Carnitine is required to bring the longer chain fatty acids into the mitochondrial matrix where it can be further broken down to produce ATP.  Carnitine Palmitoil Transferase 1a (CPT1a) is a mitochondrial outer membrane bound protein that will take in an acyl-CoA and bind it to carnitine resulting in acyl-carnitine.  

On a ketogenic diet (KD), CPT1a will increase in numbers because the balance between available glucose and fat shifts towards more fat and less glucose.  All the relevant signaling increases the capacity to get fatty acids into the mitochondria.  This not only requires more CPT1a but also a higher availability of free carnitine to bind with.

source: “New insights concerning the role of carnitine in the regulation of fuel metabolism in skeletal muscle” https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2075186/ 

The skeletal muscle is the biggest organ and needs to produce the most energy during activity.  In addition, it should rely mostly on fat because the brain cannot rely on fatty acids as an energy fuel.  By increasing its reliance on fat, the skeletal muscle can reduce its glucose metabolism and save it for the brain.  Therefore skeletal muscle holds roughly 98% of the available carnitine.

source: “Pharmacokinetic considerations for the therapeutic use of carnitine in hemodialysis patients” https://www.clinicaltherapeutics.com/article/0149-2918(95)80017-4/pdf 

The amino acid carnitine can be obtained from the diet via meat, poultry, fish and dairy products but it can also be synthesized by our bodies.  Our bodies generally depend on both.

L-carnitine is synthesized from the essential amino acids L-lysine and L-methionine.

source: “Ascorbic acid and carnitine biosynthesis” https://www.sciencedirect.com/science/article/pii/S0002916523319646 

One would think that in order to obtain sufficient carnitine, it would be simply a matter of eating carnitine or food containing L-lysine and L-methionine.

Before we can make any conclusions, let’s look at absorption first.

Absorption from food

One tracer study that radio labeled L-carnitine made the following observations.  5 subjects started taking carnitine supplementation for 15 days.  They took 4 capsules per meal, 3 times per day.  On day 5 they received an additional single dose of labeled L-carnitine and they monitored the labeled excretion via urine and feces.  After 24 hours, 23% to 24% was recovered via urine.  Fecal carnitine accounted for less than 2%.  However, fecal γ-butyrobetaine accounted for as much as 45% of the administered dose.

Finally they concluded:

In an earlier report,” we quantified the extent of carnirine degradation at two different levels of carnitine normally found in western diets (2 and 10 micromol/[kg body wt] d). We found that at these two levels of dietary carnitine, 25.3% and 36.7% of a tracer dose of [methyL3H]rcarnitine was excreted as metabolites other than carnitine. In this study, we found that at a carnitine intake of 141 micromol/[kg body wt] . d, 55% of the tracer dose was excreted as metabolites. These results further support the conclusion that the extent of absorption (percent absorbed) of carnitine is inversely proportional to the level of intake, and conversely, the degree to which carnitine is degraded in the intestinal tract is directly proportional to the quantity ingested. 

Thus, the more you take in, the higher the % you excrete.  The researchers even point out that there may have been an additional 27% excreted through volatile metabolites.  Thus a total of 83% is not ending up as carnitine in the cells. From the 141 micromol/kg only 24micromol/kg may be resulting as carnitine but that would assume 100% efficiency for what could not be traced.  I would say that is unlikely so the final amount may even be lower.

“Quantitative estimation of absorption and degradation of a carnitine supplement by human adults” https://www.metabolismjournal.com/article/0026-0495(91)90033-S/pdf 

It is possible that the absorption depends on other factors within the meal.  For example, the addition of fat may increase absorption and reduce degradation in the gut or a full protein meal may do something similar.

The following source also points out the poor absorption from oral supplementation and states absorption from food is around 54%~72% after adaptation to a high-carnitine diet.

https://lpi.oregonstate.edu/mic/dietary-factors/L-carnitine#absorption

What is also interesting is that carnitine is present in the bile.  This indicates that carnitine is involved in the absorption of dietary fat.  The carnitine is >50% in the form of acylcarnitine but this would distract us so I won’t go further into it.

