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Are Fats The Optimal Primal Fuel For Endurance Athletes?

By Dr. Marc Bubbs ND, MSc, CISSN, CSCS
May 18, 2015
Are Fats The Optimal Primal Fuel For Endurance Athletes? image

The marathon and triathlon seasons are fully underway and this year over half a million people will complete a marathon and thousands more participate in triathlons and other endurance events. Whether they’re entering an event for the first time or trying to achieve a personal best, one of the most common questions I get asked by clients is “what should I eat before my long run?” The answer may surprise you!

A common refrain amongst most endurance coaches is that you must consume a heavy carbohydrate meal before your long run or bike to perform your best. High carbohydrate breakfasts of oatmeal, cereals, and juice or the classic ‘carb-load’ pasta dinner are staple recommendations from many top endurance coaches. While this is certainly not bad advice, there is another option.

Don’t eat any carbs before your long run or ride! (Yes, you heard me correctly.)

Let’s take a moment for a quick physiology review. When carbohydrates are ingested they are preferentially burned for fuel and provide 4 calories per gram of energy. When fats are ingested, or burned from body stores, they provide 9 calories per gram of fuel. You are effectively doubling your fuel efficiency if you use fat for your primary fuel source from the outset.

By maximizing your capacity to burn fat for fuel, you’ll also be sparing precious muscle glycogen, the carbohydrate stores in your muscle. You have approximately 500g of glycogen stored primarily in your muscles (and some in your liver) that can provide you with 2,000 calories of energy during your race. In contrast, even lean individuals between 7-14% body-fat have 20,000-30,000 calories available for energy use in their fat stores. Wow! That’s a lot of fuel that could be used, if you changed your pre-race food choices.

Skeptical? I don’t blame you, but the preliminary results are impressive.

Dr. Jeff Volek PhD is a world-renowned researcher and author of the FASTER (Fat-Adapated Substrate oxidation in Trained-Elite-Runners) study. Dr. Volek and his team are investigating the impact of very low-carb diets on an athlete’s capacity to burn fat for fuel, compared to the traditional high-carb diets used by endurance athletes.

A traditional high carb diet would be broken down to approximately 60% carbs, 15% protein, and 25% fat. A low-carb diet would dramatically increase the fat intake to 70%, provide slightly more protein at 20%, and carbs would only make up 10% of the total daily energetic intake.

The initial data is sending shockwaves through the exercise community. Why?

It has been well established in the scientific literature for years that the maximum amount of body-fat that can be burned per minute is 1.0 grams, while the average athlete burns between 0.45-0.75g per minute. 1

The initial results from Dr Volek’s FASTER study smash this concept, showing the LCD athletes are burning upwards of 1.1-1.8g/minute which is way beyond what was thought physiologically possible. 2 You can see the peak fat oxidation or burning rates in the graph below.

Are Fats The Optimal Primal Fuel For Endurance Athletes? image

Are You Eating Enough Carbs For Optimal Recovery? Traditionally, it was believed that somewhere between 35-65% of your maximum heart rate you switch from burning fat for fuel to carbohydrates. 3 However, this new research is re-shaping the way we think about fueling endurance athletes. The graph below highlights the body’s capacity to burn fat on a low-carb diet at much higher training intensities than thought possible. 2

Are Fats The Optimal Primal Fuel For Endurance Athletes? image

If you ingest primarily carbohydrates your body quickly shifts over to burning the ingested carbs, and your body’s glycogen stores, for fuel. However, if you eat primarily fats and protein before exercise you’ll be able to tap into your fat reserves more effectively. This allows you to spare your muscle glycogen for further along in the race, when you really need it!

This new research is in-line with an ancestral or Paleo approach to eating. You don’t need to rely on an endless array of gels, powders, and pills to produce the best possible endurance race times. If you are engaging in endurance sports to lose weight this is critical, as your excess carbohydrate consumption is likely holding you back from achieving a better body and better health.

For example, you could start your day with a coffee and a tablespoon or two of coconut oil or MCT oil. The caffeine helps stimulate lipolysis, the breakdown of body-fat stores to free fatty acids for fuel and the MCTs provide the added instant energy source. Alternatively, you could have eggs with avocado and a serving a stir-fried kale or spinach. This will allow your body to burn fat for fuel more effectively, as well as improving your health.

Not everyone needs to follow a low-carb diet. If you are purely performance-based and striving for new personal bests than you may want to tread lightly. (Check out my previous article on Eating Enough Carbs Optimal Recovery to get your post-exercise carb fix). However, in a sport like endurance training where carbohydrates are king, this compelling new research highlights that we still have a ways to go in understanding how to most effectively fuel the body for endurance performance.

Train your body to burn more fat and watch your performance, as well as your health, reap the benefits!


1. Venables; “Determinants of fat oxidation during exercise in healthy men and women: a cross-sectional study”. J Appl Physiol (1985) 2005 Jan;98(1):160-7.

2. Defty, President VESPA, Peter. "The Emerging Science on Fat Adaptation | Ultrarunning Magazine." Ultrarunning Magazine The Emerging Science on Fat Adaptation Comments. Ultrarunning Magazine, 16 Dec. 2014. Web. 1 May 2015.

3. Lima-Silva A et al. Relationship between training status and maximal fat oxidation rate. J Sports Sci Med. 2010 Mar 1:9(1):31-5

4. Noakes, T., J.S. Volek, and S.D. Phinney. Low-carbohydrate diets for athletes: what evidence? British Journal of Sports Medicine. 48(14):1077-8, 2014.

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