The Science Behind: 'Synthetine - Lipid (Fat) Transporter'

Synthetine™ is an L-Carnitine based sterile preparation manufactured by a pharmaceutical company in accordance with the highest level of manufacturing practices. Synthetine is a highly bioavailable form of L-Carnitine that if used together with SyntheDextrin will activate a “switch” that will reduce carbohydrate oxidation and increases fat oxidation in contracting muscle, reduce fatigue, reduce muscle glycolysis and increase glycogen storage during periods that are almost always reserved for carbohydrate oxidation..

The science presented in this article is unique. It details a proven but little known protocol using Synthetine™ and SyntheDEXTRIN™ that immediately enables the regulation of muscle fuel selection in favor of utilizing fats.

The focus of this article is an attempt to describe the science with enough detail so that the readers can incorporate this knowledge into their own plans to advance their fitness and health goals. This article is not about selling the aforementioned products. In the studies a highly bioavailable sterile L-Carnitine such as S ynthetineâ„¢ was used repeatedly together with either insulin or high glycemic shake such as SyntheDEXTRIN™ in the protocol that immediately activated the switch. However a second protocol is fully described herein and it involves low bioavailable oral ingestion of L-Carnitine and high glycemic shake. This second method is a slow build up process requiring daily use for 100 days to be fully active.

I make no apologies for the length of this article. I have kept the science understandable by introducing key concepts before explaining their specific relevance to the focus of this article. I have included a table of contents to make navigation easier.

During the 1990’s a substantial amount of research was undertaken which investigated the effects of L-carnitine supplementation on exercise performance. The primary hope of the research was that increasing carnitine availability in the body would lead to increased fat oxidation during prolonged exercise, spare glycogen stores and, consequently delay the onset of fatigue as well as promote fat loss. Scientific interest in L-carnitine as a performance enhancement and weight loss tool came to an end when it became apparent that L-carnitine feeding does not alter fuel metabolism during exercise or, more importantly impact upon the muscle carnitine pool in humans.

The scientific community for the most part abandoned further research. L-carnitine’s metabolic role had been thoroughly mapped out and described in the literature. The typical carnivorous diet seemed to supply sufficient carnitine. Carnitine supplementation proved to be of no additional benefit…it was simply excreted… end of story.

Despite the failure of science, L-carnitine feeding as a tool to promote weight loss and improve exercise performance became a multimillion dollar dietary supplement industry with no genuine benefit to the end users.

Today our understanding of the transport mechanisms that permit cellular membrane penetration are much more advanced. What was once a saturated transporter and an impermeable membrane are now elements that are open to favorable manipulation, sometimes in surprisingly simple and physiologically obtainable ways. The hopes and hypes of yesterday with the scientific approaches described herein are reborn anew.


This article summarizes in understandable language the elements of metabolism that are necessary to appreciate both the mechanisms and conclusions arrived at through a series of studies published in very well respected journals by a group of scientists I’ll label the “Stephens Group”. The group of four scientists, headquartered at the “Centre for Integrated Systems Biology and Medicine” at Queen’s Medical Centre, University of Nottingham in the United Kingdom examined carnitine’s importance as a regulator of skeletal muscle fuel selection.

They came to the understanding that because carnitine is vitally involved in both fat metabolism and carbohydrate metabolism in the cellular mitochondria (where energy production takes place) and because the pool of available carnitine is restricted at that level, carnitine availability is “the switch” that toggles between these two systems for energy creation.

There are periods of times when the use of glucose (derive from carbohydrates) as a fuel increases. This occurs at high intensity exercise and when there is plenty of glucose available. When this occurs this metabolic pathway calls upon carnitine within the mitochondria for use in the process and this takes away from the carnitine that is available for fat metabolism. This results in a “switch away” from the use of fats for fuel in favor of glucose.

The Stephens Group was able to fully describe this process and discover that there exists a “switch back” which reduces glucose metabolism and increases fat metabolism even during those periods of time (high intensity exercise, carbohydrate intake and preference for glycolysis) that normally demand otherwise.

In essence they discovered that increasing skeletal muscle carnitine above a threshold inhibited carbohydrate oxidation.

They then moved forward and also discovered that increasing skeletal muscle carnitine above a threshold also increased fat oxidation.

