L-carnitine: what is it, and what are its main benefits?
The metabolic functions of carnitine have been extensively studied for the past 50 years. It is known that it’s essential for the mitochondrial generation of energy and for acetyl coenzyme A metabolism, and can be found in many dietary sources, such as red meat (e.g. beef), fish, poultry and milk.  Of course you’ve heard about this amino acid before, but are you aware of all it does? Let’s find out!
What is L-carnitine?
L-carnitine, the biological active form involved in fatty acid metabolism and present in food, is biosynthesised by the human body using the amino acids L-lysine and L-methionine as substrates, and transports fatty acid chains to the mitochondrial matrix. This way, it allows for fat breakdown and energy production from accumulated fat reserves . Since L-carnitine and its compounds help reduce oxidative stress, this substance has been suggested as treatment of many conditions, including obesity. [1, 2]
What are the benefits of L-carnitine?
It is essential for the treatment of infant and child nutritional deficiencies and inborn metabolism errors. L-carnitine has been approved by the Food and Drug Administration for the treatment of clinical diseases with this substance deficiency. [3–5] However, there are other advantages in using L-carnitine as a food supplement:
1. Cardiovascular system
70% of the energy consumed by the heart is supplied by fatty acids, and L-carnitine helps transform those fatty acids into a lasting, sustainable energy source for the cardiovascular system. Apart from delaying cardiac ageing and improving peripheral circulation (also improving peripheral vascular disease and reducing intermittent claudication symptoms), L-carnitine appears to protect people who’ve already had a heart attack in the past. [6–8]
It also possesses mechanisms to improve blood pressure in hypertensive individuals, possibly through a mechanism that involves nitric oxide, a substance with a vasodilator effect that increases blood flow. [9, 10]
2. Memory and neurological system
Due to its unique chemical properties, L-carnitine provides neuroprotective benefits to the brain. In addition to stimulating the production of acetylcholine, a neurotransmitter that plays a central role in memory and learning, L-carnitine also increases the levels of noradrenaline and serotonin, monoamines with an influence on mood, anxiety, sleep and food intake, among others.  L-carnitine has been shown to increase the levels of glucose and creatine in some regions of the brain.
Studies prove that a daily intake of 2g of L-carnitine, for a period of 6 months, is effective at alleviating the side effects of ageing. Apart from helping with age-related changes, such as memory and mood, reducing neurological decline, deterioration of mental function and chronic fatigue, L-carnitine seems to protect brain cells from the toxicity of some harmful substances in older individuals. [12, 13]
It can also help regenerate damaged nerves and is currently being investigated as treatment for Alzheimer and dementia. [14–17] In addition to increasing the release of dopamine (a chemical substance used by nerve cells) and improving receptor connection, L-carnitine seems to play an important role in preventing and reducing tremor symptoms in Parkinson’s disease. [18, 19] Moreover, L-carnitine has been shown to significantly improve symptoms in children who suffer from Attention Deficit Hyperactivity Disorder and autism, and it may be considered as adjuvant to the treatment in the future. [20, 21]
3. Reproductive system
Although L-carnitine is not able to increase the levels of testosterone, it has been shown that a daily intake of 2 g for a period of 3 weeks increases the levels of IGF-3 and, in theory, of IGF-1 e IGF-2, hormones directly related to the growth hormone.  A daily supplementation of 3 g seems to improve sperm quality, mainly when it comes to aspects related to its morphology. However, further research is needed to assess all potential biochemical, pharmacological and physiological benefits of carnitine supplementation for the reproductive system. [4, 5]
4. Cancer and hormonal system
Studies that used a daily intake of 2 g of L-carnitine registered a clear improvement in some hormones and, consequently, in body composition. [1, 2]. Since studies with older individuals presented reductions in fatigue levels (improvement of sleep disturbances related to fatigue and post-exercise effort), L-carnitine is currently being studied in the context of chronic fatigue syndrome and cachexia (muscle deterioration). 
This substance can also help with these same symptoms of fatigue, mood and sleep in individuals with advanced cancer. Although it does not prevent cancer, L-carnitine is now being studied as a complement to chemotherapy to help fight fatigue and cachexia, addressing any deficiencies that may occur. [4, 5] Regarding the endocrine system, and although it slightly improves cholesterol levels, especially in older individuals, the results are not statistically significant. Still, it seems to benefit individuals with diabetes, improving insulin (an essential hormone for the control of blood glycaemia levels) sensitivity, and patients undergoing haemodialysis. [6–9]
5. Immune system and infections
L-carnitine can help immune system function by improving the functional capacity of macrophages (cells that protect the body from infections and help eliminate strange elements). [10, 11] Many studies have analysed supplementation with L-carnitine for the treatment of individuals with Human Immunodeficiency Virus (HIV). Although the results of the study are very preliminary, they provide some evidence that daily doses of 2 to 6 g of L-carnitine can result in delayed lymphocyte death (also known as white blood cells and crucial for the body’s defence), reduce neuropathy (a nerve problem) caused by HIV and responsible for pain and numbness in hands and feet, and increase life expectancy of individuals with HIV. [12–14]
What’s more, there is also some evidence that the immune response to infection with M. Tuberculosis, the causative agent of Tuberculosis, can be balanced and improved with substances acting on the neuroendocrine system, such as L-carnitine. 
6. Musculoskeletal system
While exercising, the body requires high levels of glucose and fatty acids to produce energy. The use of these two substrates varies considerably during exercise and depends mainly on its intensity. With L-carnitine, some studies conducted under aerobic conditions have shown a greater preference for the use of fatty acids and a lower use of glucose, which will bring benefits in terms of performance and endurance and delay the onset of training fatigue. [16–19]
Besides, supplementation with L-carnitine before high-intensity training has some benefits in muscle recovery and reduces damage to the muscle tissue, also reducing muscle pain after training. [20–22] This substance is also being studied in the context of osteoporosis prevention. Treatment with L-carnitine slows down bone loss, improves bone microstructural properties by reducing bone turnover, and helps with the healing process of fractures in animal models. As such, it can be a promising substance for the treatment of patients with bone loss and osteoporosis. [23–25]
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