“Does carnitine have a role in fat absorption?” https://www.sciencedirect.com/science/article/abs/pii/0024320587906801 

Insulin

A number of trials have tried to increase carnitine by supplementing with carnitine and even intravenous administration without effect.  As we’ve seen, the absorption is very low.  Through experiments they found that adding carbs, stimulating insulin, helps to increase levels.

“Chronic oral ingestion of L-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans” https://physoc.onlinelibrary.wiley.com/doi/full/10.1113/jphysiol.2010.201343 

During an experiment they were able to increase skeletal muscle carnitine from 22 to 24.7 mmol/kg/dm. A 12.3% increase. And this was with a simultaneous 2.3-fold increase in carnitine transporter protein (OCTN2) mRNA expression.  This is a sodium dependent transporter.

“Insulin stimulates L-carnitine accumulation in human skeletal muscle” https://faseb.onlinelibrary.wiley.com/doi/epdf/10.1096/fj.05-4985fje 

24 weeks of supplementing 1g of L-carnitine-L-tartrate + 3g of leucine per day did raise plasma free carnitine and total carnitine but had no effect on the skeletal muscle content.

“Twenty-Four Weeks of L-Carnitine Combined with Leucine Supplementation Does Not Increase the Muscle Carnitine Content in Healthy Active Subjects” https://www.karger.com/Article/Abstract/529333 

Another failure with similar dosage and duration had similar endresults.  A rise in plasma carnitine but no effect on strength.  No muscle biopsy was taken.

“L-Carnitine Combined with Leucine Supplementation Does Not Improve the Effectiveness of Progressive Resistance Training in Healthy Aged Women” https://link.springer.com/article/10.1007/s12603-022-1848-y 

It makes the evidence conflicting because leucine is insulinogenic yet doesn’t seem to have the same effect.  Is it possible that leucine supplementation fails to get absorbed like carnitine?

What we can tell is that the form in which supplements are ingested have their effect.  This was investigated by the next paper and they made an interesting comment:

a recent study (39) in elderly women showed that when dietary protein intake was increased through addition of vegetable protein, postabsorptive protein breakdown was not inhibited to the same extent as that occurring when animal protein was given. The study also showed that net protein synthesis during the fed period of the day was less with feeding high vegetable protein vs. a high animal protein diet. These kinetic or leucine turnover differences were observed despite the fact that both high-protein diets supplied a generous total nitrogen intake (201–209 mg N·kg21 ·day21), and the subjects were in daily body nitrogen balance.

“Kinetics of L-[1-13C]leucine when ingested with free amino acids, unlabeled or intrinsically labeled casein” https://journals.physiology.org/doi/pdf/10.1152/ajpendo.2000.278.6.E1000 

Thus we see here that although leucine makes a difference, it matters how it is taken in.

I find the results conflicting.  Supplementation fails due to the absorption.  The addition of carbs may improve absorption but I have no evidence of that.  Assuming it doesn’t then how could it possibly be that carbs increase skeletal muscle carnitine levels?  It is possible that insulin stimulates the protein synthesis. 

“Insulin-associated changes in carnitine palmitoyltransferase in cultured neonatal rat cardiac myocytes” https://www.jmcc-online.com/article/S0022-2828(08)80054-5/pdf 

But that would be conflicting with the observation that carnitine and acylcarnitine goes up under starvation and ketogenic diet.  These conditions are marked by reduced plasma insulin levels.

“Translating the basic knowledge of mitochondrial functions to metabolic therapy: role of L-carnitine” https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3590819/#S4title 

Because insulin generally dictates ‘storage’ and carnitine is involved in consumption of that storage, there may be some opposing effect.

But I want to add one more study so that the confusion is complete.  They looked at carbohydrate + L-carnitine (CL) or carbohydrate+protein + L-carnitine (CPL) administration and looked at plasma level and also looked at the disappearance rate in the forearm by looking at the difference between arterial and venous content.

