Waterloo Chiropractor, Physiotherapist, Massage Therapist (RMT) and Registered Dietitian



keto 1

Welcome to part 2 of the my exploration of very low carb diets for endurance athletes.  My last article provided the basis for understanding this article as it explored how and when our body choses to use fat vs. carbs.  Check it out here.

In this article, I will now explore (1) do low carb diets actually enhance fat metabolism and (2) does that actually makes us faster.

Does VLCD increase fat burning capacity?

Short answer, yes, the body is forced to increase fat use to support the energy needs during exercise. Research clearly shows that after adapting to a keto diet for as little as 3 weeks results in significantly elevated rates of fat oxidation (0.6g/min to 1.5g/min) during exercise. Fat oxidation at moderate intensities (65% VO2max) in elite ultra-endurance athletes on a keto diet contributed 88% of the fuel for exercise verses 56% in athletes consuming high carbohydrate diets. Now, if we remember what we discussed in the last article, we learned that as we increase intensity we increase the amount of carbohydrates burned. This begs the question “will those high fat oxidation rates continue at intense ecercise (80% VO2max)?”. Another study investigated the fuel usage of elite race walkers at 80% VO2max, and they too found that fat oxidation was elevated to the same levels as previous research (1.5g/min).

low carb

Research Outcomes of VLCD and Performance:

The more important question in my mind (and likely yours as well) is “well that is fine and dandy- my body will burn more fat, but what will happen to my performance?!”. We will review some key research studies that have looked at fat adaption diets (high fat diets for 3-7days), keto diets and their effect on performance. The majority of high fat diet adaptation and keto diets find that performance decreased and a handful found they had no-statistically significant effect. Only two articles find a performance benefit.

Keto, Training and Performance

Louise Burke et al. (2017) conducted a large study investigating the effects of a keto diet, chronically high carbohydrate diet or periodised carbohydrate diet on race performance of elite race walkers after a 3 week intervention and training camp. Athletes on the keto diet perceived the training to be significantly more difficult and experienced an inability to complete the exercise training sessions planned. This is a very important point because if an athlete cannot train as hard as they could they won’t see much improvement in their sport.

After the 3 weeks of intense training, the keto group had higher fat oxidation compared to the two high carbohydrate groups. All groups had significant increases in their maximal oxygen uptake (VO2max) as a result of the training. As we discussed in the previous article, burning fat is less efficient and this study clearly demonstrated that at all competition race speeds there was significantly more oxygen used in the keto group and there was no change in the fraction of VO2max at various speeds. The high carbohydrate and periodised carbohydrate groups used less oxygen and were able to keep the same pace at lower fraction of VO2max. In plain English, the two carbohydrate groups improved their running economy and efficiency with the training where the keto group did not reap the benefits of the training because the cost of burning fat is so high.

Lastly, this study compared pre and post training performance walk times in a real 10km race. They found that both carbohydrate groups had a reduction in their time by 5-6% (on average 190s and 124s faster for high carbohydrate and periodised carbohydrate group). There was no improvement in the keto group and on average their times were 23s slower. There was a wide variability in performance for the keto group, ranging from 162s faster to 208s slower, meaning that keto worked for some individuals but not others.

High Fat Diet With Carbohydrate Loading

What if we don’t go into ketosis and we use a fat adaptation strategy + carbohydrate load, best of both worlds right? Havemann et al. (2006) showed that when elite cyclists consumed a high fat diet (68%) for 6 days with 1 day of carbohydrate loading that there was no significant difference in time to complete a 100km simulated bike race compared to a traditional high carbohydrate diet. However, if we look at the time to completion, we find that the high carbohydrate trial was completed on average 3 minutes 44 seconds faster (likely significant in the real world), leading us to believe that on average high carbohydrate diet may be superior to high fat diets. Again, 3 out of 8 racers on the high fat diet did improve their time compared to high carbohydrate diet, demonstrating that there may be some athletes who may respond well to a high fat diet.

