Seeing is believing.
Sometimes it takes looking at the hard data from your own body to really change your mindset, habits, and beliefs.
When I first began getting into health and fitness one of the aspects that helped motivate me was looking at numbers. It started with a simple act of tracking my heart rate, distance, and pace when I was running. As I became more physically fit over time I could see these numbers change and improve. With time I also learned to correlate my performance metrics I tracked while running with other lifestyle factors like sleep, stress, and diet.
Then I began to learn about the data you can gather in a simple blood test. Being able to correlate the way I was feeling with various blood markers was enlightening. I could make tweaks to my lifestyle and go get my blood drawn a few months later and see the changes right there in front of me!
From the time I first began paying attention to my health and fitness until now the ability to quantify the changes you are making to improve your health and fitness have evolved immensely.
CGMs, fitness wearables, glucometers, sleep trackers, and HRV were all once fringe and are now commonplace.
The reason why I like data, hard numbers, is because numbers don’t lie.
Do they tell the whole story? Should numbers define you? Are they always an accurate reflection of what you are trying to quantify?
No, no way.
However with consistency, time, and context you can at least get a good idea of the general direction you are heading.
Take the scale for example.
If you step on the scale and it says you weigh 200 lbs as a 6ft male, it really doesn’t tell you much. You could be 200 lbs and be 40% body fat or you could be 200 lbs and be 10% body fat.
Weighing 200lbs, that number, doesn’t really tell you much about your overall health.
However if we put CONTEXT around it, now that is powerful stuff!
CONTEXT is key.
This then brings me to my “fat burning” experiment I ran on myself and why I wrote the previous 2 blog posts(post 1, post 2) on carb vs fat utilization during exercise. I needed to give you some context around how the body works in order to explain what I was seeing in my experiment.
Over the years I have had a battle going on internally.
I had gone down the low carb path, I had done keto, I had seen all the benefits of being a “fat burning” athlete.
However, it did not seem to agree with my body. I could not eat enough to support my activity levels eating low carb. I needed more calories and I knew those calories had to come from carbs (I was already eating enough protein and eating fat just for the hell of it was not appetizing), but what if I lost my ability to burn fat in the process? What if I became dependent on carbs?
It was hard for me to rectify this problem. I had blood work to prove my body needed more calories, I knew it had to come from carbs, but I did not want to lose the benefits of utilizing fat as well.
Eventually I decided I had all the tools to track the right metrics to put my mind at ease.
Slowly but surely over the course of years I increased my carb intake up to around 350-400g per day.
My strength and performance improved.
I could lift more weight in the gym.
My cardiovascular fitness improved.
I was performing the best I ever had in obstacle course racing.
Yet I could still go out and run for hours without needing to slam carbs along the way.
I didn’t have cravings for processed foods.
I didn’t have energy crashes.
Blood markers that were always out of range suddenly started to look normal.
And guess what, every marker of metabolic health, fasting glucose, hemoglobin A1C, fasting insulin, c-peptide, and triglycerides never left the optimal range!
Now I should note that the carbs I was eating were based around whole foods: potatoes, squash, fruits, white rice, and select gluten free whole grains.
While going through this journey myself, I still heard many people claiming that the levels of carbs I was eating was going to cause my body to be continually burning carbs during exercise and unable to tap into fat as a fuel.
I knew this was not true, because I saw it in myself.
I knew I wanted to share this information but I also wanted to gather some more data.
Fortunately I have access to a metabolic cart so I could use that to measure the amount of fat and carbs I was burning at rest, during an easy aerobic exercise session, and during an all out max effort.
Using the metabolic cart I would do what’s called a resting metabolic rate test. This test basically involves you laying down and breathing into a mask for several minutes. At the end of the test you can determine how many calories you burn at rest (aka your metabolic rate) and what the ratio of fat vs carbs you burn at rest.
The other test I did is a V02Max test. During this test you start out at a very easy intensity and continually increase the intensity every minute until you reach a point where you can no longer sustain that level of work. It’s not nearly as fun as the resting metabolic rate test but gives you a great measure of your overall cardiometabolic fitness and also allows you to look at the amount of carbs and fats you burn at various intensities.
