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ATP, the energy currency your body needs to live, is produced from your food by your mitochondria. These organelles are present inside every cell of the body except for red blood cells. Remarkably, while mitochondria can convert one glucose molecule into 38 ATP molecules, each fatty acid can produce 129 ATP—making fat a remarkably efficient source of energy. 

Assuming you can get the fatty acids inside the mitochondria, that is. This job falls to one single compound amino acid: carnitine.

Where Carnitine Comes From 

Carnitine is an amino acid produced from lysine, methionine (required for methylation), and a series of enzymes requiring the cofactors Vitamin C, iron, zinc, and NAD+. This process occurs in the liver and the kidneys. Once produced, some 98% of carnitine resides in your muscles, where it can be used to shuttle fatty acids into the mitochondria to get turned into energy.

You can also get some carnitine from food—though as its name implies (carne), it’s only found in animal products. Carnitine levels are far and away highest in beef, though there is some in dairy, poultry, and fish as well. Plant-based foods are a negligible source, though vegetarians can compensate for this with higher bioavailability of the carnitine they do have than meat eaters. Nevertheless, deficiencies of carnitine can be dietary (lack of intake of carnitine itself, or of its precursors), due to genetic mutations in the transporters necessary for carnitine function, or due to kidney or liver disease, compromising endogenous production.

Carnitine, Energy, and Cognition 

Given carnitine’s role in energy production, it’s not surprising that it is associated with improvements in fatigue—depending upon the cause of fatigue, of course. If carnitine levels are already replete, increasing beef consumption or supplementation isn’t likely to make a big difference. But if levels are moderate to low, it may. 

This study of centenarians found improvement in fatigue and cognitive function with carnitine supplementation, as well as a positive shift in body composition toward more muscle and less fat. Along those lines, this study showed improvement in short-term memory among dementia patients as well.

This study showed improvement in both physical and mental fatigue with carnitine supplementation. 

Due to carnitine’s storage in muscles, it’s often used as an exercise supplement to boost exercise performance. This study showed that high dose carnitine prior to a workout increased the time until muscle fatigue. 

Carnitine and Fat Loss

One would think, given its role in fatty acid breakdown, that carnitine would be beneficial for weight loss. There are some studies that show this, though there are others that show more of a modest effect. 

Again, I suspect this is because there is no indication in the studies I read that subjects were screened beforehand for carnitine deficiency. My guess is that those who noticed less of an effect also had adequate carnitine levels to begin with. 

Carnitine and Metabolic Markers 

If carnitine would be expected to improve weight, one might expect similar benefits to other metabolic markers, particularly lipids. While this is the case, the studies that show a benefit require high doses, upwards of 1500-2000 mg/d.

Insulin sensitivity also improves with carnitine, and blood sugar levels drop significantly as well. This may be due to increased activity of an enzyme called AMPK, which improves efficient utilization of carbohydrates. 

Carnitine and Fertility

While the number of mitochondria per cell varies greatly by cell type, the low end is a few hundred, while oocytes top out the list at over 100K! While oocytes are dormant throughout most of their existence, once fertilized, they have the massive job of cellular division into a new human, which requires an incredible amount of energy. Mitochondrial damage accumulates with time, which is the primary theory for the inverse relationship between maternal age and fertility. Carnitine is only one supplement that may be helpful in supporting mitochondrial function in fertility, but it has been shown to be helpful.

Sperm, likewise, require a great deal of energy once released, and motility plays a big role in male fertility. Carnitine has been shown to support this, as well.

The Upshot

Anytime you read a list of various benefits of a given supplement, it’s temping to conclude, “wow, shouldn’t everyone be on that?” 

The answer is, it depends. All of the aforementioned problems that carnitine has been shown to assist can have many other causes, too. The question is whether you have those symptoms, and whether you are also deficient in carnitine as well (or toward the low end of the reference range). If you are, then supplementing may be helpful to you. If not, you likely won’t notice much difference. 

That said, carnitine supplementation is generally safe with few side effects, and typical doses range from 500-2000 mg per day. If you would like to try it, the best option for exercise tolerance, physical energy, or metabolic markers would be L-Carnitine,

while a better choice for mental fatigue and cognition would be Acetyl-L-Carnitine, as it crosses the blood brain barrier better.