For runners, there are few things more frustrating than running out of energy when mentally you are still good to go. Fatigue is the enemy of endurance, the cause is bioenergetics – how the body uses food for fuel. Fortunately the amino acid derivative L-Carnitine has been shown to improve this process and help keep fatigue at bay.
Bioenergetics – food, fuel and storage
To understand fatigue in exercise better, we should recap how our food becomes our fuel. When digested, food is broken into more simple components; glucose (from sugar/carbohydrates), amino acids (from protein) and triglycerides/fatty acids (fats), before entering our muscles via the bloodstream.
Inside a muscle cell, these simple components travel to the mitochondria, the ‘powerhouse’ which is responsible for turning food into energy. Within the mitochondria, glucose, amino acids and fatty acids can eventually be metabolised through a process called cellular respiration, leading to the production of the molecule ATP. Some ATP can be stored in the mitochondria and provides the muscles with quick, explosive energy. However, only some of our food makes it to become ATP while the rest is stored for later use in one of the body’s two main energy reserves, glycogen and adipose tissue (fat).
Glycogen depletion and the onset of fatigue
Glycogen (found in the liver and muscles) is where excess glucose is stored (along with the occasional amino acid). Glycogen is a highly valuable energy reserve as it can easily be converted back into glucose for easy energy production. Glycogen is limited however and when it runs out, fatigue takes hold and you are likely to “hit the wall”.
The second reserve we have is adipose (body fat) and this is where excess fats can be stored. Adipose contains massive amounts of energy in the form of fats, but unlike glucose from glycogen, fats require adequate oxygen to metabolise, as well as a good supply of L-Carnitine (more on that later).
Powering a run
For the first few seconds of a run, the muscles are powered by ATP, but as this depletes, the body will start breaking down glycogen into glucose. The body will initially turn to glucose rather than fat because this can be metabolised into energy without oxygen (anaerobically) as in the early stages of a run there isn’t enough oxygen circulating in the body. Only glucose can be reliably turned to energy/ATP in this manner. Anaerobic respiration also produces lactate, which can induce the formation of a muscle cramp and muscle fatigue. After about three minutes, our blood oxygen levels climb thanks to our elevated breathing rate and the body is able to start to utilise fats as an excellent energy source, hence sparing glycogen (the stored form of glucose).
Sparing glycogen – Intensity matters
Our body’s choice of fuel reserve also depends on the intensity of the exercise. During low to moderate intensity exercise, we’re likely to use fats as a main energy source as our oxygen levels keep up with the demands of aerobic respiration. During intense exercise however, we use up our oxygen and may have to revert again to anaerobic respiration, where our glycogen will be used.1,2 By encouraging fat metabolism, we can spare our limited glycogen stores and delay the onset of fatigue. This is where L-Carnitine comes in.
Tap the fat
As mentioned, fats are an excellent, and effectively unlimited, energy source, but they are difficult to access. Not only does fat metabolism (known as beta-oxidation) require oxygen, but unlike glucose, fatty acids also find it difficult to get through the mitochondria’s membrane. If fatty acids cannot get into the mitochondria, they cannot be used for energy.
To overcome this, fatty acids require a complex delivery system that involves the amino acid derivative L-Carnitine. This mechanism is known as the ‘carnitine shuttle’ as it effectively transports fats into the mitochondria, enabling the body to burn fat and transform it into energy. Carnitine is necessary in order for the body to be able to turn fat into energy.
Many robust clinical trials show that by increasing the body's Carnitine stores through supplementation, the body is able to transport the fatty acids more rapidly into the mitochondria, allowing for a more efficient energy metabolism and hence delayed fatigue.
L-Carnitine in endurance & recovery
The influence of Carnitine supplementation on exercise endurance was explored in a 2011 randomised control study. In the trial, two groups of volunteers were given either L-Carnitine L-Tartrate supplements or a placebo before exercise and various biomarkers associated with exercise were monitored. In the results, the L-Carnitine L-Tartrate group used 55% less glycogen than the placebo group and experienced a 44% drop in lactate build-up, all leading to an increase in exercise output. The trial also showed that L-Carnitine supplementation encouraged fat oxidation (fat burn), even at high-intensity exercise.3 In a separate study, athletes given L-Carnitine before strenuous exercise experienced a decrease in exhaustion and cardiac exertion versus athletes given a placebo.4
Carnitine’s benefits don’t stop at supporting endurance, it can also aid recovery from exercise. A recent study evaluated the effects of daily L-Carnitine L-Tartrate supplementation for 5 weeks on recovery and fatigue.5 They found that it was able to improve recovery and fatigue based on reduced muscle damage and soreness after exercise. Part of the reason why our muscles are sore after exercise is exposure to oxidative damage. Our mitochondria produce free radicals during the respiration process, which in excess can damage our muscle tissues. In another trial featuring athletes, L-Carnitine supplementation reduced oxidative stress and signs of muscle damage resulting in better recovery, due to L-Carnitine’s antioxidant action. 6
How to choose an L-Carnitine?
L-Carnitine is found in several forms, though for athletic performance, the L-Carnitine L-Tartrate form is favoured when looking at the available scientific evidence. A premium L-Carnitine L-Tartrate product known as Carnipure® is an ideal option due to being produced under strict quality assurance measures. Carnipure® also has an approval from the European Food Safety Authority (EFSA). Considered a high grade of pure L-carnitine, 1000mg of Carnipure is found within the recommended daily dose of RunStrong (3 capsules).
Especially for long-distance runners, the goal is to be able to run for as long as possible without depleting the body's energy reserves. Endurance depends on preserving our limited glycogen reserves in favour of using our practically unlimited fat reserves for fuel. In order to support the body's ability to 'tap the fat', supplementation of high-quality L-Carnitine can help - and that’s why you can find it in RunStrong!
1. Westerblad, H., Bruton, J. D., & Katz, A. (2010). Skeletal muscle: energy metabolism, fiber types, fatigue and adaptability. Experimental cell research, 316(18), 3093–3099. https://doi.org/10.1016/j.yexcr.2010.05.019
2. Constantin-Teodosiu, D., Saris, W. H., & Wagenmakers, A. J. (2001). The effects of increasing exercise intensity on muscle fuel utilisation in humans. The Journal of physiology, 536(Pt 1), 295–304. https://doi.org/10.1111/j.1469-7793.2001.00295.x
3. Wall, B. T., Stephens, F. B., Constantin-Teodosiu, D., Marimuthu, K., Macdonald, I. A., & Greenhaff, P. L. (2011). Chronic oral ingestion of L-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans. The Journal of physiology, 589(Pt 4), 963–973. https://doi.org/10.1113/jphysiol.2010.201343
4. Orer, G. E., & Guzel, N. A. (2014). The effects of acute L-carnitine supplementation on endurance performance of athletes. Journal of strength and conditioning research, 28(2), 514–519. https://doi.org/10.1519/JSC.0b013e3182a76790
5. Stefan, M.; Sharp, M.; Gheith, R.; Lowery, R.; Ottinger, C.; Wilson, J.; Durkee, S.; Bellamine, A. L-Carnitine Tartrate Supplementation for 5 Weeks Improves Exercise Recovery in Men and Women: A Randomized, Double-Blind, Placebo-Controlled Trial. Nutrients 2021, 13, 3432. https://doi.org/10.3390/nu13103432
6. Parandak, K., Arazi, H., Khoshkhahesh, F., & Nakhostin-Roohi, B. (2014). The effect of two-week L-carnitine supplementation on exercise -induced oxidative stress and muscle damage. Asian journal of sports medicine, 5(2), 123–128. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4374610/