Having identified this “switching mechanism” they then discovered that increasing muscle carnitine content in healthy humans at rest reduced glycolysis, increased glycogen storage and increased fat oxidation.

They then came to the understanding that increasing muscle carnitine content alleviated the decline in fat oxidation rates during high intensity exercise and reduced muscle glycogen utilization. They were able to reference in vitro (out of body) studies that reported that increasing muscle carnitine substantially delayed the onset of fatigue.

Having established the “switching” mechanism and its potential positive benefits they then set about discovering a method for increasing carnitine in muscle. It is important to remember that this had never been accomplished before.

Their studies discovered two protocols. One protocol resulted in an immediate and rapid increase in muscle carnitine levels to the switching threshold. This protocol involved a highly bioavailable method that increased the influx of carnitine into muscle cells. The second protocol involved a slower day to day build up of carnitine levels and took 100 days to arrive at the switching threshold. This protocol involved lower bioavailability but more convenient methods.

The first protocol might be considered by bodybuilders, athletes and fitness enthusiasts while the second might be better suited for the public at large.

Table of Contents

I. Introduction to Fat Oxidation
II. Summary of the two roles played by Carnitine
A. Role 1: Energy Pathway “Fatty acid transport/oxidation” – Two pools of carnitine transport
B. Role 2: Competing Energy Pathway “Glycolysis” and Carnitine Buffer
III. The Switch
A. Turning down the rate of fat oxidation
B. Reversing the Switch – Turning up the rate of fat oxidation & turning down the rate of carbohydrate oxidation
IV. Athletic Performance, fatigue & the first few seconds of muscle contraction
V. The Stephens Group Research: Summary of Findings
VI. How do you increase carnitine in muscle?
A. How carnitine is normally transported into the cell
B. How insulin increases the flow of carnitine into muscle
VII. Methodology and results
VIII. Alternative Methodology
A. Obtaining a lower insulin threshold
B. Orally ingesting lower bioavailable L-carnitine together with high glycemic index carbohydrates
C. Extrapolating an accumulation strategy
IX. Final Note

Introduction to Fat Oxidation

To sustain life the production of energy is required. This process necessitates the acquisition & concurrent use of both oxygen and a fuel source. Fuel sources are available from either consumption of carbohydrates, fats, and rarely proteins or the release of stored fuels from within the body. The ingestion of dietary fat is an initial energy acquisition process called consumption while the process called oxidation is the final step of conversion into human energy. Between the initial process of consumption and the final step of conversion are the processes of storage and eventual release for conversion into energy.

Whether the middle processes of fat storage or fat release are activated depends primarily on the state of energy balance at any point in time. If there is a surplus of fuel sources from ingested carbohydrates or fats then fat will not be released from storage in fat cells in appreciable quantities and the body will use its preferred source of energy carbohydrates followed by newly ingested fats to meet its energy requirements.

When there is a surplus of energy from consumption (i.e. eating outpaces physical activity) the human body will not readily convert excess carbohydrates into fat stores but will use them for energy. Carbohydrate ingestion does not always lead to increased fat stores but may do so by being excessive and by crowding out concurrently ingested fats’ potential to be utilized as energy. As a result ingested fats during periods of surplus energy consumption will generally be stored in fat cells.

Ingested fats are broken down and converted into free fatty acids, which are then stored in fat cells in a form known as triglycerides where they remain as potential energy units until called for by negative energy states.

When energy balance is in a deficit (i.e. physical activity outpaces eating) fat oxidation will increase. In order for this final step of oxidative conversion into human energy to occur the middle step of release of fat stores (triglycerides) must take place. This process will result in loss of fat mass.

Various hormones will trigger the release of the triglycerides from fat cells. These triglycerides, through a process labeled lipolysis are broken down into two compounds and released into the bloodstream. The first compound glycerol is primarily converted to glucose by the liver and provides energy for cellular metabolism. The second compound fatty acids are transported to the mitochondria, the portion of a cell that produces energy within each cell.

This is the stage where carnitine plays an essential role in fatty acid oxidation. It is not possible for the newly liberated fatty acids to penetrate the mitochondria membrane and enter the mitochondria without the help of carnitine which acts as a transport mechanism.

In general, carnitine transports long-chain acyl groups from fatty acids into the mitochondria where they are broken down through beta-oxidation in a process that ends up creating adenosine triphosphate (ATP), the energy-producing fuel.

