Laugavegur Ultra-Marathon 2010

Are you a runner and looking into high-fat, low-carb diet? Then you must read this.

The high-fat, low-carb (carbohydrates; HFLC) diet seems to be popular in every corner of the world. No wonder, as it can be a tool to tackle obesity, sugar addictions, diabetes and other metabolic syndromes. Having said that, it is my true belief that no one should change over to this diet without being monitored for physical changes. For healthy individuals, with bodily systems working properly, a balanced diet with food in moderate amounts from all food groups, is a safe way for a long term health and success.

How does the HFLC diet work for athletes? And does it matter what type of sport a person participates in? Let’s take a look.

What substrates (source of fuel, i.e. food) the body uses for energy depends on several factors:

  • Intensity of exercise
  • Duration of the exercise
  • Physical shape of the participant
  • Temperature, both of the body and atmospheric

All of these, and more, play a role in whether the body uses fat (fatty acids), carbohydrate (carbs) or a mixture of both during exercise. I will not be talking about proteins in this post as we can control only to a very limited extend the percent utilization of proteins during exercise.

Before we go further, it may be necessary to explain what the body, and the exercising muscles, actually use to “make” the energy we use for exercise. All substrates (i.e. food) are broken down into energy rich compounds called adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine monophosphate (AMP), where ATP gives the most energy of the three when broken down. Obviously, the body does more with food than to only make ATP but for this article let’s focus on this part only. The energy rich ATP, ADP and AMP compounds are then actually used by the working body to move muscles and during workouts. But to make these we have to have carbohydrates, fats, proteins or alcohol available in our body (the four energy yielding nutrients) for the body to convert into the phosphate-energy rich compounds. Several steps are involved in the conversion of our food into ATPs and many enzymes, minerals and other factors are needed for this processes to work as it should.

During endurance exercise such as running, swimming, bike riding, and many other types of endurance exercises, we are mostly breaking down our food for energy through aerobic pathways, i.e. oxygen is available and is a major player (different from 100m sprint, 50m swim race and more short bouts which are generally anaerobic). How much oxygen we need depends on many factors but for this post the one thing that makes the greatest difference is exercise intensity. The more intense the exercise becomes, the greater need our bodies have for oxygen transport in the blood and the oxygen supply becomes the limiting factor on how long we can keep on going during the exercise/workout (Figure 1).

Substrate vs. Intensity, Copyright ©2009 The McGraw-Hill Companies, Inc, All Rights Reserved.

Photo credit: Copyright ©2009 The McGraw-Hill Companies, Inc, All Rights Reserved.

But where do fats and carbs come in and is one better than the other for endurance activities? Well, breaking down fat (beta oxidation) will provide us with fewer high energy compounds (ATPs) for the working muscles than does breaking down carbs (glycolysis) per each oxygen molecule consumed through breathing. In other words, when the oxygen supply is the limiting factor during an intense exercise it is better for the body to use carbs to make ATPs rather than to use fat. Let me explain why this is.

For each “metabolic round” in the energy generating systems from fat and carbs (beta oxidation, glycolysis and the Krebs cycle) the body makes a number of ATPs in the breakdown of each compound, i.e. breakdown of fat and breakdown of carbohydrates. The body makes from our food 14-17 ATPs from each “fat metabolic round” and 30 ATPs from each “carb metabolic round”. For each “metabolic round” to take place, a supply of oxygen has to be in place. When oxygen is a limiting factor, the body prefers energy production from carbohydrates as it will make more energy under oxygen limiting conditions –> the metabolism of carbohydrates (resulting in the formation of ATP) is more efficient with regards to the oxygen consumption under these conditions.

The body likes using fat to make energy rich compounds, but just not during high intensity exercise. To make this even more confusing, high intensity is observed at a different level for each person. In other words, a highly trained individual will be able to utilize more fat for energy at a certain intensity level when compared to an untrained individual at the same relative intensity level. Some scientists call this “exercise induced physiological adaptations.” I will talk about this in detail in a later post.

How can we now interpret this? It´s hard because we are all so different when it comes to physical workload and scientific support for recommendations one way or the other is lacking. But my conclusion is the following:

For most healthy adults who are in general good physical conditions, and who are not new to running, the diet can easily include more fat and less carbs if expected time in a 10K run is approximately 60-70 minutes or more. For most, who run a 10K below this time, a mixture of carbohydrate rich diet with good fats is a good choice. For those who plan on running 10K under approximately 40-45 minutes, a diet rich in carbohydrates and relatively low in fat, but inclusive of omega-3 fat, is the best bet. Again, I must state that scientific support for my recommendations is lacking as every individual is different when it comes to running pace or exercise intensity. But as a general rule, this may help some runners, and other endurance athletes, decide whether a diet richer in fat and lower in carbs, will suite their needs.

Furthermore, when exercise periods are long and competitions go beyond approximately two hours, a greater focus on fat consumption may become necessary (see figure 2). Unlike carbohydrate stores, the energy supply provided by fat is virtually inexhaustible in most individuals and focusing on fat utilization may become necessary to finish these ultra-long workout or competitions.