Finally I wanted to do several 5000 meter rows at a very low intensity while manipulating the diet I was eating to see how it affected the amount of fat and carbs I was burning. Below are the six conditions I rowed the 5000 meters under.
Normal Diet: 150g of protein, 350-400g of carbs, 90g fat
Condition 1: Normal diet day before, 100g of carbs in meal before 5k
Condition 2: Normal diet day before, done first thing in the morning, about 12 hours before dinner the night before
Condition 3: Normal diet the day before, done after 15.5 hours after eating dinner the night before
Condition 4: Low carb the day before (less than 50g), done first thing in the morning about 12 hours after dinner the night before
Condition 5: Normal diet the day before, 100g of carbs in the meal before the 5k, plus 2 shots of HVMN Keto IQ (20g of ketones)
Condition 6: Low carb the day before (less than 50g), done about 15 hours after dinner the night before, 2 shots of HVMN Keto IQ (20g of ketones)
Would the science and research we talked about in the previous blog posts (post 1, post 2) play out or had I severely hindered my ability to burn fat by consuming large amounts of carbohydrates?
Let’s find out!
First up was the resting metabolic rate test.
Here is a graph of the amount of carbs vs fat I burnt during rest.
The light green line is carbs, the dark green line is fat. Overall I had a mean fat burn of 92.7% and I burnt a mean of 7.3% carbs.
The verdict: clearly I am burning mostly fat in a fasted state at rest despite the amount of carbohydrates I am eating.
That looks all good.
Now onto the 5000 meter tests.
Below is a table I put together comparing all 6 conditions. To make things easier to reference I also included the description of all 6 conditions below the table.
|
Condition |
Avg HR (BPM) |
Avg Watts |
Total Breaths |
Mean Fat % |
Mean Carb % |
Total Fat (kcal) |
Total Carb (kcal) |
|---|---|---|---|---|---|---|---|
|
1 |
98 |
93 |
302 |
57 |
43 |
171.6 |
130.5 |
|
2 |
105 |
93 |
347 |
56 |
44 |
161.8 |
129.2 |
|
3 |
93 |
95 |
386 |
78.5 |
21.5 |
217.5 |
60.1 |
|
4 |
102 |
91 |
321 |
81.8 |
18.2 |
236.6 |
52.9 |
|
5 |
96 |
90 |
314 |
71.7 |
28.3 |
209.2 |
83.1 |
|
6 |
90 |
84 |
394 |
75 |
25 |
214.8 |
72 |
Normal Diet: 150g of protein, 350-400g of carbs, 90g fat
Condition 1: Normal diet day before, 100g of carbs in meal before 5k
Condition 2: Normal diet day before, done first thing in the morning, about 12 hours before dinner the night before
Condition 3: Normal diet the day before, done after 15.5 hours after eating dinner the night before
Condition 4: Low carb the day before (less than 50g), done first thing in the morning about 12 hours after dinner the night before
Condition 5: Normal diet the day before, 100g of carbs in the meal before the 5k, plus 2 shots of HVMN Keto IQ (20g of ketones)
Condition 6: Low carb the day before (less than 50g), done about 15 hours after dinner the night before, 2 shots of Keto IQ (20g of ketones)
As you can see the intensity of the row based on heart rate and watts was relatively the same between all 6 conditions. This is important because as we have discussed in the previous blog posts intensity is one of the main determinants of fuel utilization. Since intensity was relatively the same that variable is taken out of the mix.
The only variable that is changing is diet.
In conditions 1 and 2 we can see the effect diet has on my fat burning. In those conditions I burned a lot more carbs. According to the percentages it was still mostly fueled by fat but for arguments sake it was 50/50 carbs and fat. It makes complete sense based on what we talked about in the first blog post of this series. If you have plenty of carbs around both in the muscle and in the bloodstream the muscle will prioritize them due to needing to dispose of them anyways.
