Diary of a bodacious dietitian, Fitness

VO2 Max and the Fat Burning Zone

Efficient Exercise

We’ve all seen those color coded zone diagrams on the fitness equipment at the gym. Work this hard and you are in the Fat Burning Zone, a bit harder and you are in the Aerobic Zone. We all want to exercise and work more efficiently. Do more in less time. See results with less work.

My husband, Dr. P, likes to think of it as conservation of energy for better things- like if we exercise or need to clean the house or have the obligatory spousal conversation about the status of our relationship in less time, then we have more time and energy to party, man. Sounds good to this girl!

To figure out how to best exercise, we should identify where our bodies switch from one energy system to the next- more on this in a minute. When the exercise machine tells us that Zone 1 (the fat burning zone) becomes Zone 2 (the aerobic zone) becomes Zone 3 (the intense fitness zone), what it is really doing is guessing when our energy systems are changing. Think about a light jog (Zone 1) vs a hard run like an 800 meter dash (Zone 2) vs a quick, hard Usain Bolt sprint (Zone 3).

Working through from one zone to the next helps us increase our fitness levels faster and also likely helps us lose weight faster by maximally challenging our bodies.

An exercise machine can’t accurately tell us where our unique zones start and end. We need a fitness test for that.

weight rack

The Fitness Assessment + Korr Machine

I recently started doing independent contracting with a boutique fitness studio in DC, and we administer fitness tests to determine VO2 max and AT by using a Korr machine. The AT is the point where we switch from Zone 1 to Zone 2. AT is also called the lactate threshold (LT)- more on this in a moment.

When I first saw this machine, I wondered- how does it figure this out? I am always one to be curious. I looked it up and it actually monitors gas exchange. As our need for O2 increases, we create more carbon dioxide (CO2). As we cross the LT, we also release bicarbonate (CHO3) which is a metabolite of lactate. When the ratio of CHO3 + CO2 to O2 reaches a certain point, the machine identifies the AT based on an equation using metabolites (CO2/CHO3) relative to O2 in the breath.

The guy who discovered this was named Wasserman, and he dubbed this equation the Wasserman’s theory of gas exchange equation. Very clever indeed.

The machine notices that our bodies are going anaerobic and at this point in the test, it matches it with a heart rate zone and the AT is identified.

Coming back to the fitness assessment- also called a graded exercise test. We have clients run on a treadmill for 12 minutes while hooked up to the machine with a face mask that looks like a gas mask contraption. We measure their breathing while we try to tire them out by steadily increasing their speed and incline. We essentially make them run their arse off on a steep incline until they fatigue.

Sounds fun, right? It is actually super awesome!

When they reach exhaustion, we give them a breather, sit them down and tell them what their AT and VO2 max. What does this mean, they ask?

Excellent question. Let’s go over cardiac output and VO2 first, then we’ll delve more into energy systems and the AT threshold.

Cardiac Output + VO2 Max

While we exercise, a couple things happen. We start breathing deeper and faster to get more O2 to our working muscles -about 80% of blood flow during exercise is used up by the muscles. Our hearts start to pump harder- called stroke volume- and it also starts to beat faster- called heart rate. When we multiply stroke volume x heart rate, we get cardiac output.

Cardiac output is one of the factors affecting our fitness. The other major factors are the ability of our lungs to absorb O2 from the air, our blood cells to carry it (think iron and anemia connection) and the ability of our muscles to take it up from the blood. If we have healthy lungs and are in shape, we will have more mitochondria in our muscles to handle the O2 supply.

Mitochondria are like little factories for energy creation. They can work with O2 using fat for energy or without, using sugars for energy. When we exercise a lot, the number of mitochondria in the muscles increases and that means we can better utilize O2 to make more energy and therefore we can run longer and harder while staying aerobic- ie, we can run a marathon faster, longer, and harder because we can work at a higher level while still being in the aerobic zone.

As we stay motivated and keep up with our program, our heart actually remodels (called an Athlete’s Heart). The remodeling of our heart occurs by thickening of our cardiac chambers which enables us to pump more blood. This is different than the thickening of the heart when we have heart disease. An Athlete’s Heart is more flexible, while a sick heart is not- big difference (think of bending stick vs a rubber band).

This is why in shape people have lower resting heart rates- their heart is stronger and doesn’t need as many beats per minute.

This heart remodeling can be seen in less than a year, and when this happens, we are in noticeably better shape. Our VO2 max is higher. Increases in mitochondrial density, however, occur first and therefore we can increase our VO2 max and AT in just a month or so. Again, another reason to not give up on our exercise programs. Progress always occurs.

So now we know: VO2= cardiac output x O2 utilization. VO2 max is our top ability to consume O2. And we can measure this and track it over time.


