Today I’m kicking off a new blog segment called the “Journal Club.”
In the Journal Club, we’ll examine a single research paper that holds significance to you as a cyclist. Whenever possible, I’ll select open-access research and include a downloadable link to a personally annotated PDF so you can dig into a full-text version and follow along.
In today’s Journal Club we’ll check out a brand new paper examining the performance implications of a Low Carb High Fat or “keto” diet. Is going “keto” likely to improve your cycling? Let’s find out.
Burke, L.M., et al., Crisis of confidence averted: Impairment of exercise economy and performance in elite race walkers by ketogenic low carbohydrate, high fat (LCHF) diet is reproducible 
Before we cover a few highlights from the study, let’s start with a bit of context.
Interest in Low Carb High Fat (LCHF) or “keto” diets as a method to improve endurance performance goes back decades . While there are various approaches to “going keto”, the goal when applied to endurance sport is the same: increase your ability to use fat as an energy source.
Since fat is in nearly limitless supply even on lean athletes, the hope is that increased fat oxidation will free up glycogen for the most crucial moments of competition, ultimately improving endurance performance.
Let’s take a look at the specific keto strategy evaluated in this study .
Low Carb High Fat (LCHF) Diet “Keto”
1. Three week diet
Spend three weeks adapting to diet.
2. Restrict daily carbs
Keep daily carbs under about 50g. That’s the equivalent of around 2 gels, for the entire day.
3. Eat a ton of fat.
Get about 75-80% of your daily calories from fat.
Now that we’ve got a bit of perspective into a typical keto diet, let’s dig into the primary highlights from the study.
Q – What was the purpose of the study?
Researchers sought to reproduce findings from their earlier study that found while a LCHF diet did improve rates of fat oxidation, it did not improve performance in their group of world-class race walkers .
In practical terms, race walkers who crushed carbs were faster than those going keto.
On a personal note. If I was placing a bet on two otherwise identical athletes: one following a LCHF diet and the other following a typical high carbohydrate availability diet, the best evidence to date suggests the cyclist crushing carbs is the safest bet to win .
Q – Why did these researchers go to the trouble to reproduce findings from research they had already done?
The researcher’s original study received wide-spread interest in addition to criticism. In short, reproducing results inspires confidence in the quality of research.
Q – What did researchers find?
Researchers essentially found the same thing as before: that even though a LCHF diet can drive significant increases in fat oxidation, this improved ability to burn fat does not translate into improved performance.
Researchers also found that increased fat oxidation also drives an increase in oxygen demand. Put another way: carbohydrate is a more efficient source of fuel in terms of providing energy per unit of oxygen. The greater efficiency of carbs might explain why the LCHF group performed significantly worse than the carb group.
Q – If you go on a LCHF diet, then add in carbs before a race, can you reap the benefit of increased fat oxidation in addition to the benefit of fueling with carbs?
This was one of the additional questions researchers added from their original study design. In short, there didn’t seem to be any “carryover effect” from having previously done a LCHF diet. In other words, sticking to carbs the whole way through resulted in a net 7% performance difference (Carb group increased 5%, LCHF group decreased 2%).
Q – What are the implications for me as a cyclist?
The best evidence we have suggests that sticking with adequate carb fueling is the dietary approach most likely to result in your best performance.
While LCHF diets improve your ability to utilize fat during exercise, they also seem to reduce your ability to utilize carbohydrates at higher intensities.
Increasing fat oxidation at the expense of carbohydrate oxidation is like putting knobby tires on your road bike. Yes, you’ll gain the flexibility to do more off-road riding, but when it comes to going fast, the additional rolling resistance of your beefier tire will slow you down.
Q – Is there any situation in which a HCLF diet might be helpful to a cyclist?
Perhaps. If you’re doing an ultra-endurance cycling event where regularly fueling might be difficult, or if the intensity demands of your ride are consistently low, trading improvement in fat oxidation, for a decrease in efficiency at higher intensities might be a worthy trade.
Q – How might I apply these findings to my training?
For most cyclists who ride at a variety of intensities, this study should offer greater confidence that if you follow a high carbohydrate availability or periodized nutrition approach to your cycling, you’ll be giving yourself the best chance to ride well.
- Burke, L.M., et al., Crisis of confidence averted: Impairment of exercise economy and performance in elite race walkers by ketogenic low carbohydrate, high fat (LCHF) diet is reproducible. PLoS One, 2020. 15(6): p. e0234027.
- Phinney, S.D., et al., The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation. Metabolism, 1983. 32(8): p. 769-76.
- Burke, L.M., et al., Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. The Journal of physiology, 2017. 595: p. 2785-2807.