Of all the sports supplements, even amongst the most well-read sports science and nutrition experts, carnitine has to be up there with the most widely debated and this is because the research is often conflicting, the results dependent on the context and population of the study and other factors that influence whether carnitine is effective or not. We are going to focus in this article on carnitines role as a potential aid to fat loss. Although carnitine in its different forms may have other health or performance benefits, carnitine (usually in L-Carnitine form) is often marketed as a fat loss supplement on its own and part of other fat loss products.
Firstly, we will take a little look as to why carnitine was, and continues to be, investigated for potential fat loss benefits and this fundamentally relates to what carnitine does in the body. Carnitine is a water soluble amino acid that is mostly stored in the muscles, holding over 95% of the body’s total stores. Carnitine can be created in the liver and kidneys from other sources if we are running low, and is also obtained from the diet and is found in good amounts in meat and dairy. Carnitines interest from a fat loss perspective is that it is an essential component of enzymes that are used for the transportation of fatty acids (that we would obtain from the diet or as a result of body fat (adipose) tissue breakdown) into the mitochondria of the muscle cell. In the mitochondria, these fatty acids are used to generate energy and this is why they are considered they are considered the power plants of the body. In theory, if we could increase muscle carnitine levels beyond normal physiological levels then this could allow more fatty acids to be shuttled into the mitochondria and these would be preferentially used as a fuel, equating to higher rates of fat loss. ‘In theory’.
Unfortunately, it appears that when supplementing with carnitine even for long periods, this doesn’t have an impact on muscle carnitine levels. It also assumes that carnitine levels are the determining factor that will govern the use of fat as a fuel, and this is a very complex system that is reliant on many steps and under the influence of many factors such as hormone levels, and a variety of enzymes that could all act as the ‘rate limiting step’ in the process. A rate limiting step is basically a point at which a process can only work so quickly, so for example an enzyme that is responsible for a part in a process can only do its job so fast - think of this as someone (our enzyme) trying to pick and pack orders at a factory. To a certain extent you can increase the amount of orders and the packer will be able to cope, but at some point the process will stop when they become overwhelmed (the individual enzyme has no reached its capacity to do its thing!) Now the company (the DNA of a cell) can produce more workers (enzymes) to help with the packing process (in our case using fat for fuel) however, even then this can only occur to a certain point as there are only so many applications for the job and positions you can afford to fill. In carnitines case, it appears that the amount we can get into the muscle is limited and supplementation might only be of benefit if someone has some kind of deficiency.
So if these two pretty big factors are limiting the effect carnitine can have, why are there still debates? Well the assumption that carnitine cannot be increased in muscle is false; it can, but it is dependent on the co-ingestion of carbohydrate. It also appears that carnitine is linked to the rate limiting step in the process of fatty acid oxidation (using fat as fuel source) and that elevated muscle carnitine levels are associated with preferential use of fatty acids as a fuel source, switching more from carbohydrate to metabolism to be more fat dominant.
Carbohydrates are important, as it appears high amounts of insulin are required to transport carnitine into the muscle in the first place and this requires large doses of carbohydrates, with study’s showing requirements of 80g of carbs and upwards. Straight away you can see that any potential fat loss benefits will not be gained in those on a ‘keto’ or very low carb diet as this amount of carbohydrate would comfortably pull you out of ketosis. Another issue is that carbs come with calories, and although 80g of carbohydrate may only contain 320kcal, this still needs to be considered. So for those who are looking to restrict calories and are interested in trying carnitine as a fat loss aid, keeping your carbs for use with L-carnitine (especially around training when demand for fatty oxidation goes up when we require more energy) is probably the best way to increase muscle carnitine levels and potentially increase fat loss.
Again, here I have to use the potentially and again I have to highlight why. Unfortunately, the study’s that have shown increased muscle l-carnitine concentrations and corresponding increases in fatty acid oxidation, especially during exercise, have been in populations where they were eating a normal or increased amount of calories and in these populations they have found that those who supplemented with carbohydrate and l-carnitine maintained their weight and fat mass, whilst the people who had just the carbohydrate increased body weight and fat mass. This hints at the potential for carnitine to be used as a fat loss aid but there needs to be more research done when we can compare groups that are placed on a calorie restricted, fat loss diet and to see if it is effective without the use of exercise in these populations.
However, this does show one potential use that many people would not consider carnitine for! If on a calorie surplus and we are using more fat over carbohydrate and maintain weight, then in a muscle building phase this could mean with higher calories there is the opportunity to make leaner gains. It appears L-carnitine has some form of nutrient portioning effect meaning that carbohydrates will be stored as glycogen in the liver and muscles whilst dietary fat, instead of being stored whilst we would be using our carbs for fuel, is then preferentially used as a fuel source leading to less body fat gain. Obviously calories do still matter as carbohydrates can be converted to fat (there’s only so much the body can store before it does), but by creating an environment in which we are not storing dietary fat as fat (which is actually where most stored body fat comes from) we may be able to stay a little leaner.
On a final point, carnitine may also be useful for athletes as by being able to more efficiently use fat as a fuel, then we can preserve muscle glycogen for when we really need it and this might be particularly important for endurance athletes and research in this area shows some promising performance results.
In summary, L-carnitine supplementation under the right nutritional circumstances has shown some really interesting benefits in reducing fat storage, but these have not been thoroughly tested and confirmed as an effective fat loss aid as part of a fat loss diet. Hopefully through this minefield of information this article has at least cleared up one thing… And that is hopefully that you can now see why people still argue about the merits of l-carnitine as a fat loss supplement!