The research studies have examined the possibility that greater amounts of fatty acids could be oxidized if carnitine levels were elevated through supplements. Carnitine increases via supplementation were determined to have no effect on fatty acid oxidization.

It is important to note that what I have described concerning energy balance and fat storage versus release is a generalized net (or overall) effect. Fat is constantly being stored in and released from fat cells no matter what the current energy state however the overall net effect very much depends on the state of energy balance, or as is the focus of this paper L-carnitine can be made to oxidize fat even in the presence of a positive energy balance.

Summary of the two roles played by Carnitine

In order to understand the relevant conclusions drawn from the Stephens Group’s research detailed from their studies herein it is necessary to understand a few elementary essentials concerning carnitine’s role in skeletal muscle fuel metabolism and briefly mention the “competing” metabolic pathway and a second function of carnitine, which is to act as a buffer during carbohydrate metabolism. When free carnitine is engaged in its role as a buffering agent for carbohydrate metabolism long-chain fatty acid oxidation diminishes.

Role 1: Energy Pathway “Fatty acid transport/oxidation” – Two pools of carnitine transport

The Mitochondria (the intra-cellular area where oxidation and energy production occurs) membrane is impermeable to fatty acyl-CoA (i.e. the long-chain fatty acid liberated from fat cells bonded to an enzyme named coenzyme A (CoA).) But this is not true if it is bound to carnitine. Carnitine enables the fatty acid to penetrate the membrane and it does so by binding to it and forming acylcarnitine.

However there are two separate pools of carnitine that need to be utilized to move fatty acids into the mitochondria. One pool located outside of the mitochondria membrane and one pool located inside the mitochondria matrix. The one outside the mitochondria is the one that binds to fatty acids and transports them through the first of 2 layers that make up the membrane and up to the 2nd inner layer but not through the mitochondria membrane. The pool of carnitine inside the mitochondria is known as “intra-mitochondria free carnitine”. It moves to the membrane from the inside and is “handed” the fatty acyl-CoA that was delivered “to the door” by outside carnitine. The handing over process is mediated by an enzyme called Carnitine palmitoyltransferase I (CPT1) which resides on the 2nd layer of the membrane. We don’t need to introduce all the various proteins and enzymes involved in the process. Simply understand that CPT1 is akin to a bouncer at a nightclub who takes a note from someone outside the doorway and gives it to someone inside the doorway.

In this way two carnitines (one from outside & one from inside the mitochondria) do the work of transporting the fatty acyl-CoA.

Inside the mitochondria matrix the newly formed acylcarnitine (thanks to the hand off) is reduced back to two individual components: free carnitine and the long chain fatty acyl-CoA.


So in summary the fatty acyl-CoA thanks to two carnitines has been transported into the mitochondria where it will be oxidixed and cleaved of the coenzyme A (CoA) which will take two carbon atoms with it. This process is known as beta-oxidation and results in acetyl-CoA (note: acyl goes in but acetyl comes out).

Acetyl-CoA enters the TCA Cycle (ie. citric acid cycle — also known as the Krebs cycle) where energy is produced (i.e. ATP is synthesized).


Role 2: Competing Energy Pathyway “Glycolysis” and Carnitine Buffer

While the aforementioned metabolic pathway oxidizes fats and is the route for fatty acid metabolism, the glycolysis pathway is the carbohydrate metabolic pathway. Glycolysis is the metabolic pathway that converts glucose into pyruvate.

This is accomplished in the Pyruvate dehydrogenase complex (PDC) which is a complex of three enzymes that transform pyruvate into acetyl-CoA through a process called pyruvate decarboxylation.

Acetyl-CoA is the same end product arrived at through the fat oxidation pathway and is also “fed to the fire” to produce energy.

Acetyl-CoA enters the TCA Cycle (ie. citric acid cycle — also known as the Krebs cycle) where energy is produced (i.e. ATP is synthesized).


Under certain circumstances PDC (Pyruvate dehydrogenase complex) activity greatly increases and makes acetyl-CoA at a rate faster then the TCA cycle can consume. At that point free carnitine inside the mitochondria acts as a buffer and binds to excess acetyl from acetyl-CoA and removes it or holds it as a reservoir. This is carnitine’s other function, the removal of excess acetyl groups thereby ensuring a sufficient pool of CoA for the continuation of PDC and TCA cycle reactions.