More importantly, this research included 1km sprints throughout the ride to simulate a race like situation and found that the power output was significantly lower in the high fat diet group which lead to slower sprint times. Despite having lower power output in the high fat trial, they perceived they were working as hard as they were in the high carbohydrate trial. There was no difference in muscle recruitment during the sprints, meaning the high fat trial worked just as hard as the carbohydrate trial but did not achieve the same results in the sprint performance. The researchers thought that the high fat diet + a carbohydrate loading period would result in glycogen sparing due to increased reliance on fat for fuel, thus improving sprint times as sprinting relies on glucose to provide fuel. This was not the case and it is possible that high fat/fat adaptation diets reduce the ability to effectively burn carbohydrates.


  • VLCD do result in higher rates of fat oxidation during exercise
  • VLCD may reduce response to training
  • VLCD decreases economy in elite athletes
  • VLCD decreases ability to work at maximal effort which is important when there is change in work intensity- ie running up a hill, breaking away from the pack
  • Most studies show that on average VLCD negatively affect performance in endurance athletes, however there are some that may respond well
  • Remember that VLCD are not the same as training fasted or temporarily low carb diets to train your body to use fat more effectively, as this is an effective training method

In my final article in this series, I will explore the roll of supplements, the keto diet, and how that relates to athletic performance.


Volek JS, Noakes T, Phinney SD. Rethinking fat as a fuel for endurance exercise. Eur J Sport Sci. 2015;15(1):13- 20.

Volek JS, Freidenreich DJ, Saenz C, Kunces LJ, Creighton BC, Bartley JM, Davitt PM, Munoz CX, Anderson JM, Maresh CM, Lee EC, Schuenke MD, Aerni G, Kraemer WJ, Phinney SD. Metabolic Characteristics of keto-adapted ultra –endurance runners. Metabolism. 2016;65(3):100-10.

Burke LM, Ross ML, Garvican-Lewis LA, Welvaert M, Heikura IA, Forbes SG, Mirtschin JG, Cato LE, Strobel N, Sharma AP, Hawley JA. Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J. Physiol. 2017;595(9):2785-2807.

Havemann L, West SJ, Goedecke JH, Macdonald IA, Gobson ASC, Noakes TD, Lambert EV. Fat adaptation followed by carbohydrate loading compromises high intensity sprint performance. J. Appl. Physiol. 2006;100: 194-202.

Leckey JJ, Ross ML, Quod M, Hawley JA, Burke LM. Ketone diester ingestion impairs time-trial performance in professional cyclists. Front. Physiol. 2017;8(806).

53650544 - foods high in carbohydrate on rustic wooden background. top view

There has been a burst of social media hype around the idea of using very low carbohydrate (ketogenic or keto) diets for endurance events. The keto diet usually consists of 5-10% of total kcal (~50g or less) from carbohydrates per day, 75% from fat and 10-20% from protein, although there is no set standard of carbohydrate level. In this article I want to talk about how the body uses energy during exercise before we get into the research.

If we look at the stores of energy in the body, we know that our ability to store carbohydrates as glycogen is limited. We can store carbohydrates in our muscles and liver and the more trained an individual is the more they can store, however it is still limited and can only supply about 1500-2000kcal and muscle glycogen is depleted within 1hour of intense exercise. Fat on the other hand is very calorically dense and we can store 65 000 kcals (could fuel about 20 marathons!) in our adipose (fat) tissue and within the muscle. Fat can supply significantly more energy than carbohydrates without supplementation. This diet is seen as attractive because it promotes the idea that we can max out our capacity to burn fat as a fuel while running, reducing the amount of food or energy we need to take on the run. Therefore, it seems logical to adopt a keto diet, max out our ability to burn fat as a fuel, right? Well let’s look at this topic a little deeper.

low carb


Understanding Energy Production During Activity

Our body uses two processed to produce energy in the form of adenoside triphosphate (ATP). The first method is anaerobic (without oxygen) and the second is aerobic (with oxygen). There are 3 energy systems that the body uses depending on a few factors, such as intensity, length of event, and availability of oxygen.

(1) Phosphocreatine System

This system is the quickest way to produce energy and is the first system to turn on to crank out ATP. Let’s say as you are reading this article your fire alarm stated to ring, you would immediately jump up and run out the door. In this fight or flight response you are using mostly the phosphocreatine system. It works by taking the phosphocreatine that is stored in the muscles, and through an enzymatic reaction the phosphate is split off and added to adenosine diphosphate to make ATP. This is a very simple reaction, and the body can use this system for about 8-10 seconds before it’s tapped out. This system can recover; it takes about 4 minutes before the system is ready for another intense bout.

