In our Part 1 blog on Carnitine, we highlighted research showing the positive implications for endurance performance through upregulated muscle-carnitine loading.
Tangible metabolic improvements in fatty-acid utilization (and subsequent glycogen retention) at lower intensities, and lower lactate production at higher intensities - are material and meaningful advantages, for endurance performance.
It's important to note again that Carnitine is produced by the kidneys, liver and brain - but not by muscle tissue. While the heart and skeletal muscle tissues stores about 95% of whole-body carnitine, they do not produce Carnitine. To that end, when the metabolic need arises, Carnitine needs to be transported from the producing organs, and then actively transported into the muscle (skeletal or cardiac) tissues. It's here that it can apply its work of transporting long and medium-chain fatty-acids across cell membranes, and into the mitochondria for oxidation - and subsequent energy release.
STUDIES ON MUSCLE CARNITINE LOADING
Again, in our Part 1 blog, we highlighted how earlier attempts of muscle-loading of Carnitine had been largely unsuccessful. Nevertheless, these earlier efforts provided insight into possible mechanisms, of Carnitine muscle-loading.
In 2011, researchers (1) demonstrated upregulated muscle-loading of Carnitine through excessively high (80Grams/twice daily) carbohydrate co-ingestion. This diet (impractical) driven approach to elevated insulin levels was effective in increasing the transport of Carnitine into muscle cells. However, the high intake of carbohydrate and subsequent insulin response would blunt fatty-acid oxidation - ultimately defeating the beneficial outcomes being sort.
However during 2023, there have been two hallmark studies which begin to provide material and practical methods for effective muscle-carnitine loading.
Firstly, in one study (2), it was shown how the transport of labelled supplemental Carnitine into the muscle cell, could be increased, via by acute muscle contractions. It was shown that as the muscle mitochondria used acetlycarnitine to buffer excess acetyl-CoA (from fat oxidation and glucose breakdown in support for muscle contractions), an acute intracellular deficiency of carnitine arose, thereby increasing the demand and transport of Carnitine from the blood plasma into the muscle cell. An interesting note within this study was that, the increase in Carnitine uptake, took time. The study found that at 30minutes of muscle contractions there were minimal increases, however at 60minutes Carnitine levels had risen 1.5 times the baseline. Researchers also highlighted how labeled Carnitine uptake was distributed in the more oxidative (aerobic, more mitochondrial dense) muscle fiber types (type I and type IIa), versus the more glycolytic type IIb muscle fiber types.
Second, in a follow-up study (3) led by several of the same researchers from the 2011 study, the team sort to test whether caffeine (also a stimulant of Na/K ATPase pump, like insulin) would raise Carnitine clearance from the blood. In the final 90minutes of the 5hour infusion and testing procedure, blood levels of Carnitine had dropped by ~15% suggesting also an effective method of muscle-carnitine leading. It is important to highlight the caffeine dosage used (total of 9mg/Kg body mass) is a material (~high) dose of caffeine. Applied to a 75Kg/165 pound athlete, this would translate to 675mg of Caffeine, over a 5 hour period.
LEARNINGS AND PRACTICAL TAKEAWAYS
Based on these studies, we can apply these learnings to both supplemental dosage, and consumption methods in triggering material muscle loading of Carnitine - in support of improved endurance metabolism. This includes,
- Consume supplemental Carnitine just prior, or during physical exercise,
- Exercise (muscle contractions) should be minimally for 30minutes, and ideally for 60minutes or more to trigger metabolic shifts which raise Carnitine uptake from the blood, and into the muscle cells,
- Co-ingest Carnitine with Caffeine, to further raise blood clearance (ie. loading into the muscle).
OPTIMAL METABOLIC GUIDANCE
To that end, our guidance would be, 1-2 Sachets of SFuels PRIMED prior-during aerobic zone 2, or zone 4 interval-tempo workouts of 60minutes or more.
- Chronic oral ingestion of L-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans. Wall. B et al. Sports Medicine. Journal of Physiology Feb 2011
- Stable isotope-labeled carnitine reveals its rapid transport into muscle cells and acetylation during contraction. Furuichi. Y et al. Heliyon. Mar 2023. (Note this article is under creative commons 4.0 deed license - article can be found here)
- Caffeine ingestion stimulates plasma carnitine clearance in
humans. Wall. B et al. Physiolological Reports Feb 2023.