So increased PDC activity can lead to down-regulation of long-chain fatty acid oxidation because it makes use of carnitine in its second role as a buffer, leaving less carnitine available to act in its role as transporter.

At the beginning of high intensity exercise (but not low intensity) skeletal muscle free carnitine content is reduced by 75% as a result of it acting as a buffer. This occurs to a greater extent in Type I muscle fibers.

Skeletal muscle free carnitine content is also reduced during moderate intensity exercise when muscle glycogen content is elevated.

Increased PDC activity whether it is brought about by high intensity exercise or carbohydrate metabolism results in more carnitine acting as a buffer which reduces its availability to transport fatty acid and thus long-chain fatty acid oxidation rates go down.

The Switch

Turning down the rate of fat oxidation

Reducing the muscle free carnitine pool during conditions of high PDC flux limits the ability of CPT1 (the mediator enzyme “bouncer at the door”) to transport long-chain acyl-CoA into the mitochondrial matrix and thus the rate of fat oxidation.

Support for this understanding is well established and not limited to the Stephens Group’s research. Van Loon et al. (2001) demonstrated that a 35% decrease in the rate of long-chain fatty oxidation that occurred at an exercise intensity above 75% VO2 max,was paralleled by a 65% decline in skeletal muscle free carnitine content.

Roepstorff et al. (2005) showed a 2.5-fold decrease in the rate of fat oxidation, compared to control, during moderate intensity exercise (65% of VO2 max) when free carnitine availability was reduced by 50% as a result of pyruvate, and therefore acetyl-CoA, production being increased as a result of pre-exercise muscle glycogen content being elevated.

Further support for the understanding that free carnitine availability may limit fat oxidation comes from Achten & Jeukendrup, (2004). Muscle free carnitine content has been shown to decrease from approximately 11 to below 5.5mmol (kg dm)-1 between the exercise intensities of 60 and 80% of VO2 max, and it has been calculated that maximal and minimal fat oxidation rates during exercise are achieved at exercise intensities of around 65% and greater than 80% of VO2 max, respectively.

Reversing the Switch – Turning up the rate of fat oxidation & turning down the rate of carbohydrate oxidation

Putmanet al. (1993) supply evidence by demonstrating that during bicycle exercise at 75% of VO2 max to exhaustion, both muscle free carnitine content and fat oxidation rates were markedly higher when pre-exercise muscle glycogen content was lowered compared to control.

In one of the Stephens Group’s studies they found that a 15% increase in skeletal muscle carnitine content… resulted in a 30% decrease in muscle PDC activity and a 40% decrease in muscle lactate content, leading them to conclude “These results suggest that an acute increase in human skeletal muscle total carnitine content results in an inhibition of carbohydrate oxidation in conditions of high carbohydrate availability, due to a carnitine-mediated increase in fat oxidation.”

As an explanation

Philip Randle in the 1960s undertook a series of landmark and controversial studies detailing the workings of the balance between fatty acid oxidation and glucose oxidation in what he called the glucose–fatty acid cycle (Randle et al. 1963, 1964; Garland et al. 1963; Garland & Randle, 1963). Therein he laid down the fundamental concept of reciprocal substrate competition between glucose and non-esterified fatty acids (the major fuels that are oxidized to provide ATP in mammals) in normal physiology in muscle.

In describing the competition between glucose oxidation and fatty acid oxidation he specified that an increase in beta-oxidation would result in an elevation of muscle acetyl-CoA concentration and, consequently, an increase in muscle citrate and glucose-6-phosphate content. This, in turn, would result in the down-regulation of carbohydrate flux [activity], due to product inhibition of PDC, phosphofructokinase and hexokinase, respectively.

The Stephens Group acknowledge Randle’s important work in their elaboration of the results of their own study stating “In support of [Randle’s description] muscle long-chain acyl-CoA content returned to basal overnight during the L-carnitine infusion visit (whereas it remained suppressed during the control visit), which suggests that Beta-oxidation was indeed increased…while there was a 30% decrease in muscle PDC activity”

Athletic Performance, fatigue & the first few seconds of muscle contraction

It is well established that there is a lag in oxidative ATP delivery at the onset of exercise and muscular contraction. This is attributable to a lag in mitochondrial ATP production brought about by a lag in PDC activity which results in an insufficient acetyl-CoA supply to match the demands of the TCA cycle (Krebs cycle – energy producing). The fuel supply is lacking at that moment in time.