CPL saw greater plasma levels than CL (3.5 mmol/L vs 1.9 mmol/L).  Oddly the control group with just flavored water saw a higher increase than CL with 2.1 mmol/L.  This indicates that absorption is increased with added protein in the diet and carbohydrates have no effect on absorption.

As for the A-V difference, CL saw a 4.1 micromol difference while the control and CPL saw -8.6 and -14.6 micromol difference.  This looks to say that insulin increases absorption but, purely speculating, glucagon may blunt absorption.

“Protein ingestion acutely inhibits insulin-stimulated muscle carnitine uptake in healthy young men” https://academic.oup.com/ajcn/article/103/1/276/4569332 

To conclude, it seems that you need protein to maximally absorb carnitine from the food and then you need insulin without the protein to absorb the carnitine into the muscle.

Endogenous synthesis

The previous link from Oregon State University also points out that L-carnitine is primarily synthesized in the liver and transported via the bloodstream to cardiac and skeletal muscle.

This is interesting to know because we can look at levels observed in the bloodstream.

https://lpi.oregonstate.edu/mic/dietary-factors/L-carnitine#metabolism-bioavailability

But there’s more to endogenous synthesis than just making sure you eat protein…

Ascorbic acid

Ascorbic acid (AA) is mainly known for its antioxidant function and collagen formation.  It is however also a necessary factor in two hydroxylation reactions on the pathway to carnitine.  These 2 enzymes are trimethyllysine hydroxylase and γ-butyrobetaine hydroxylase.

As noted in the study below, because of its involvement in carnitine synthesis, it is believed that the earliest signs of AA deficiency, leading to scurvy and ultimately death, are a general fatigue, unwillingness to be active.  This is suspected to be due to a lower energy production from fatty acids due to lack of carnitine.

“Ascorbic acid and carnitine biosynthesis” https://www.sciencedirect.com/science/article/pii/S0002916523319646

The next paper shows how sensitive skeletal muscle ascorbic acid concentration is to dietary intake, measured via biopsies. 0.5 or 2 kiwis per day increased concentration 3.5 fold in the vastus lateralis.

“Human skeletal muscle ascorbate is highly responsive to changes in vitamin C intake and plasma concentrations” https://academic.oup.com/ajcn/article/97/4/800/4577085

The fiber type is important because carnitine concentration needs to be higher in red meat (type I fiber) where there is a higher reliance on fat metabolism.  White meat is more glycolytic.  The vastus lateralis muscle of both the men and women contained approximately 41% I, 1% IC, 1% IIC, 31% IIA, 6% IIAB, and 20% IIB.  Keep in mind these are averages.

“Fiber type composition of the vastus lateralis muscle of young men and women” https://journals.sagepub.com/doi/10.1177/002215540004800506 

Looking at guinea pig hepatocytes, because they don’t produce ascorbate endogenously like humans, we find that supplementing ascorbate results in enhanced carnitine synthesis and enhanced fat metabolism and ketogenesis in this in vitro setting.

The highest supplementation of both AA and γ-butyrobetaine resulted in the highest production of beta-hydroxybutyrate.  What is interesting here is that there seems to be a dose response curve with a dependency on both elements for maximum stimulation.

“Ascorbate indirectly stimulates fatty acid utilization in primary cultured guinea pig hepatocytes by enhancing carnitine synthesis” https://www.sciencedirect.com/science/article/pii/S0022316623058005 

An ex vivo study using guinea pig livers showed the impact of ascorbate deficiency in carnitine synthesis.  With prior administration of ascorbate they were able to restore the synthesis rate to the control values.

