(Yet another) reason why low carb diets can make you slower

Low-carb, high-fat. Teach your body to burn more fat so you can be more “efficient”, right? No.

Low-carb diets ruin running economy. And cycling efficiency. I’ve written about this before:

Fat Burning – Why it’s overrated for the competitive endurance athlete

What’s your running economy? A tale of two runners

A recent study came out that offers some more data on how dietary changes can affect running economy.

Effects of ad libitum Low Carbohydrate High-Fat Dieting in Middle-Age Male Runners.

In short, researchers had recreationally-trained endurance athletes follow a low-carb, high-fat diet for three weeks and measure what happened when they ran at various intensities/paces. These participants  were men around 40 years old, who compete in triathlon, marathons, Ironman, etc., but are definitely not elite athletes. Research is hard and it’s easy to nitpick aspects of study design, but no need to do that here. The researchers did a fine job studying what they were investigating. I want to zero in on one area, and show you the side-effects of going on a low-carb diet if you’re an endurance athlete. 

The participants ran at paces corresponding to their 5 km, 10 km, half-marathon, marathon, and ultra-distance running times, first after consuming their habitual moderate/high-carb diets and then after three weeks on the low-carb, high-fat diet.

The researchers measured a number of different variables related to fat burning, temperature regulation, breathing rates, thirst, and perceived exertion. But what I’m most interested in are the changes in oxygen consumption and, in turn, running economy.

Yes the low-carb group burned more fat. Was it beneficial?

Oxygen Consumption

When running at each of the running paces, we can see a significant increase in VO2 (oxygen consumption). This means that on the low-carb diet, people used more oxygen to run the same speed.

Because our VO2 max is the greatest amount of oxygen our body can process, this means that people in the low-carb group were exercising at a higher percentage of their maximal capacity, to run the same speed.


Running Economy

We can quantify running economy by calculating the amount of oxygen required to go 1 km, normalized for bodyweight. Our units for running economy are milliliters of oxygen per kilogram of bodyweight per kilometer. 

Participants in this study actually started out with pretty good running economy, but we can clearly see what happens on a low-carb diet. It costs more oxygen to go the same speed!

This is not surprising, this is biochemistry. We also see the same thing in cycling, where high-carb diets offer improved efficiency.

Participants in this study also were tested on 5 km performance, with no significant differences seen between the groups. Some individuals did better with low-carb, and others with high-carb. There was also significant weight loss in the low-carb group, so that’s certainly a good thing. However that should have improved their running economy, so it’s plausible that their economy would have been even worse with just the dietary change. Also worth noting is that participants were given a book that was clearly in favor of low-carb diets and athletic performance, potentially introducing a biased view.

Glycogen “Sparing”

This study didn’t address this, but because it often comes up I wanted to mention it. Low-carb, high-fat diets don’t spare glycogen, but instead reduce your ability to use your glycogen. A small but huge difference. You can read more about this here and here.

I am not one who advocates consuming a ton of carbs all the time, instead I prefer a periodized approach to carbohydrate intake. There are times we want to burn fat, but not when we’re trying to go as fast as we can!

So given the choice between using more oxygen and less oxygen to run the same speed, which would you choose?

Measuring your running economy

What gets measured gets managed, and we can measure your running economy during an exercise test. You can contact me to schedule a test, or I can help you find a testing facility in your area and I can turn your test results into practical recommendations.



Sign up to learn more about interesting topics in sports nutrition!

What are you most interested in?

Leave a Reply

Your email address will not be published. Required fields are marked *