Exercise Intensity and Fat Burning

There is a common misconception that more exercise intensity burns more fat. It is true that more exercise intensity burns more calories but at some intensity level exercise burns more carbohydrates than fat. At even higher intensities there is no fat burned at all. Back to my chart of %VO2max (on the horizontal axis) vs calories of fat (blue) or carbs (brown).

The maximum amount of fat burning is at about 52% of VO2max. At around 78% of VO2max equal calories of fat and carbs are burned. After that point fat drops off quickly and carbs take over. At around 98% of VO2max all the energy comes from carbs. At this point the energy from carbs is over 20 kcals per minute which is nearly 2x the max energy that came from fat.

This is the reason that I have a hard time performing at high intensities. I just don’t have the carb stores to sustain longer high intensity efforts. This is why I changed my training to work more at the lower intensities where I can exclusively use fat as fuel.

It should be possible to push the blue curve over to the right farther through training. Zach Bitter’s numbers (Zach Bitter – Another FASTER participant) show that he uses fat for 98% of his energy at 75% of his VO2max. At that point I am nearly 50-50. I don’t think that this is just dietary fat adaptation but exercise adaptation. Zach can mobilize more fat energy than I can and training explains the difference.


Zach Bitter – Another FASTER participant

Zach Bitter was in the FASTER study (Jeff S. Volek, Daniel J. Freidenreich, Catherine Saenz, Laura J. Kunces, Brent C. Creighton, Jenna M. Bartley, Patrick M. Davitt, Colleen X. Munoz, Jeffrey M. Anderson, Carl M. Maresh, Elaine C. Lee, Mark D. Schuenke, Giselle Aerni, William J. Kraemer, Stephen D. Phinney. Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism, Volume 65, Issue 3, March 2016, Pages 100-110.).

Zach wrote about his experience in the FASTER study (Takeaways from the FASTER Study) where he gave some additional information about the FASTER tests. At least in Zach’s case (and probably in FASTER10 as well as FASTER43) the termination condition was inability to continue.

During the test, the researchers gradually increased both the speed and incline on the treadmill until I could no longer continue, and my rates of fat and carbohydrate metabolism at various intensities were measured.

Zach continued:

I can pinpoint where my fat metabolism and carbohydrate metabolism peak at varying intensities, and I can see the ratio between the two at any given percentage of my VO2 Max.

Zach had a very nice maximum fat oxidation rate and a very high rate of fat usage at a much higher % of his VO2max.

My fat metabolism peaked at 1.57 grams/minute. At this point in the test, my VO2 uptake was at 49.4. By dividing this number by my eventual VO2 Max of 66.1, I can calculate at what intensity I burn the most fat: 74.4%. At that intensity, I was burning 98% fat 2% carbohydrate (1.57 fat grams/minute and .07 carb grams/minute).

% VO2 Max
Fat Usage
Carb Usage

What  Zach is saying is that at 74.4% of his VO2max he is burning nearly 98% fat (his chart shows that point as 75%). We don’t have the rest of the data to see where he was in lower intensities. That may well be a product of his level of training? Clearly he was very fat adapted as well.

Zach’s 50-50% cross-over point (where he burned 50% carbs/50% fat) is much higher than mine. My crossover point was at 75% of VO2max. By my calculations, Zach’s cross-over point was 89% of his VO2max.

My own max fat oxidation was at about 50% of my VO2max where at Zach’s was 50% higher.

Fat and Carb Burning

One interesting thing to look at in VO2max testing is how many grams of carbs and fat are oxidized vs heart rate. [Heart Rate is used as the surrogate for %VO2max]. My chart looks like:

The blue dots are fat oxidation. The brown dots are carbohydrate oxidation. Remember that carbs have 4 kcals per gram and fat has 9 kcals per gram.

Both curves are parabolas. The carbohydrate oxidation curve ends up being the limiting factor in exercise at high rare rates. My sweet spot is 120 bpm. My MAF number is 112-120 bpm. No coincidence there.

Normalizing the graph for calories is interesting since it shows the calories are somewhat comparable.

Here is Ben Greenfield’s data (in kcals also). It is quite similar to mine except Ben has more calories oxidized. Unfortunately I don’t have Ben’s heart rate data but VO2max is directly related to Heart Rate.