“Carnitine biosynthesis from gamma-butyrobetaine and from exogenous protein-bound 6-N-trimethyl-L-lysine by the perfused guinea pig liver. Effect of ascorbate deficiency on the in situ activity of gamma-butyrobetaine hydroxylase” https://www.jbc.org/article/S0021-9258(18)90577-6/pdf 

Zinc

As we could see above, in the steps of how L-carnitine is synthesized, in the final step γ-Butyrobetaine hydroxylase is a zinc binding enzyme.  

https://en.wikipedia.org/wiki/Carnitine_biosynthesis#%CE%B3-Butyrobetaine_hydroxylase

source: “Trimethyllysine: From Carnitine Biosynthesis to Epigenetics” https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7764450/ 

The role of zinc is important as it helps fold proteins so that they can obtain their active form.

There could be other relevant factors but so far I’ve only come across AA and zinc as factors required for carnitine synthesis in which humans tend to develop deficiencies in.

Carnitine transport in the muscle cell

We’ve already touched on it under “Insulin”.  For plasma carnitine to enter skeletal muscle cells, it makes use of OCTN2 which is a sodium-dependent carnitine transporter.

Studies show a correlation between uptake of L-carnitine and OCTN2 protein expression and a higher expression in type I muscle fiber.

OCTN2 expression is increased by PPARalpha which in turn is activated by AMPK.  This indicates that adaptation to exercise results in higher reliance on fat oxidation by increasing fat import capacity.

“AMPK activation increases fatty acid oxidation in skeletal muscle by activating PPARα and PGC-1” https://www.sciencedirect.com/science/article/abs/pii/S0006291X05027506 

“Clofibrate treatment up-regulates novel organic cation transporter (OCTN)-2 in tissues of pigs as a model of non-proliferating species” https://www.sciencedirect.com/science/article/abs/pii/S0014299908000605 

Dietary intake of protein also releases glucagon.  I don’t know in how far this is relevant in an acute setting but since OCTN2 depends on sodium, it is good to know that glucagon promotes water and salt depletion.

“Influence of Glucagon on Water and Electrolyte Metabolism” https://link.springer.com/chapter/10.1007/978-3-642-69019-8_28 

This could potentially reveal the difficulty with the results from insulin and protein feeding.  What could be missing is the exercise component to stimulate the natural sequence for a greater import.

It also tells us that stimulating insulin through carbohydrates works contrary to increasing reliance on fat metabolism.  This could further explain why it is so difficult to increase muscle carnitine in such trials.  Acute ingestion may work thanks to insulin but the added carbs over time may reduce the effect.

RCTs

The diet that the test subjects are on, is important to understand how much carnitine is in the diet, what the source of the protein is, how much ascorbic acid is in the diet and what is the status before and after the intervention.  

Someone who is deficient may see improvement but can someone with sufficient levels see a further increase?  And if not, is it because the intervention substance failed or are there some (unknown) additional factors missing?  This makes study interpretation difficult if you want to come to conclusions.

Carnitine was supplementation for 25 weeks in older exercising healthy individuals.  Their skeletal muscle carnitine content in the vastus lateralis increased 20% and their fat oxidation also rose with 20% during a 50% VO2max exercise.  The mixture was either placebo or L-carnitine with an insulinogenic beverage.

Here we see that, on the right, the L-carnitine supplementing group shows an increase in total energy contribution during exercise from “Other fat” and a reduction from carbohydrate, comparing before intervention and at the end of the intervention.

This other fat was shown to be mainly from intramyocellular lipids (IMCL) which are lipid droplets in the skeletal muscle.  They examined and found a higher turnover from IMCL.

“Increasing skeletal muscle carnitine content in older individuals increases whole‐body fat oxidation during moderate‐intensity exercise” https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7884033/ 

Interestingly we find a very similar increase, 21% increase in total carnitine, when younger healthy males volunteered 24 weeks of either 80g carbs (placebo) or 80g carbs + 2g of L-carnitne-L-tartine (Carnitine) twice daily.  At the end of the intervention, the Carnitine group utilized 55% less muscle glycogen at 50% VO2max.  At 80% VO2max pyruvate dehydrogenase complex activation was 38% higher, acetylcarnitine was 16% greater (yet not statistically significant!) and muscle lactate was 44% lower in the Carnitine group.

