010, s. 31.