According to a study by Roberts et al. (2002) a lag in acetyl group provision (predominately in the form of acetylcarnitine) occurs during the initial 20 seconds of contraction. Remember acetylcarnitine is created as a result of its role as a buffer during high PDC activity and held as an acetyl reserve. If PDC activity is not high enough at the start of contraction there will be very little acetyl group available to feed the cycle that produces energy (ATP).

So at the onset of contraction there is a lack of fuel in the form of acetyl groups.

“This is a rate-limiting step in the rate of rise in mitochondrial ATP re-synthesis in skeletal muscle at the onset of exercise, which in turn will dictate the magnitude of oxygen-independent ATP delivery, and thereby the rate of fatigue development during intense exercise.” – Stephens Group

This can be overcome by “priming” through manipulating muscle carnitine pools at rest so as to make available sufficient energy substrate and by activating the PDC prior to the event by warming up before intense exercise.

The Stephens Group Research: Summary of Findings

How to increase muscle carnitine

  • Plasma carnitine is not specifically lacking
  • The transport mechanism and membrane gradient for carnitine flow into muscle cells are restrictive
  • Increasing the amount of carnitine is of no value in and of itself
  • Increasing the transport mechanism/membrane gradient + the amount of carnitine = increase in muscle carnitine
  • The methodolgy for increasing flow of carnitine in to cells is via changing the membrane permeability to allow more carnitine transport


    • Insulin is a method for effecting this increased flow
    • Intravenously administered L-carnitine + insulin = increased carnitine in muscle cells
    • There is a threshold concentration for the stimulatory effect of insulin on skeletal muscle accumulation
      • blunts PDC activity (carbohydrate oxidation)
      • reduces muscle lactate content
      • increases glycogen content in muscle (i.e reduces oxidation of glucose in favor of storage)
      • reduces muscle glycolysis
      • increases fat oxidation

Results: Using this methodology carnitine content increases by 13% to 15% and:

Alternative Methodology

    • For the reason that the determined insulin threshold is low enough to be reached via non-pharmacological methods, oral ingestion of glucose may be used.
    • For the reason that the carnitine turnover rate in skeletal muscle is low (190 +/- 20 hours) daily carnitine increases will result in a continued build up of total muscle carnitine.
    • This allows the use of low bioavailable methods such as daily ingestion of oral carnitine w/ glucose load to increase carnitine in muscle at an incremental rate such that within 100 days carnitine content will have increased by 10%.
      • blunt PDC activity (carbohydrate oxidation)
      • reduce muscle lactate content
      • increase glycogen content in muscle (i.e. reduce oxidation of glucose in favor of storage)
      • reduce muscle glycolysis
      • increase fat oxidation

Results: This amount of carnitine is sufficient to:

How do you increase carnitine in muscle?

Studies have consistently failed to increase skeletal muscle carnitine content either through oral supplementation or intravenous L-carnitine administration. Watcher et al (2002) fed 2 grams of L-carnitine twice a day for 3 months to normal people and failed. Similar studies by Barnet et al. (1994) and Vulkovich et al. (1994) demonstrated similar failures with oral feedings of l-carnitine for 3 months.

Intravenous infusion of L-carnitine for up to 5 hours similarly failed to have any effect on muscular carnitine content (Brass et al. (1994); (Stephens Group, Insulin stimulates… (2006)).

How carnitine is normally transported into the cell

The reason for these failures is very simple. Normal people have no deficiency in circulating plasma levels of carnitine. What they have is a fully saturated transport mechanism. No amount of carnitine load is sufficient without a concurrent increase in the ability of the transport mechanism to transport carnitine across the cellular membrane.

The cellular membrane is a lipid bilayer easily permeable to water molecules and a few other small, uncharged, molecules such as oxygen and carbon dioxide but little else. The cellular membrane is not permeable to ions such as K+, Na+.

In the normal course of things molecules and ions move about spontaneously down what is known as their concentration gradient (i.e., from a region of higher to a region of lower concentration) by diffusion.

