L-carnitine is advertised and described as a safe dietary supplement that helps burn body fat and build muscle. It is especially popular and is used in a number of fitness programs for weight loss and strengthening and is a regular ingredient in most energy drinks , but few know what the active ingredient actually is and how effective it is.
Carnitine is a compound present in almost every cell of the human body. It is a natural substance formed by the amino acids lysine and methionine and is responsible for the transport of fatty acids to mitochondria. 
Mitochondria are intracellular structures whose function is to produce energy and can be thought of as the energy centers of cells. Carnitine transports fatty acids to the mitochondria, where they are "burned" by oxidative processes that release energy. There is another important function related to transport: it removes waste and toxic substances from the mitochondria, preventing their accumulation.
L-carnitine is one of the three forms of the compound and is used in medicine to treat carnitine deficiency - a condition in which the body synthesizes very small amounts of any form of carnitine. The deficiency can be caused by a genetic predisposition, as well as by taking drugs or hemodialysis , which can lower the levels of the transport compound in the blood. It is also used in the treatment of cardiovascular disease , chest pain , diphtheria and high cholesterol
Unfortunately, these are the only scientifically based reasons to take L-carnitine. Any other suggestion, whether for health or fitness, is either a publicity stunt or a blatant fallacy: there is no independent and reliable scientific evidence to support the benefits of taking the compound for purposes other than those listed above. 
Despite the lack of sufficient evidence, the compound is especially popular as a means of increasing muscle mass and strength, losing excess fat and reducing exercise fatigue.
When taken orally, L-carnitine reaches the intestines, where intestinal bacteria break it down and the resulting products are absorbed into the blood. Reaching the liver, these products interfere with the natural metabolism and shift its balance , leading to the accumulation of plaque on the walls of blood vessels and to a slow but sure rise in blood pressure, leading to hypertension . A US study reveals that only vegetarians can consume L-carnitine as a dietary supplement without experiencing side effects because they have a very different intestinal microbiome adapted to a plant-based diet.
Whether taken in capsule, powder, solution, tablet or other form, levocarnitine , the active ingredient in L-carnitine products, has been shown to increase blood pressure with systemic use, with rapid heartbeat often accompanying the condition. Abdominal pain and discomfort, diarrhea, headache, nausea and vomiting, increased body odor, depression, impaired vision, sweating of the palms and soles and insomnia are also among the side effects of levocarnitine products.
If, according to individual preferences, the benefits of taking L-carnitine outweigh the potential risks, its use should not exceed 2 g per day and should be combined with strenuous exercise. If the compound is taken without consuming active and daily training activities, it causes its side effects more often and faster and accelerates the accumulation of fat and plaque.
100 g of mutton contains about 190 mg of L-carnitine, 100 g of beef - 90-130 mg, 100 g of pork - 28 mg, and chicken and seafood contain from 1.6 to 6 mg. These amounts should be in accordance with the daily intake of food supplements containing L-carnitine and should not exceed 2000 mg per day.