Science Behind Ketogenic Diet: How Endurance-Athletes Thrive on a High-Fat Low Carb (HFLC) Diet

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Since everybody is talking about ketogenic diet. It is important to understand what this diet really is and how does keto work?

Today, we will dig deeper into the ketogenic diet (what is keto?) science. We will also discuss how our body uses food as energy source.

In the later section of the post, we will be reviewing some interesting research on fat adapted athletes and exogenous ketone esters. Don’t worry about the term that I just mentioned. I will explain it when we do literature review. In my opinion this the best diet for endurance runner.

4 ways body can tap energy

Food that we eat, can be directly used as energy source.

Whatever we eat, carbohydrates, protein and fat can directly be used for energy. If we eat more than what is needed, then you guessed it right. Body will store it in the form of fat or glycogen. Why? So that it can tap into this energy source whenever needed.

Te question that we must be asking here is where is glycogen produced and stored? It is produced in liver and stored in liver and skeletal muscles.

Body store glycogen in muscles and liver. This can be used when required.

Simply put, storage form of glucose in human is glycogen. There is a limit how much human body can store glycogen. It is around 2000 calories.

Once you use this energy, body goes into crisis mode and start tapping fat for the energy requirement. But energy supply from the fat is slow.

Have you seen people bonk around mile 20-22 in a marathon. This is what precisely happens. Their store of glycogen is depleted and there is no energy left from the stored glycogen.

However, if you train your body to use fat, then it will use it efficiently. This can be done by limiting your carb intake. This way your body to go in the state of ketosis. Ketone bodies provide energy when we switch to a ketogenic diet.

Body fat can be used when energy demand is very high.

Now, lets say you are running an ultra marathon or doing Ironman (Running, swimming, biking-total distance 140.7 miles). Your glycogen level is almost depleted. Body will start using fat stored in your belly and other organs. Since the leanest athlete has energy from fat in excess of 50,000 calories. You can use this stored fat to run or exercise longer. Body will go into ketosis which means that one will have ketone bodies floating in the blood. As discussed in my previous post, you can go into state of ketosis using several methods. The best way to go into ketosis is nutrional ketosis which means eat keto approved foods only.

Muscles can also be used to survive when one goes into starvation mode.

Once your storage of glycogen and fat is empty, body will start using muscles for fuel. To do that, body will break skeletal muscles and use amino acids for energy production.

So now, we know that carbohydrates, fat can be used for energy directly from the food or from the glycogen or fat storage. How about protein we consume?

Protein we consume can either be used by body to build muscles or it will be stored in the form of glycogen or fat.

So, who is the winner as far as stored energy is concerned. The order is here 1. Fat 2. Carbs and 3. Protein. Fat having more than 50,000 calories and glycogen only about 2000.

No, you do not want to use your muscles (protein) for energy. No matter how many calories your muscles can provide.

What is used and when?

Human body can use glucose or fat for energy. However, glucose is a preferred energy source. This is because it can be utilized quickly and we can get energy instantly.

Fat on the other hand is not a preferred source of energy. It takes body lot of time to produce energy using fat. These molecules go through lot of cycles in human body. However, one gram of fat will produce 4X energy as produced by glucose. Let us take some notes before we dive deeper into fat metabolism

  • Body use glucose as a primary energy source.
  • Glucose can either be consumed directly or it can be taken in the form of carbohydrates.
  • Carbohydrates are basically lot of glucose connected together. Fibers are also considered carbs but they do not provide energy.
  • Body breaks down these carbohydrates into glucose
  • Then the glucose molecules will go to all kind of cells
  • Cells of the body will take that glucose and provide us energy..boom!

Why Glucose is preferred

This is because, human body cells can quickly absorb glucose and mitochondria in the cells can quickly use it. Fat on the other hand takes longer time to reach mitochondria.

What happens when energy is low from carbohydrates

Let us know say that you started a keto diet. Your consumption of carbs has gone down significantly. Body will start tapping into stored fat for energy. Fat is stored in human body as adipose tissues. These are nothing but lot of fat around various organs. When demand arise, lipase will attack this fat and break this down into free fatty acid.

As I described in my previous article that fatty acid are either found in the form of di or triglycerides. Which means that two or three fatty acids are connected together by a glycerol bond.

Lipase break that bond and release fatty acids in the blood stream. These fatty acids will then go to liver and liver will transform then into beta-hydroxybutyrate (BHB). BHB is what we call blood ketone.

The amount of ketone bodies determine if someone is in the state of ketosis or not. BHB then travel to cells of body and can be used mitrochondria for energy production.

BHB as well as free fatty acids are used for energy production in cells. However, there is an important point to remember here.

Human brain can not use free fatty acid because of its large size. It prefers smaller molecules. BHB is a small ketone and can directly be used by human brain. The process involved in the fat metabolization is very complex. Therefore, I have skipped some steps between breakdown of fat into free fatty acids to energy production in mitrocondria.

Now, you have trained your body to use fat properly. It will do it efficiently when you are doing strenuous exercise such as ultra marathon. As mentioned earlier, human body has fat stored in excess of 50k calories. This huge pool of energy can now be used with proper diet and training.

Watch this very nice video about fat metabolism that explains the process in detail.

Recent studies on ketogenic diet:

The utilization of ketone by human body have been reported to help in many contexts such as

  • Neurological disorder (1)
  • Cancer (1)
  • Heart failure (1)
  • Exercise performance (2-4)

The first three benefits are out of scope of this article. So we will focus on the third point here. There have been many research studies on mice and human alike. All points toward to the enhanced fat oxidation in the subject that are on high fat low carb diet.