It is worth noting that Ben’s cross-over point is around 68% of his VO2max. Ben’s fat max oxidation is very close to my own rate (a little less actually).

Also, Ben barely goes to zero on his carb oxidation and it is at a very low 40% (approx) of his VO2max. Ben has said that his normal carbohydrate consumption was somewhere around 100 grams during the time preceding this test. Ben was relatively low carb for a high intensity athlete like himself.

Here is my data with the x-axis as %VO2max. Ben’s greater access to carb stores may help him.

How much fat was I burning?

It took some maths for me to figure out my own fat oxidation rate from my VO2max testing. Here’s the math (REE from VO2max).

At one point I got over 1.2 g/min of fat oxidation. Not bad considering that high numbers are often 1.6 g/min sort of numbers. Here’s the classical curve – an inverted parabola – of heart rate (x-axis) vs fat oxidation in g/min (y-axis).

The R^2 is 0.87 which is a pretty decent fit.

My sweet spot for maximum fat burning is about 120 bpm. My MAF range is 112-122 which nicely straddles this sweet spot of fat burning found during VO2max testing.

If you do the same maths on VO2max data you can find your sweet spot. But it won’t work well for fat burning if you are not fat adapted already. Or just use the MAF 180 formula.


Another Look at the Lambert Endurance Study

In this post I looked at a study on (Fat Adapted Athletes). I wrote the original post well before I had my own VO2max tests so I didn’t have a way to apply it to my own situation.

I found a link to a paper which is critical of the study (Asker E. Jeukendrup. High-carbohydrate versus high-fat diets in endurance sports.  Schweizerische Zeitschrift für «Sportmedizin und Sporttraumatologie 51 (1), 17–23, 2003).

The practical relevance of the improved endurance capacity at 62% VO2max reported by Lambert et al. [Lambert et al. 1994], however, is questionable since no endurance events are completed at these low exercise intensities.

I think this is a valid criticism of the work capacity of Low Carbohydrate athletes. There is a sweet spot where the diet is the most efficient.

However, in its favor the Low Carb diet lowers the RER and increases the VO2max point where carbs are oxidized. In my own VO2max testing, my RER crossed over the 0.7 line (From 100% fat burning just starting into burning carbs).

My own crossover point was about 59% of my VO2max which is quite close to the point noted above. That is a sweet spot since it has as high of possible fat oxidation with zero carbohydrate oxidation.

It is probably true that there are few competitive sports where operating at this point offers an advantage. Endurance activities of the Long/Slow sort may be an exception, particularly in long ultra-marathons.

In theory, a fat adapted person could perform very long activities at this level. This number is essentially the MAF number in my case. But that’s partly because I am already fat adapted.

What does this look like for the vegan FASTER study participant? His VO2max was an impressive 63.4. So, 63% of that is 39.9. At that value Damian’s %CHO was 88%.  That means 88% of Damian’s energy was coming from carbohydrates. That puts him at a performance advantage if the activity is within his carbohydrate stores but a disadvantage if the activity for a longer time where he needs to access his fat stores.

It’s also an interesting discontinuity in his data at the next point. There his VO2max dropped and his carbohydrate oxidation went way up. Not sure if this was a walk/run transition. Perhaps Damian is a much more efficient runner than fast walker?

I would say that it seems pretty clear to me Damian is not very good at burning fat. His RER never gets below 0.85 which is a mixed fuel mixture. Not sure how low a dietary fat level he is normally at?

Damian’s MAF Number

Damian was 32 years old. His MAF is 180-32 = 148. Unfortunately, there is no Heart Rate column in his data and I can’t figure out what the correlation to the RR BPM column means.

Bought a new toy

I just bought myself a Concept 2 Rower. I used one at Crossfit a couple of times.

I bought it with a Polar H7 Heart Rate strap.

This allows rowing with heart rate monitoring.

I will be using the same MAF heart rate number of 180 minus age (range of plus 0 minus 10). For me, at 58 years of age, that’s 112 – 122 bpm.


Gaining Muscle During a Cut

Leucine seems to be the central amino acid in muscle protein synthesis.