Fat vs Carb In Condition 1
If I extend the time between my last meal after an overnight fast or ate low carb the day before I can get my body to use around 80% fat. This is what we see represented in the data from conditions 3 and 4.
One of the puzzling parts of this experiment was the drastic difference in fat utilization just by extending my overnight fast by 3.5 hours as I did between condition 2 and 3. It could be that when I did the row after a 12 hour fast that day something else was affecting my physiology, maybe it was my sleep, stress, or something else entirely, it’s hard to say.
None the less condition 3 and 4 shows that my body can still access and utilize fat as a fuel if I restrict the amount of carbs I eat around exercise.
Fat vs Carb In Condition 3
Conditions 5 and 6 is where I introduced exogenous ketones in the mix.
Now I will preface this and say that we honestly don’t know if the metabolic cart data is that accurate in the presence of ketones. The algorithms used to calculate fat and carb utilization when using a metabolic cart were not developed considering ketones so it could be that it is not an accurate representation of fuel utilization (Frayn, n.d.).
That said I do think condition 5 is of particular interest because it’s comparable to condition 1 where I ate my normal diet plus had 100g of carbs beforehand but the fat and carb usage is drastically different.
Fat vs Carb Condition 5
As we discussed in the first blog post, ketones get priority in terms of fuel utilization and that would mean it would blunt the utilization of carbohydrates even if there are plenty around to be burned. As we discussed, one of the evolutionary advantages of ketones was the sparing of carbohydrates. Naturally ketones would only be present in the body in times where there was not an excess of carbohydrates available. In other words, ingesting a bunch of exogenous ketones in the presence of carbohydrates is a completely unnatural state to be in, but it appears that the body will always prioritize using them over anything else….it thinks it needs to conserve glucose.
The question is whether I really burned as much fat as the metabolic cart thought I did or if those were actually ketones? Unfortunately we can’t tell.
The main take away for me from condition 5 is that if you are looking to conserve carbs in the body because you will be moving for a long period of time, exogenous ketones may be something you want to try.
In condition 6, where I ate low carb the day before, extended my fast, and took exogenous ketones I did not see any greater utilization of fat than the exact same condition minus the ketones in condition 3. This leads me to believe that if you are already in a state where your body is primed to utilize fat, ketones won’t increase the utilization of fat…they don’t make you a better “fat burner”.
The last thing to look at is the VO2Max test and how my fat and carb utilization looks when I go all out. My diet leading up to this test was “normal” as I described above.
The bright neon green line is my heart rate.
The purple line is the watts I was producing on the rower. As you can see every minute during the test I increased the wattage until I reached a point where I maxed out and could no longer hold the wattage I was rowing at.
The dark green line is my fat utilization and the light blue line is my carb utilization.
Up until the 600 second mark I am using predominantly fat for the test. At that point my heart rate was 140 beats per minute and I was producing around 190 watts on the rower. From that point on carbs start becoming the predominant source of fuel. As we learned in the first post the reason for this is because fat can no longer keep up with the rate of ATP needed by the muscles.
It is also interesting to look at my breathing frequency.
In the above graph the purple line is my breathing frequency, how many breaths I am taking per minute. There is something interesting that happens once I get to around 50 breaths per minute, my fat utilization drops to virtually 0. Around 50 breaths per minute is when you start hyperventilating, meaning you are removing a ton of carbon dioxide from your body. Carbon dioxide is needed to get oxygen into the muscle so without carbon dioxide you are basically starving your muscle of oxygen. Without oxygen it’s impossible to burn fat. Once I hit this point it was only a matter of time before I had to stop the test. The accumulation of hydrogen ions from metabolizing carbs without oxygen is going to eventually cause me to stop and sure enough, I quit not too long after that point.
That all being said, the fat and carb utilization from my test is pretty good and indicates I can utilize fat and carbs under the appropriate intensities.
