For the past several months, I’ve been looking into various nutrition and metabolic research trying to figure out how to optimize physical health and athletic performance through diet.

Now, it might seem likes professional athletes have got it all figured out, but when it comes to nutrition, the research is very conflicting and often wrong.

Take, for example, the Ancel Keyes hypothesis that fat causes heart disease.

This hypothesis was the catalyst for numerous radical and outright crazy new products and policies capitalized on by big food companies.

Dietary fat is not a primary cause for heart disease and has been disproven numerous times.1 And whether saturated fat plays a role in heart disease is also very unclear.2

Now, whether carbohydrates are necessary for athletic performance has been bothering the heck out of me for some time now. I’ve spent a lot of time listening to experts explain, discuss and tear at each other’s throats, but I haven’t ever really looked into the literature myself.

Time to give this all up? I don’t know about that one…

Before I go too far into things, I’ll do a little TL;DR on my thoughts.

Nutrition is complicated. It seems that everywhere I look there are experts with confliction opinions and, more importantly, research with conflicting findings.

It seems like eating carbohydrates has a positive effect particularly on anaerobic or higher intensity athletic activity. And because Orienteering includes a lot of tough terrain and hilly running, I don’t see any reason to stop eating carbohydrates (particularly those consistent with evolutionary dietary habits).

Now let’s dive into some of the studies:

The FASTER study is widely cited by low carb advocates and athletes showing that “fat-adapted athletes” show no decreased markers for performance in endurance running.3

Despite not being a double-blind RCT (randomized controlled trial) and being only n = 10 subject in each group, the FASTER study is widely cited.

The most suprising finding was that in both groups, the muscle glycogen levels were the same before exercise, post exercise, and post consumption of a recovery drink with the appropriate macros.

This is interesting because for a long time people thought that the depletion of muscle glycogen was strongly associated with fatigue.

And that muscle glycogen is best maintained by a high-carb diet. This seems to have been disproven in the study.

What was significantly different between the two groups was the oxidation of each preferred fuel source:

As expected, fat-adapted athletes burned more fat and less carbohydrates, however it’s interesting to keep in mind the shape of the curves and that both group burn BOTH fat and carbohydrates.

It’s easy to look at these results and say that fat-adapted athletes shouldn’t have any problems with athletic activity compared to typical carbohydrate fueled athletes, but now I’m going to attempt to disprove that assumption.

Zach Bitter, “fat-adapted” ultra-endurance world record holder

There is a decent body of evidence that suggests anaerobic or high intensity activity is impaired by a diet low in carbohydrates.4,5,6

This is a pretty big deal considering that intense activity is a huge part of athletics in almost every possible situation, including endurance events.

Sweden winning the WOC 2021 Sprint Relay

Anthony Colpo’s article does a great job explaining why high intensity activity is critical for success in competition.

Just imagine. Even in an ultra-marathon there are going to be steep hills, rocky trails, and maybe even a sprint finish. So even if your low intensity speed is not limited by a low carbohydrate diet, the lack of energy to run at higher intensity is still a limitation.

This limitation is further exaggerated in track, cross-country and orienteering, despite being endurance sports (that I love).

My time behind the leaders in the WOC 2021 sprint relay.

Why do low carbohydrate diets impair high intensity athletic activity?

It’s not entirely clear. The traditional explanation is that the aerobic system uses “glycosis” (metabolization of glucose) to create energy (ATP) in the absence of oxygen (at higher intensities).7

This makes sense under the assumption that high carbohydrate diets optimize carbohydrate metabolism and glycogen stores in your muscles.

As we saw in a previous study, however, muscle glycogen stores in fat-adapted athletes seem to be very similar to those of high-carboydrate athletes during sub-maximal exercise.

Hmmm… so shouldn’t fat-adapted athletes have similar levels of glycogen to use for glycolysis?

Another potential explanation is that circulating levels of blood glucose is lower on a low carbohydrate diet, thus limiting the rate of glycolysis and inhibiting performance at higher intensities.

But it isn’t clear that blood glucose levels typically vary by that much between the two diets.8

So what is it actually that limits anaerobic performance? It seems that fatty acids and fat stores cannot be metabolized fast enough during glycolytic activity to make ATP at the same rate as metabolized carbohydrates.

If that isn’t enough to make you run away (literally) from the idea of low carbohydrate diets for performance, there is also the idea that carbo-loading or a diet high in carbohydrates increases glycogen stores AND perhaps increase utilization of glycogen stores for energy.

And the idea that carbohydrates produce more ATP per unit of oxygen than fats.7

Whether this is important for purely aerobic activities, like ultra-running, cycling, and iron-man triathlons, is unclear.

What is clear, however, is that higher intensity bouts are limited by low carbohydrate intake and it takes a long time (2 – 4 weeks minimum) to adapt to a high fat diet which may cause short term performance deterioration.

So basically… nutrition is still complicated. But it seems to me that the common belief held by most nutritionists, coaches and athletes is largely correct: carbohydrates are important for athletic activity.

Yes, athletes can become “fat-adapted” and it seems like it works fine for ultra-endurance sports that don’t require much high intensity activity.

But when it comes to most sports, high intensity activity is inevitably involved. And without carbohydrates to quickly metabolize, activity is severely limited.

This doesn’t mean that if you’re an athlete, you should cram down as many carbohydrates that you possibly can. It’s important to get the proper amount of calories, protein and micronutrients for your level of activity. The source of carbohydrate also matters, but that’s a story for another time.

Anyways that’s enough for now.


1. Mozaffarian D, Micha R, Wallace S “Effects on Coronary Heart Disease of Increasing Polyunsaturated Fat in Place of Saturated Fat: A Systematic Review and Meta-Analysis of Randomized Controlled Trials” PLOS Medicine

2. DiNicolantonio, James J et al. “The Evidence for Saturated Fat and for Sugar Related to Coronary Heart Disease.” Progress in cardiovascular diseases vol. 58,5 (2016): 464-72.

3. 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, 2016, Pages 100-110, ISSN 0026-0495

4. Louise M. Burke and Bente Kiens “Fat adaptation” for athletic performance: the nail in the coffin?
Journal of Applied Physiology 2006 100:1, 7-8

5. Zinn, Caryn et al. “Ketogenic diet benefits body composition and well-being but not performance in a pilot case study of New Zealand endurance athletes.” Journal of the International Society of Sports Nutrition vol. 14 22. 12 Jul. 2017

6. Michalczyk, Małgorzata Magdalena et al. “Anaerobic Performance after a Low-Carbohydrate Diet (LCD) Followed by 7 Days of Carbohydrate Loading in Male Basketball Players.” Nutrients vol. 11,4 778. 4 Apr. 2019

7. Davidson, Mackenzie “Fats vs. Carbs as Energy Sources” LinkedIn Articles April 1, 2019

8. Yuan, X., Wang, J., Yang, S. et al. “Effect of the ketogenic diet on glycemic control, insulin resistance, and lipid metabolism in patients with T2DM: a systematic review and meta-analysis.” Nutr. Diabetes 10, 38 (2020).

Leave a Comment

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s