There’s plenty of interest in gaining muscle while cutting fat. Here’s an interesting item from a paper on that subject (Donald K. Layman, Jamie I. Baum. Dietary Protein Impact on Glycemic Control during Weight Loss. The Journal of Nutrition, Volume 134, Issue 4, 1 April 2004, Pages 968S–973S):

During catabolic periods such as energy restriction, supplementation with leucine or a complete mixture of the 3 BCAAs, leucine, isoleucine, and valine, stimulates muscle protein synthesis (35-37).


The three references (35-37) are:

35. Li, J. B. & Jefferson, L. S. (1978) Influence of amino acid availability on protein turnover in perfused skeletal muscle. Biochim. Biophys. Acta 544:351–359.
36. Buse, M. G. & Reid, S. S. (1975) Leucine. A possible regulator of protein turnover in muscle. J. Clin. Invest. 56:1250–1261.
37. Hong, S. C. & Layman, D. K. (1984) Effects of leucine on in vitro protein synthesis and degradation in rat skeletal muscle. J. Nutr. 114:1204–1212.

All three were rat studies. From 36.

The data presented indicate that leucine may act as a regulator of the turnover of protein in muscle cells. They are compatible with the hypothesis that leucine inhibits protein degradation and promotes protein synthesis in muscle.

Human Studies

Lundholm K, Edström S, Ekman L, Karlberg I, Walker P, Scherstén T. Protein Degradation in Human Skeletal Muscle Tissue: The Effect of Insulin, Leucine, Amino Acids. Clin Sci (Lond). 1981 Mar;60(3):319-26.

Satoshi Fujita,et.al. Effect of insulin on human skeletal muscle protein synthesis is modulated by insulin-induced changes in muscle blood flow and amino acid availability  Am J Physiol Endocrinol Metab. 2006 Oct; 291(4): E745–E754.

Changes in muscle protein synthesis were strongly associated with changes in muscle blood flow and phenylalanine delivery and availability. In conclusion, physiological hyperinsulinemia promotes muscle protein synthesis as long as it concomitantly increases muscle blood flow, amino acid delivery and availability.


Increasing Fat Oxidation Rates in Exercise

Fat Oxidation rates can be increased through training. This was demonstrated in this study (Hetlelid KJ, Plews DJ, Herold E, Paul B Laursen, Stephen Seiler. Rethinking the role of fat oxidation: substrate utilisation during high-intensity interval training in well-trained and recreationally trained runners.  BMJ Open Sport & Exercise Medicine 2015.). The study compared well trained (WT) to recreationally trained (WT) athletes on the same HIIT test. Both groups had similar carbohydrate oxidation rates but the WT athletes oxidized substantially more fat than the RT athletes.

Fat oxidation (0.64±0.13 vs 0.22±0.16 g/min for WT and RT, respectively) accounted for 33±6% of the total energy expenditure in WT vs 16±6% in RT most likely very large difference in fat oxidation (ES 90% CL=1.74±0.83) runners.

Despite similar RPE, blood lactate and carbohydrate oxidation rates, the better performance by the WT group was explained by their nearly threefold higher rates of fat oxidation at high intensity.

Note that this was not comparing Low Carb to High Carb athletes but it does offer the possibility that increased fat oxidation can lead to an increase in performance.

2018-07-08 Update: Found this interview with one of the study authors (Paul Laursen, PhD, and Dr. Phil Maffetone: Rethinking The Role of Fat Oxidation At High Intensities, Plus Practical Diet and Training Applications To Yield Results and More).

I don’t think this study invalidates anything related to RER and the VO2max testing. The FASTER study curves shows that some athletes (low carb ones in particular) are better at burning fat at higher intensities.

This does reinforce the value of training to increase fat oxidation rates. Increased volume is only possible through longer training which is done at a lower heart rate.


CrossFit Competition – Festivus Games

Last month, I competed in my first CrossFit competition, the Festivus Games.

I did not do all that great but it was fun. And hard. Mostly hard. A little fun.

That’s 518th out of 666 entries. The only reason I placed that high is that I made it into the finals. The top five people in every box got into the finals and CrossFit Pittsburgh wasn’t all that crowded.


Exogenous Ketone Experiment Results

I finished off the jar of exogenous ketones that I bought and recorded my own experiences.