If we compare to research (Assessment of Metabolic Flexibility by Means of Measuring Blood Lactate, Fat, and Carbohydrate Oxidation Responses to Exercise in Professional Endurance Athletes and Less-Fit Individuals, n.d.) done on elite cyclists, moderately active individuals and individuals who have metabolic syndrome, I am clearly not as good as the elite athletes, but it looks fairly similar.
https://pubmed.ncbi.nlm.nih.gov/28623613/
It is obvious in the figure above that when you have metabolic syndrome you are completely carb dependent and will start utilizing carbs for fuel even at very low intensities. This was the fear I had when I started to increase my carbs, I feared I would no longer be able to utilize fat. Turns out all that was false. This is why I am saying CONTEXT is important. If you are already unhealthy or you are consuming more carbs than you need (more on this below) you can end up not being able to tap into fat as a fuel, but it is not as black and white as people make it seem.
I am also willing to bet that many of the low carb athletes could be leaving something on the table in terms of performance if they are too low carb for too long.
Their ability to burn fat may be much higher throughout a VO2Max test but they may be unable to tap into carbs once they start to reach higher intensities. This limits their ability to perform as well as they otherwise would.
Here is one such example I found on highlighting the “benefit” of being low carb during what is supposed to be an all out test.
https://vespapower.com/vespa-r-d-team/vespas-research-development-philosophy/
Notice this person reaches their VO2Max but has not even crossed over into predominantly carbs!
You might say “WOW! how can I do that?”
To me though this looks like a potential serious problem.
For one, we have no idea what their VO2Max was so it’s impossible to say if it is good or bad.
Regardless of that though, what if this person could have done better if their body could burn more carbs? In theory the body is working harder to fuel this level of intensity with fat. If they could utilize carbs more they could not work as hard at the same intensity and push even harder!
The main takeaway for me from this experiment is that despite consuming 350-400g of carbs a day for several months I was still able to maintain the metabolic flexibility to utilize fat under the right conditions. I think this is an important point.
Eating the proper amount of carbohydrates for you is not going to inherently make you unhealthy or turn you into a carb burner.
Now everyone is different, so just because I can eat this amount of carbohydrates and still maintain my health and perform the way I want doesn’t mean you can.
So how do you know if you have started to push your carbohydrates too high?
Unsurprisingly, health markers related to carbohydrate consumption and your ability to continue to utilize fat and carbs during exercise are connected.
If you start to see a rising fasted glucose number, a rising hemoglobin A1c, a rising C-Peptide, and a rising fasted insulin number I bet you would also see an inability to utilize fat as a fuel under a variety of conditions.
I do think though that people who are training hard and eating whole foods who are already metabolically healthy can probably stand to eat more carbohydrates than they currently are eating. They would likely see improvements in performance without any deleterious health impacts.
Experimentation is key, and you do not need to go to the extreme I have in order to test this.
I have written blog posts on the importance of measuring and tracking fasting blood sugar as well as how you can enhance your metabolic flexibility. You can easily use the information in those posts to assess your ability to burn fats and carbs and then actions you can take to address any limitations you find.
When we make health and fitness black and white it makes things seem so easy. Eat carbs and you are a carb burner. Restrict carbs and you burn more fat. Burning more fat makes it sound like you will lose body fat, and with so many people interested in looking good it gets a lot of attention. However just because you burn more fat during your exercise does not mean you will lose more fat or any fat at all and as we have demonstrated it also may make your workouts worse. My goal is to inject nuance into the discussion around health and fitness, because health and fitness is not black and white. I also strive to give simple actionable steps you can take that allow you to reach your goals but are based around this nuance.
If you are looking for a more balanced approach to health and fitness sign up for my newsletter using the form below. Each week I will provide you with free information meant to help you reach your goals in a balanced, sustainable manner.
References
Assessment of Metabolic Flexibility by Means of Measuring Blood Lactate, Fat, and Carbohydrate Oxidation Responses to Exercise in Professional Endurance Athletes and Less-Fit Individuals. (n.d.). PubMed. Retrieved March 24, 2023, from https://pubmed.ncbi.nlm.nih.gov/28623613/
Frayn, K. (n.d.). Calculation of substrate oxidation rates in vivo from gaseous exchange. PubMed. Retrieved March 23, 2023, from https://pubmed.ncbi.nlm.nih.gov/6618956