Molecules and ions are capable of moving against their concentration gradient, but this process requires a process known as active transport.

It is the active transport that is lacking in regard to carnitine movement and unless this is changed additional carnitine will not be allowed to enter the cell.

Active transport is the pumping of molecules or ions through a membrane against their concentration gradient. It requires: a transmembrane protein (usually a complex of them) called a transporter and energy. The source of this energy is ATP.

The transmembrane protein responsible for carnitine transport into skeletal muscle is OCTN2. The half-saturation concentration of L-carnitine uptake by OCTN2 is 4.34 umols (Tamai et al. (1998)). In the normal state skeletal muscle carnitine uptake is saturated since plasma total carnitine concentration is 50 umols.

OCTN2 has a high affinity for carnitine and sodium ions (Na+) and readily binds to both and so carnitine is transported into skeletal muscle against a substantial concentration gradient via a transport process involving sodium Na+ flow. In essence carnitine hitches a ride on OCTN2 which hitches a ride on Na+.

A detailed description of this process is beyond the scope of this article so a general reduction will suffice. One method of direct active transport across the cellular membrane is the Na+/K+ ATPase pump.

The concentration of potassium ions (K+) is as much as 20 times higher inside the cell then outside. Conversely, the fluid outside the cell contains a concentration of sodium ions (Na+) as much as 10 times greater than that within the cell. Because of this difference a concentration gradient amenable to flow exists and the Na+/K+ ATPase pump effects the transfer of these two ions pushing out 3 Na+ ions for every 2 K+ ions pumped back into the cell. This activity establishes a net charge across the plasma membrane with the interior of the cell being negatively charged with respect to the exterior.

So with this basic understanding that OCTN2 is a cotransporter of sodium & carnitine and that under normal conditions it is fully saturated and thus unable to benefit further carnitine inflow via Na+/K+ ATPase pump activity, lets examine how insulin overcomes this equilibrium and brings about an increased inflow of carnitine into skeletal muscle.

How insulin increases the flow of carnitine into muscle

The Na+ dependent, active transport of carnitine into human skeletal muscle is mediated via a high-affinity, transporter OCTN2.

The Stephens Group found that the combination of increased carnitine and increased insulin (above a threshold) increased skeletal muscle OCTN2 mRNA expression by 2.3 fold (Stephens Group, Insulin stimulates… (2006)) in addition to increasing the activity of Na+/K+ ATPase pump. This results in an increased availability of transporter which potentially increases the amount of carnitine that may be carried into the muscle cell.


The action of insulin however is most important in changing the membrane permeability in favor of carnitine inflow.

It has been demonstrated that the Na+ dependent uptake of other nutrients into skeletal muscle is increased by insulin, for example amino acids (Zorzano et al. 2000) and creatine (Green et al. 1996; Steenge et al. 1998).

Insulin is able to increase the flow of carnitine into skeletal muscle as follows.

Insulin increases Na+/K+ ATPase pump activity by increasing translocation (or movement) of alpha2 and beta1 pump subunits from an intracellular storage site to the plasma membrane (Sweeney & Klip, 1998), and through an increase in the sensitivity of the Na+/K+ ATPase pump to intracellular Na+ (Clausen, 1986, 2003; Ewart & Klip, 1995).

OCTN2 has equal affinity for sodium ion (Na+) and carnitine and bonds to both. With an increased Na+/K+ ATPase pump activity brought about by an increase in circulating insulin concentration intracellular Na+ concentration is lowered which increases the electrochemical gradient for Na+ and therefore increases Na+/carnitine cotransport.

This results in an increase of carnitine inside the muscle cell.

Methodology and results

The Stephens Group undertook a series of experiments building on each to create an overall understanding. In Insulin stimulates L-carnitine accumulation in human skeletal muscle, they were able to increase muscle total carnitine content by 13%. They achieved this by using what I will call an “overkill amount of L-carnitine” administered by infusion. They administered a 15mg/kg bolus w/in the 1st 10 minutes rapidly achieving a supraphysiological plasma concentration of about 500 umol/L. This was followed by 10mg/kg infused over the next 290 minutes to maintain hypercarnitinemia.

In addition they infused insulin at a dose I will call an “overkill amount”. The aim of study was to determine whether insulin could increase Na+/dependent skeletal muscle carnitine up-take in healthy human subjects as a result of increasing Na+/K+ ATPase pump activity. They were very much successful. The positive results of the study are incorporated in the previous section.