Substrate vs. Time, Copyright ©2009 The McGraw-Hill Companies, Inc, All Rights Reserved.

Photo credit: Copyright ©2009 The McGraw-Hill Companies, Inc, All Rights Reserved.

The higher-fat dietary habits are therefore practiced by many ultra-marathon runners. One example of this can be taken from the Thames Ring 250 held in the UK. This is a 250 mile run (appr. 400 km) which takes about 70 hours to complete for the fastest entries. In an endurance event this long, a diet richer in fat is a must for the individual to be able to finish. Actually, as I have stated publicly before, the biggest challenge these participants face is not whether most of the energy is coming from fat or carbs but for them to be able to cross the mental barrier of such a long event and for them to be able to consume enough food, any kind of food, throughout the race.

So when your friend tells you how his HFLC diet is working great for him, you can say that he is obviously not running fast enough and not nearly as fast as you 🙂

This justification becomes more difficult to use if your friend is an ultra-marathoner 😉

Want to know more about nutrition for running and food supplements for runners? Take a look at this book in Amazon Kindle Store.



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Comments (11)

  1. Great article and website!

    Interesting. One question, you explain how the body utilizes fat and carbs for energy. It almost seems you are talking about dietary intake and not stores. Our carb storage is miniscule compared to fat. Why would dietary intake of fat help with performance if you already have adequate fat storage?

    I believe our fat storage is an evolutionary marvel that allowed us to survive long periods of famine and perhaps enabled us to walk for long periods of time to search for food or shelter. Any more activity than that and the body would prefer glucose for energy. That’s why when you are carb deprived and ingest any high carb food you have a huge surge in energy levels.


    • steinar swooshr

      Thank you G. Suarez
      This is a great comment. When the body uses fat for energy it is mostly in the form of free fatty acids (ffa) available in the body / blood system. Dietary changes to a high fat diet (and low carb) in regularly trained individuals will result, some studies suggest, to an increase in fat oxidation (use of ffa) during exercise. So if I am correctly interpreting the studies I have read so far on this topic, there is a physical adaptation that takes place during this high fat, low carb phase that makes our body more capable of using fat (ffa) via beta oxidation for exercise, but only under conditions of low intensity. This fat adaptation will not be of any help if the intensity is high as the preferred substrate under intense conditions is carbohydrate. The level where the body switches from fat to carb for energy utilization is not an “on-off” switch, but more like a “dimmer light”. So even if intensity is higher than during a normal walk, as long as it is not “too” high, improved utilization of ffa may be beneficial. Therefore, a dietary modification which will include more fat (omega-3 should be a major focus), may benefit some runners, especially those you run at a slow pace.

      I hope this was of some help. Please let me know if there is anything else.


  2. Great article. Just wanted to add some more to the discussion.
    First of all, the Figure 1 in the article is typical for a non fat adapted athlete. Once you are fat adapted, your MEP will be at much higher %VO2 max. For more info on this look for the work of Bob Seebohar, Dr Phinney & Dr Volek.

    Here are few more advantages that a fat adapted athlete can benefit from…
    – Since you are less reliant on glycolytic pathways, you produce less Radical Oxygen Species. Hence, less oxidative damage and quicker recovery times.
    – Once you are fat adapted your body doesn’t catabolize muscle mass to convert to glycogen unless you are in complete starvation. Quicker recovery.
    – You become bonk proof. During aerobic exercise, you spare your stored gycogen and use fat preferentially. This essentially makes you bonk proof.
    – Mental clarity is another advantage. Carbs are not the only fuel your brain can use. Ketones, another product of fat metabolism, is another brain fuel that might actually be better for the brain. Look up Grain Brain – Dr Perlmutter.
    – Since you rely primarily on stored fat, you don’t need to eat as much. I know fat adapted athletes that can race on as little as 100 gms of carbs/hour. This significantly reduces your chances of getting an upset stomach from ingested foods.

    • Thanks Neeraj for your comment and additional insights 😉

    • dprice81

      I tried this stuff it doesnt work. If it actually worked youd see people filling their water bottles with coconut oil. Don’t get scammed. Aid stations are loaded for carbohydrates for a reason. High carb diet=happiness Low Carb diet=suicidal. I didnt want to leave my bed let alone train for a marathon. I’m back on the carbs and smashing out 45 mile weeks. Couldn’t do this on very low carbs. It just doesn’t work. You want to get rid of the fat that has been stored on your body so work out in fat burning mode but don’t add more fat to your body then it has just worked off.

      • Personally I would have to agree with you. However, there are runners that have been successful while on this diet regimen, but it is highly unlikely that you would find a fast pace 5K-full marathon runner that has been on this diet as energy utilization is mostly linked to intensity. Thanks for your input dprice81.

  3. Sure, not a problem at all.

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  6. Hi this is a cracking article, would you be happy forme to post about it on social media?

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