(2) Anaerobic glycolysis

This is the second system to turn on to support energy production. To continue with out analogy from above, after our initial jumping and sprinting for the door, after the 8ish seconds our body tends to rely more heavily on the anaerobic glycolysis system. This system uses carbohydrates (glucose) and through a series of steps breaks down 1 glucose molecule into 2 pyruvate molecules. Without oxygen, this pyruvate can be further broken down into lactate and a hydrogen ion and ATP. This system will last about 1-2 minutes. In repeated sprints, such as hockey shifts, research shows that subsequent sprints rely on anaerobic glycolysis to provide 50% of the energy.

(3) Oxidative phosphorylation (Aerobic system)

This system takes a while to get warmed up and to get going once you start exercising. In untrained individuals, it can take a few minutes for this engine to get running fully, but in elite athletes it can fully turn on within about 30-60seconds. In our analogy above, lets say we lived in the country and we needed to go run to the fire station down the road, after our initial sprint and quick getaway, our aerobic system turns on and produces a majority of the energy needed. This system can last forever; at rest we are aerobically oxidizing mostly fats to provide energy for the body at rest.

The aerobic system can burn carbohydrates, fat and protein depending on availability and intensity. Protein used in very small amounts to provide energy for exercise and we wont discuss it further here. I want to discuss the differences between fat and carbohydrate oxidation.

The classic study from Romijn et al. showed that the fuel sources changed depending on the intensity of exercise. They looked at 25% VO2Max, 65% VO2Max and 85% VO2Max. The found that as intensity increased, so did caloric expenditure and the more intense exercise relied more heavily on carbohydrate use. This is because at higher intensity of exercise the fat breakdown and transport into the mitochondria and oxidation rates are too slow to keep up with the energy demands.


Carbohydrate is also the preferred fuel during intense activity as it provides 5.5% more kcal/L of oxygen compared to fat oxidation, meaning it is a more efficient fuel source. Research shows that elite level marathoners fuel use is 85% carbohydrate and 15% fat oxidation.

One thing to note is, you do not only use one system at a time, your aerobic system is always running in the back ground. Think of these systems like dimmer switches, they can be turned up or turned down depending on the situation. For example, if we take a cyclist who is riding on a flat surface, they have their aerobic system pumping out most of the ATP to cover the cost of their cycling. When they hit a hill, there is an increased demand for ATP. The aerobic system take a little while to adjust, and therefore the anaerobic system has to kick in to provide some quick energy, and that means the phosphocreatine system turns on, and the anaerobic glycolytic system turns on to meet the ATP needs of the body.

Key points:

  • Fat is slower to provide energy, and therefore intensity is lower when using fat as a fuel source
  • Fat is less efficient; it uses more oxygen to produce less ATP
  • High burst of exercise- ie the energy change when cycling or running up a hill is usually covered by anaerobic glycolysis which uses glucose (carbohydrate)
  • Most elite athletes run at high %VO2 and burn a large amount of carbohydrates


Romijn JA, Coyle EF, Sidossis LS, Gastaldelli A, Horowitz JF, Endert E, Wolfe RR. Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. Am J Physiol. 1993;265:E380-E391.

O’Brien MJ, Viguie CA, Mazzeo RS, et al. Carbohydrate dependence during marathon running. Med Sci Sports Exerc. 1993;25:1009–1017. doi: 10.1249/00005768-199309000-00007

Bosch, AN, Goslin BR, Noakes TD, Dennis SC. Physiological differences between black and white runners during a treadmill marathon. Eur J Appl Physiol. 1990;61:68-72.

training and performance

The days of my protein rambling have almost come to an end! In my last post, I discussed how much protein should be ingested/day and how that protein should be distributed. The article before that discussed whether acute protein consumption before and activity enhances performance.iron sources

In my final article in the protein series, I want to discuss acute protein ingestion related to an acute bout of exercise. This is something that I get asked about all the time at the club: is it important to quickly ingest protein directly after a given activity to enhance recovery from that bout of training?

CLICK HERE to read the rest in the RW Magazine.

Book your appointment

Contact us to book your next appointment

Call Us 519 885 4930