The key to enhance performance on a high-fat diet

There is a professor (Louise Burke) who looked at athlete performance extensively in elite race-walkers. Shamefully the high-fat community has criticized her and attacked her because the results that she came up with showed that when it comes to world class athletes, there is no performance benefit and more likely there is a slight decrease.

That critique has been met with more research and the results remained the same.

Rather than criticizing and recognizing the facts for the given circumstances, I started to wonder why there could be an issue with performance. We’ve seen great benefits from fat adapted athletes meaning improved performance and markers but not at the high intensity that needs to be sustained during a race.


We know that under high intensity the contribution of fat to energy diminishes. It is not a great amount but still, it reduces.

the same data represented as a graph

The figure above comes from people who were not fat adapted. They have their peak fat oxidation at around 55% of VO2Max. When we look at a study from Jeff Volek then we see that maximum fat oxidation can shifts towards a higher intensity. The high carb group shows the same results as before, max fat oxidation at 55% while the LC group (very low carb) reached the peak at 70%.

That is an impressive result so why doesn’t it result in improved performance at the highest intensities?


Without going into too much details, carnitine is needed to bring the long chain fatty acids (LCFA) into the mitochondria so that the fat can be used to produce ATP.

The following presentation helps greatly to understand why carnitine availability is an issue.

In short, at higher intensities there is not enough available carnitine to support the import of LCFA so that there is even a negative effect on its ability to import those fats.

That is a major blocking point if you are primarily fueled by fat.

In high carb athletes we see a strong reduction after 65% of VO2Max intensities. My guess is that we’ll see a similar effect at around 70~75% for the fat-adapted athletes.

This is supported by another study looking at free carnitine. Subjects started at an average of 15.9 mmol per kg of dry weight. Exercise at 70% and 100% VO2Max resulted in a drop to 5.9 and 4.6. Less than a 3rd remains.

“Muscle carnitine metabolism during incremental dynamic exercise in humans”

Increase carnitine content

So naturally people will have the reflex of supplementing carnitine and indeed, when they succeed we see a number of changes. It translates into a greater reduction in lactate which shows that more fat is used for energy production.

When testing the results of a 20% increase in carnitine content in a time trial effort of 30 minutes at 80% VO2Max we see that all participants improved and on average had an 11% increase in performance. The study subjects were recreational athletes so the results may not equal in a similar performance gain for top-level athletes but it shows promise and 11% is a very big deal if this could be achieved in such athletes.

“Chronic oral ingestion of L-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans”


Naturally if it offers a performance enhancement we’re interested. It turns out that it is not as simple as taking a carnitine supplement. Bacteria in the gut seem to love it and not even intravenous supplementation made any change.

What researchers did find out is that it requires insulin and works via a sodium transporter. So they recommend to take carbohydrates along with the supplement.

But research comes with varying results. For example 3 months of supplementation shows no increase while 6 months do.

Three months of dietary supplementation with a combination of carnitine and CHO had no effect, but after 6 months muscle carnitine content increased by 21%. The necessity of using a very long supplementation period demonstrates the difficulties involved and explains the failure of previous studies with shorter intervention periods.

source: “Boosting fat burning with carnitine: an old friend comes out from the shadow”

If you however want to stay on a ketogenic diet and be as fat adapted as possible, I think there are alternative ways to increment muscle carnitine levels.

I know we are not chicken but when looking at what they did, they were able to increase the body mass while reducing the fat content. How they achieved this was by varying protein content in the diet and supplementing with carnitine. It looks like the combination is a great way to increase the absorption and usage.

“Effects of dietary L-carnitine supplementation and protein level on performance and degree of meatness and fatness of broilers”

From the youtube presentation it was clear that insulin is required. They experimented on one hand with 80gr carbs and later on also had experiments with 40gr carbs mixed with protein and had similar results.

Purely a guess but I think that the insulin response to a meal is already sufficient for the absorption so low carb athletes could make sure that they take a supplement during meals.

But also here the results are conflicting. The inclusion of carbs and protein seem to blunt the uptake in an acute phase. Conflicting because from the youtube presentation there was successful results using a mix of 40gr carbs and 13gr protein in 24 weeks.

“Protein ingestion acutely inhibits insulin-stimulated muscle carnitine uptake in healthy young men”

The following study, although a very specific case seems to hint at a redistribution mechanism. This may indicate why it could take some time before muscle carnitine increases. It is possible that there is a lot of tissue in the body that takes up carnitine so regular and prolonged higher intake of carnitine may be necessary to create a saturation effect before muscle carnitine increases.

“Muscle and plasma carnitine levels and urinary carnitine excretion in multiply injured patients on total parenteral nutrition”

There is also a trend towards carnivore eating, it would be interesting to compare carnitine levels in these people/athletes to see if natural sources are sufficient to increase levels.

So clearly more research is required and hopefully we can see another step up in athlete performance.

  • What are the best tolerable ways to increase carnitine in the shortest period of time?
  • How high can carnitine levels be pushed in athletes?
  • Does increased carnitine provide a performance benefit for high-fat athletes and does that benefit still hold if we also increase carnitine levels in high-carb athletes?
  • Do we have any non-invasive proxies so that we can avoid muscle biopsies to check carnitine levels?
  • Where lays the true limit of oxygen availability?

A final word on the fat part

One other way to overcome the limitations that carnitine levels impose is to enhance the availability of short and medium chain fatty acids in the body. However, levels must be sustained for the whole duration of the race. It is likely that the absorption of these fatty acids into the mitochondria goes faster and will be depleted or reduced quicker. Fueling with fats during the race is not that straightforward.

A study looking at fat oxidation noted a quite clear difference. This was achieved through infusion so not a practical approach during racing but it shows the potential.

Furthermore, the percentage of oleate uptake oxidized decreased from 67.7 +/- 2.8% (40% VO2peak) to 51.8 +/- 4.6% (80% VO2peak, P < 0.05), whereas the percentage of octanoate oxidized was similar during exercise at 40 and 80% VO2peak (84.8 +/- 2.7 vs. 89.3 +/- 2.7%, respectively)

“Regulation of plasma fatty acid oxidation during low- and high-intensity exercise”

When looking at high performance and circulating fat we see that there is also a reduction in circulating fatty acids which also support the idea that in case of highest performance you need to fuel fatty acids.

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“The Effects of a Ketogenic Diet on Exercise Metabolism and Physical Performance in Off-Road Cyclists”

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7 thoughts on “The key to enhance performance on a high-fat diet

    1. A problem with taking l-carnitine supplements is that it doesn’t get absorbed into the muscle cells that easily. That is why research yielded disappointing results until they started to found ways to increase intracellular levels. Once increased, performance did improve. The way they got the levels to increase is by combining intake with carbs to stimulate insulin which is needed to get it into the muscle cells. So if you want to optimize for performance.. either take it with carbs or as part of a meal.


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