Effects on Weight

Here’s my weight on the day taken in the morning before and the morning of the day after I took the PerfectKeto Exogenous Ketones.

Date Wt before Wt after Wt Change Calories
5/10/2018 167.95 169.35 1.4 2219
5/11/2018 169.35 170.5 1.15 2243
5/16/2018 165.69 166.1 0.41 2221
5/17/2018 166.1 167.05 0.95 2153
5/21/2018 162.59 164.71 2.12 2504
5/22/2018 164.71 166.51 1.8 2317
Average 1.31 2276
Std Dev. 0.6 112.7

Why Weight Gain?

In my n=1, there was a consistent weight gain (1.3 +/- 0.6 lbs) the day after I took the exogenous ketone supplement. So if you are already on a ketogenic diet and are expecting exogenous ketones to help you lose weight you could be really surprised and disappointed. I wasn’t disappointed – I did this experiment since my body weight is fairly stable and I am not concerned about gaining a lb or two.

The question is especially pertinent since higher ketone levels are usually indicative of fat burning and weight loss. There’s some belief that exogenous ketones cause your own body to reduce its generation of ketones. I can’t say whether or not I would have lost weight on those days but I can say that there was a strong correlation to weight gain on the days I supplemented with the product.

It wasn’t extra calories consumed on those that caused the weight gain. My average calorie count is right in line with my maintenance calories. At the very least there was nothing in the food consumed that day that would come anywhere near a 1.3 lb weight gain.

Also, there’s only 15 calories in each product serving so there weren’t enough calories in the supplement itself to cause such a consistent and significant day-to-day weight gain.

The ingredients list might be a clue. Magnesium, Potassium, Cocoa, Stevia leaf, Vitamin C.

They don’t list Sodium as an ingredient although some exogenous ketones include Sodium. If there was Sodium then that could be the reason for the temporary weight gain. But the binding salt could be the Magnesium or Potassium. There’s no reason to think that Potassium alone should cause temporary weight gain. I wonder if there is Sodium? That could provide the most plausible explanation for the temporary weight increase. The Perfect Keto site states:

Ketone salts are formed when the ketones are bound to a salt, typically sodium or calcium, potassium, or magnesium, to improve absorption rate.

This site (BHB Salts) states:

The “BHB salt” is simply a compound that consists of sodium (Na+), potassium (K+), and the ketone body β-hydroxybutyrate.

The weight gains were temporary and quickly reversed in the days after I discontinued the product.

Rat studies showed rats gained less weight than controls when were fed ketone supplements (Shannon L. Kesl, Angela M. Poff, Nathan P. Ward, Tina N. Fiorelli, Csilla Ari, Ashley J. Van Putten, Jacob W. Sherwood, Patrick Arnold, and Dominic P. D’Agostino. Effects of exogenous ketone supplementation on blood ketone, glucose, triglyceride, and lipoprotein levels in Sprague–Dawley rats. Nutr Metab (Lond). 2016; 13: 9.).


The Keto Base Chocolate Sea Salt product costs $59 for 15 servings. That’s pretty expensive. That’s about $4 a serving which seems like a lot compared to my other supplement costs.

Mood Changes?

I think I saw a very small mood improvement but I really can’t quantify this. It may have had more to do with having more water from salt?

I did notice I developed a tiny bit of an addiction to the product. Perhaps it was just the chocolate salt mix taste that I was looking forward to. It wasn’t hard to stop so the addiction wasn’t that strong and the $4 a serving made it easier (that’s about the amount of money I spend each day on food anyway).

Performance Increases?

I didn’t perform any particular exercise benchmarks to determine if I had any performance increases. I also took the supplement early in the morning in my first coffee so I wasn’t exercising around the time. I also don’t know if it would have any time effect with respect to exercise.

Advantages of Exogenous Ketones?

I can imagine a potential use of exogenous ketones in people who are just transitioning into the ketogenic diet. It could allow them to make the transition easier and may ease the keto flu. It would be interesting to see if it has any better effects than electrolyte supplementation – which is a lot cheaper.

However, this is an application that I can imagine few taking due to the cost. People “try” diets and starting this diet with a $60 product might be too much for most people to swallow (pun intended).