Aerobic Threshold (AT) + Energy Systems

Three energy systems work in concert all the time to make sure we always have oxygen: the aerobic system, the anaerobic glycolysis system, and the anaerobic phosphagen system .

The 3 energy systems

The aerobic system-meaning “with oxygen”- is very efficient. Fat has more calories than carbs, so it is more efficient for our bodies to use fat. It produces energy from fat, glucose, and glycogen, and it burns more fat than the other 2 systems combined.

It’s like driving a Pruis vs a mack truck- staying aerobic is waaaay more energy efficient than going anaerobic.

There is evidence that working at about 54% of our maximal heart rate- in Zone 1 here- maximizes  this fat burn.  So doing a fast walk or light jog is actually scorching fat- excellent news for those of you that don’t love super intense exercise!

When we don’t have enough O2, the anaerobic system- meaning “without oxygen”- starts to take over. It mostly breaks down glycogen to create blood sugar. We produce substantially less energy when we are anaerobic, and it is not sustainable due to toxic metabolites that build up.

Lastly, the anaerobic phosphagen system uses a molecule called creatine phosphate to fuel very, very short intense bursts of energy. This is the first energy system to be activated when we start exercising, but it runs out very quickly because we only store a tiny amount of creatine phosphate. More on energy systems here.

Through exercise, we can increase our AT and therefore stay aerobic for longer. This can be measured and tracked over time with fitness testing, and we can actually see ourselves getting in better shape with increases in AT.

AT is unique to everyone based on size, gender, level or fitness, and genetics- hence the need for an individual test. And important to retest, because we can raise our AT 10-15% within a few months.

Top tier athletes have a super high AT that can reach 90-95% of their VO2 max which means they can peddle, swim, and run harder and for longer periods of time due to their body’s superior ability to use O2 aerobically.

Important to note, the 3 energy systems are all working, all the time- it is a question of which one dominates at any given time.

Moving from one Zone to another: Periodization Training

So what causes us to move from being aerobic to anaerobic?


A couple things we should notice. First,  the lactate threshold- or as I mentioned, the AT- is when lactate first starts being produced. You know that burning feeling in our muscles when we exercise hard? That is lactate buildup, and it stops our muscles from being able to continue working. This is one of the anaerobic toxic metabolites.

So when lactate is starting to be produced, this is when we know that we are no longer primarily aerobic. We are moving out of Zone 1 . We are no longer using fat as our primary fuel source. Instead, we are using a higher percentage of glycogen and free blood sugar.

Secondly, check out OBLA on the graph. This is the onset of blood lactate accumulation and this means that we are starting to have trouble maintaining effort because too much lactate is wreaking havoc on our poor muscles.

We are even more anaerobic here. Think of the Usain Bolt sprint again. It is difficult to maintain. But maintaining difficult exercise overloads our bodies.

Overloading is the key ingredient to increasing both VO2 and AT, and in Zone 2 and Zone 3 is where we do this. This is why it is efficient to work in all three zones- we maximize fat burn and fitness with interval training.

So we need a workout that takes us up and down through these zones to maximize our fitness. Working through these zones is called periodization training.

And here is a fantastic example of a weekly periodization training schedule you could try. After Day 3, take a day off and start over:

Day 1: Work Zone 1-Low intensity day with steady state cardio in fat burning zone for 60 minutes at 50-75% heart rate reserve (HRR). This could be elliptical, bike, light jog or fast walk. Talking should be easy.

Day 2: Work Zone 2- Complete fast paced runs in 1- 2 minute intervals with 1 minute light jog or walk in between. Work at 80-85% HRR during the run. This should be difficult- talking will be hard. Repeat pace runs over 20-30 minutes.

Day 3: Work Zone 3- High intensity day using sprint work on treadmill, elliptical, or bike. Fast intervals of 15-20 seconds working at 90-95% HRR. This is very difficult and talking is impossible. Complete a series of 5-10 20-second sprints.

Since some of us can’t access a graded exercise test, we can guesstimate our heart rate zones. But the creator of the MHR equation did caution that it shouldn’t be necessarily used to determine training intensity. It is often used, but remember, the only accurate way to know your AT and zones is through fitness testing.

You can also use the talking test, as mentioned in the workout descriptions, where perceived exertion is measured based on your ability to talk. I find this is fairly accurate at estimating as well.

We can also measure our approximate VO2 max based on an equation, but this isn’t necessarily accurate. Research has shown tons of variation here. If interested though, check out this post.

Thanks for staying with me! Hope this was helpful and engaging. Remember, exercise does get easier as your body adjusts. So just start somewhere. That is my version of JSS! (Inside joke for you Walking Dead fans!)


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