“Chronic oral ingestion of l-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans” https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3060373/ 

Table 1 in this review shows a mixed bag for the younger athletes which could be due to all the variables not being aligned.  In contrast, the trials listed using older individuals all show benefit from L-carnitine supplementation.  As mentioned earlier, possibly they are a group who are deficient to some degree and maybe their level of intensity also allows for better adaptation.

“l-Carnitine Supplementation in Recovery after Exercise” https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5872767/ 

Deficiency

Rodents

Casein, which is the main protein source in rodent studies, appears to be a bad source of ϵ-N-trimethyllysine.  

“є-N-Trimethyllysine Availability Regulates the Rate of Carnitine Biosynthesis in the Growing Rat” https://www.sciencedirect.com/science/article/abs/pii/S0022316623077982 

As an example, the following study generated cardiac fibrosis in mice on a KD.  In the supplemental material you can find the composition of the diet.

Casein was only 9% of the ketogenic diet while the standard diet received double.  The level of vitamin C in the diet is unknown but all diets probably received the same.

“Ketogenic diets composed of long-chain and medium-chain fatty acids induce cardiac fibrosis in mice” https://www.sciencedirect.com/science/article/pii/S2212877823000455 

Under similar conditions, now that we understand carnitine is mainly produced in the liver and kidneys, there must be a shortage of carnitine in the liver resulting in a fatty liver.

The KD was composed of 8.5% protein, 4.3% fibers, 79.1% fat, 4.3% ashes, 3.8% carbohydrate of calories.  Vitamin C and choline were adequately supplied.  In this study, the standard chow contained 18% protein.

“A low-carbohydrate diet induces hepatic insulin resistance and metabolic associated fatty liver disease in mice” https://www.sciencedirect.com/science/article/pii/S2212877823000091 

Finally

What we could not find out is if dietary intake competes or is synergistic with endogenous production or perhaps there is no effect on each other at all.

Dietary intake

It looks like dietary intake should come from animal protein instead of supplements.  For dietary intake, we also could not establish if fat helps to increase absorption but this is not important because on a KD you will need dietary fat as your source of energy if you are not looking at weight loss so you have no choice.

Endogenous synthesis

We don’t have much to go on except for the in vitro study in guinea pig hepatocytes.  

However, given that on a KD there is a higher demand for fatty acid import and metabolism, dietary protein which would feed both L-carnitine directly as also provide a source of lysine and methionine, could benefit from ascorbate supplementation to stimulate carnitine availability and synthesis to its maximum potential.

L-carnitine supplementation is expensive and poorly absorbed thus dietary intake from animal protein + dietary vitamin C seems recommended.

Because the synthesis is mainly occurring in the liver (and kidneys), bovine liver also provides a good source of both γ-butyrobetaine and ascorbate.

source: “Ascorbic acid and carnitine biosynthesis” https://www.sciencedirect.com/science/article/pii/S0002916523319646 

Ascorbic acid

I cannot find sufficient evidence but I suspect that the ratio of free AA is also important.  Namely for the simple reason that if you need to produce more carnitine then it requires a higher amount of available AA. 

We cannot assume that it means the need for a higher dietary intake.  We need a full picture because on a KD, there is more of the endogenous antioxidant glutathione, potentially sparing more AA thus it requires experiments to find out the required dietary intake.

However, given that AA has a very safe profile, one could assume insufficiency and obtain some extra intake.  Very high dosages have been administered intravenously for long periods of time thus the daily addition of a vitamin C-containing piece of fruit or supplement should not pose a problem.

To conclude:

I’ll be conducting my own n=1 experiment.  

1. Starting with a fatty breakfast
2. A run session right before lunch
3. A glass of freshly squeezed lemon, eggs & some fat and salt as the basis for my lunch.
4. Another training session in the afternoon
5. Before dinner another freshly squeezed lemon and protein & fat with salt as the basis for dinner.

This way, I’m aiming for both a good absorption, a good endogenous synthesis and a good transportation into the muscle.

We’ll see if anything noticeable changes.  If you try a similar approach, I’d would be great to hear about your results.

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