I have complied data from some research papers. I will discuss two studies that focus on the endurance athletes. You may call this a mini keto diet review

Study 1: Volek et. al., Metabolism Clinical and Experimental, 2016, 100-110

Study 2: Cox et. al., Cell metabolism, 2016, 24, 256-268

The summary of both studies is here.

Study 1: How does endurance athletes perform on keto diet

Volek et al. studied 20 elite ultrarunners and ironman athletes. He found some interesting results. The focus of his research was-how does high fat low carb diet affect the performance of elite athletes.

There were two groups. One group was on traditional high carb low fat diet and another group was on unconventional high fat low carb diet for over a year. They asked both group athletes to run for 3 hours on the treadmill.

They measured three 4 blood markers in all athletes

  1. Fat oxidation- this means that how efficiently, the athlete can use fat for energy.
  2. Carbohydrate oxidation-how efficiently, an athlete can use carbs.
  3. Both of the above markers before and after the run.
  4. Serum fatty acids-how much fat is floating around in the blood of an athlete.

Results from the study:

  • High fat oxidation was observed in athletes who were on high fat low carb diet. This means that they were able to use fat as fuel more efficiently than the other group.
  • Not surprisingly, high carb group were able to oxidize carb faster than the high fat group.
  • Blood ketone levels in high fat group was higher than the high carb group.

Study 2: What is the effect of exogenous ketones (ketone supplements) on the endurance performance?

Cox and coworkers studied some cyclists. They were given different energy drinks. Researcher took blood and urine samples of all endurance athlete at different period of time. Authors performed 5 very interesting experiments. I will explain 2 of the experiments in a simple way so that readers can follow.

Experiment 1: Athletes were asked to drink ketone esters (95% ketone and 5% carbohydrates). Ketone esters are subunits of fatty acids that can be used for energy production.

They were then asked to exercise at 45% and 75% of maximum heart rate. Then the researchers collected their blood and urine samples. The visual example of that experiment is shown here.

  • Athletes did not do anything meaning they were at rest and data was collected
  • They were asked to exercise at 45% of their maximum heart rate (very comfortable pace for all of the endurance athletes).
  • In the last step, all of the endurance athletes were asked to exercise at 75% of their maximum heart rate. This is tough to do, its hard! How do i know? Because that is my tempo run almost every week.

Nevertheless, from the data they collected scientists found out the following

  1. The amount of ketone esters in the blood when they were at rest was higher. This is expected because consumed exogenous ketones will be flowing in abundance.
  2. The amount of ketone esters is lower when they excercise at 45% of their maximum heart rate. This is because now body is using both ketones as well as glucose coming from muscle/liver glycogen.
  3. At 75% of their maximum heart rate, the amount of ketones in blood has decreased further. This indicates that body requires energy very rapidly. This demand can be easily met by using glucose as energy source.

Remember, glucose is preferred source of energy and it can provide energy almost instantly. That is why level of ketones has gone down when athletes exercised at 75% of maximum heart rate.

Experiment 2: Researchers asked endurance athletes to consume 3 types of drinks 1. 95% carb drink 2. 95% ketone drink (ketones that can provide energy quickly) and 3. 95% Fat drink (this drink may be nothing but full fat). They were then asked to exercise and researchers collected their blood plasma samples. The visual example of that study is shown in the image below.

a. Athletes did not do anything meaning they were at rest.

b. They were asked to run at 75% of their maximum heart rate for 60 minutes.

  • Like previous experiment, plasma ketone were higher when they were at rest.
  • It got lower when they exercise at 75% of their maximum heart rate. But the level of ketones were way higher than amount of fat and carbs on their plasma.


This means that consumed ketone esters will be more readily available in comparison to carbs and fat.

  • Another very interesting finding was that the plasma lactate was lowest when ketone esters were consumed. It was highest when carb was taken before the exercise.


The amount of lactate in blood is directly related with muscle tiredness. Have you heard of lactate threshold in tempo run? This means that if we drink ketone esters before exercise, less lactic acid will be generated and we must feel less tired! Very interesting.

  • Plasma Free Fatty Acids was higher when athletes consumed fat drink. This is expected because they drink fat. No rocket science here.
  • The amount of plasma insulin was highest in case of carb drink consumption. This make sense, right? More glucose in the blood and more insulin-very straightforward. However, the level of insulin level off in the later part of the exercise.

There are three more experiments that researchers did. I will explain them in more details in my coming posts along with some more relevant studies.

Although, more research is needed to establish many facts. But, initial data shows that there is lot of potential in a keto diet.


Scientific References:

  1. Patrycja Puchalska and Peter A. Crawford, Cell Metabolism, Cell Metabolism, 2017, 25, 262-284.
  2. Jeff S. Volek, Daniel J. Freidenreich, Catherine Saenza, 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,
    Metabolism Clinical and Experimental, 2016, 100-110
  3. Pete J. Cox, Tom Kirk, Tom Ashmore, Kristof Willerton, Rhys Evans, Alan Smith, Andrew J. Murray, Brianna Stubbs, James West, Stewart W. McLure, M. Todd King,6 Michael S. Dodd, Cameron Holloway,
    Stefan Neubauer, Scott Drawer,4 Richard L. Veech, Julian L. Griffin, and Kieran Clarke, Cell Metabolism, 2016, 24, 256-268.
  4. Kaito Iwayama, Reiko Kurihara, Yoshiharu Nabekura, Ryosuke Kawabuchi, Insung Park, Masashi Kobayashi, Hitomi Ogata, Momoko Kayaba, Makoto Satoh, Kumpei Tokuyama, EBioMedicine 2 (2015) 2003–2009.

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