Episode 169 – Low Carb Diets for Endurance Athletes

Key Topics Covered

Traditionally, higher carb diets are preferred for endurance athletes due to a high carb utilisation in training and events. However, there has been interest in low-carb, higher-fat diets for these athletes for several years. Today we will be unpacking a low-carb diet for endurance athletes. Firstly, we will outline why high-carb diets are typically preferenced for endurance athletes, why low-carb diets theoretically might make sense, a summary of the research and our interpretation and recommendations for athletes.

Why high carb approaches are typically used

Carbs are the most efficient fuel source for higher intensity activity.
They can be metabolised into ATP more easily than fat can.
Research comparing higher carb approaches vs normal intakes consistently has found improved outcomes. Particularly when looking at things like carb loading and intra-race carbs.

Reasons why low carb could make sense

Storage of glycogen is limited.
- Even with a full carb load, we are still likely to run out of glycogen at some stage in an event like a marathon, or beyond.
On the other hand, eating fewer carbs improves our body’s ability to use fat as a fuel source. Including better mitochondrial efficiency and fat oxidation.

Those things aren’t debatable. But there are a few other proposed benefits that are a bit more debatable including:

Improved body composition.
Reduced inflammation.
& Lower intake of high sugar products like sports gels, leading to a more nutritious diet in general.

Broad summary of the research so far

The broad summary is that low carb diets have not outperformed high carb diets for performance. The difference has not been big, but on average they have underperformed.
At lower intensities, say 60-65% of Vo2 max, it doesn’t seem to be a disadvantage. But at higher intensities, it does.
The biggest difference appears to be the metabolic cost is higher to move at the same pace.
The simple way of explaining this is that fat metabolism requires more oxygen than carbohydrate metabolism to produce the same amount of ATP.
At 80% VO2 max, oxygen delivery to muscles is a limiting factor. The greater oxygen demand for fat oxidation can lead to quicker onset of fatigue. This also explains why RPE is higher in some of these studies that involved set intensities.

Interpreting the research

While that broad summary sounds like a nail in the coffin, there is way more that needs to be unpacked here
There is a lot less research than you’d expect on this topic.
- & sample sizes are always small, typically between 5-16 participants.
We often talk about how small sample sizes skew training studies because 1) Genetics and individual responses 2) and what if somebody just has a good day on the day of testing?

Supernova Study

Let’s use the 2017 Supernova Study, linked in the show notes, as an example since it is one of the more well known studies.
They had a high carb group, periodised carb group and low carb group.
The high carb group improved their race times by 6.6%. The low carb group had no change in performance.
Digging into the numbers though, somebody in the low carb group improved by 5.3%, while somebody else in that group got slower by 8.5%

Supernova Study – Part 2:

Another thing to consider for this study is the testing conditions.
There were 4 testing days, 2 for each of the study groups which was pre and post intervention.
There was quite a large discrepant in wind and temperature between testing days.
- The lowest temp was 17 degrees whilst the highest was 28 degrees.
- With a mix of low, mild and moderate wind.

The researchers did somewhat factor this into their results.
But it’s worthwhile being aware that one group having their pre-intervention be at 28 degrees when all the other testing was done 17-20 degrees has the potential to massively change outcomes.
This is part of why more research with large sample sizes has so much potential to help.

How long does fat adaptation take?

Researchers in this space have typically used short study durations. For example the Supernova study only involved being low carb for 3 weeks.
Critics of this research often point to the short durations. I also see logic in that criticism.
There are two main responses to this criticism. One is around logistics. To really answer this question, researchers are often trying to study high level athletes, since those are who this question matters for most.
How are you going to keep a high level athlete in a controlled environment for >12 weeks? It’s much more reasonable to do a 3-6 week training camp somewhere and test this. Longer timeframes are tough with travel + also factoring in that an athlete won’t want to commit to trying something for a long timeframe if there is a chance it will be detrimental.
The other one is how do we MEASURE fat adaptation. Researchers highlight that all measurable changes appear to occur within a few weeks.
Other arguments, which I also understand, suggest that it likely takes 2-3 months or more to achieve full adaptation. If you also think about it from the athlete perspective, it likely also takes a while to figure out the best pacing strategy etc too if things did feel noticeably different.

Ultra Endurance

The longer the event, the more likely low carb approaches are to have benefits.
For example, if a full carb load would get you to around 90 minutes of glycogen storage while running at marathon pace, that covers a half marathon if performing at a high level.
It helps for a full marathon. But for a 100km+ event, you could start to question, does it help that much?
Events like Ultra-marathons typically involve a lot of time spent at ~65% of Vo2 Max, where you are less likely to see disadvantages of low carb approaches.
Theoretically, the longer the event is, the more likely it is to be beneficial, although we are not yet at a point where we think it WILL be beneficial for sure.

Anecdotal Cases

There are many athletes who compete at high levels while on low carb diets. So let’s discuss that.
The first part can be body composition changes. If somebody reduced their calorie intake and got leaner, this could lead to improved performance.
Secondly, it’s worth acknowledging that the differences in performance are not actually huge. My interpretation of the research is that for something like a marathon, it is less than a 3% difference on average. And that’s average, not even factoring in individual variation.
Many people can and should be getting better just by training. If somebody trained for 6 months in a low carb state, in many cases they should get better, because they would have gotten better anyway.
There’s individual variation. Some people might perform better on low carb diets than others.
People who struggle with things like gels from a GI perspective likely also would get more benefits out of this switch than those who don’t.
There are many examples of anecdotal cases. But for what it’s worth, if we wanted to go anecdotal, there are many more cases of people performing well on high carb diets.

Summary:

Overall, the research at this stage is in favour of high-carb diets over low-carb diets for endurance performance. Although there are gaps in the research, there is individual variation, and there are cases where low-carb diets might have advantages.

Relevant Blogs/Research

Relevant Studies:

Relevant Blogs:

Relevant Podcasts:

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