Using an identical protocol and intravenous L-carnitine & insulin amounts they undertook another study reported in An Acute Increase in Skeletal Muscle Carnitine Content Alters Fuel Metabolism in Resting Human Skeletal Muscle with the broader aim of determining the effect that an increase in skeletal muscle carnitine content would have on the integration of muscle fat and carbohydrate oxidation during and after hyper-insulinemia.

As in the previous study total carnitine content increased in skeletal muscle, this time by 15%.

This resulted in a 30% decrease in muscle PDC activity (carbohydrate metabolism) and a 40% decrease in muscle lactate content. After an overnight fast, muscle glycogen and LCA-CoA (long-chain acyl-CoA) content had increased by 30% and 40% respectively, in the carnitine group compared with control. The difference between the control and carnitine visits was not attributable to a difference in the amount of carbohydrate administered.

“Taken together, these findings lead us to conclude that the increase in muscle carnitine content observed in the present study inhibited glycolytic flux (decrease in lactate) and carbohydrate oxidation at the level of the PDC, thereby diverting muscle glucose uptake toward glycogen storage (nonoxidative glucose disposal).”

“The reciprocal relationship between carbohydrate and fat oxidation in skeletal muscle would suggest that the apparent decrease in carbohydrate flux observed was the result of, or resulted in, an increase in fat oxidation. Thus, these findings could be of major importance in the treatment of insulin-resistant states, such as obesity and type 2 diabetes, because both conditions are associated with an impaired ability of skeletal muscle to oxidize fatty acids, both at rest and during exercise. Furthermore, reducing or preventing intramuscular lipid accumulation increases insulin sensitivity.”

The implications of these results should be clear and having read the previous portions of this article and examined the figures, self-explanatory. To reiterate the decrease in PDC activity indicates a substantial drop in carbohydrate metabolism, while the decrease in muscle lactate indicates a decrease in glycolysis activity. The increase in muscle glycogen storage given the constants of the study indicate that glucose was preferentially stored not metabolized. This also indicates that existing muscle glycogen stores where enhanced rather then drawn upon.

The reciprocal relationship between carbohydrate oxidation and fat oxidation indicates that fuel sources utilized for energy where fats.

The meaning of the one item that may not be readily apparent is that of LCA-CoA (long-chain acyl-CoA) increasing overnight. Remember from the early discussion in this article that carnitine transports long-chain acyl groups from fatty acids into the mitochondria where they are broken down through beta-oxidation. The fact that these groups had increased strongly indicated that carnitine is increasing its activity as a transporter in fatty acid oxidation and that fatty acid oxidation is increased.

In the words of the Stephens Group in an overall review of their work:

“…the apparent reduction in glycolytic flux and carbohydrate oxidation… (decreased PDC activity and lactate content, and increased glycogen accumulation), in the face of high carbohydrate availability, could have been caused by a carnitine-mediated increase in skeletal muscle long-chain fatty acid oxidation, i.e. an increase in long-chain acyl-CoA translocation into the mitochondrial matrix via CPT1, resulting in an increase in beta-oxidation.

According to Randle’s glucose–fatty acid cycle (Randle et al. 1963, 1964; Garland et al. 1963; Garland & Randle, 1963), a concept proposed in the 1960s from experiments involving rat heart and diaphragm muscle, an increase in beta-oxidation would result in an elevation of muscle acetyl-CoA concentration and, consequently, an increase in muscle citrate and glucose-6-phosphate content. This, in turn, would result in the down-regulation of carbohydrate flux, due to product inhibition of PDC….

Indeed, the decrease in PDC activity observed in our study was paralleled by a reduction in muscle lactate content and resulted in an accumulation of muscle glycogen overnight, conditions which are both consistent with the premise that glycolytic flux, and therefore carbohydrate oxidation, was inhibited. In support of this… muscle long-chain acyl-CoA content returned to basal overnight during the l-carnitine infusion visit (whereas it remained suppressed during the control visit), which suggests that beta-oxidation was indeed increased.”

Alternative Methodology

Obtaining a lower insulin threshold

In an attempt to discover the lowest amount of insulin needed to drive carnitine into muscle and activate the switch from carbohydrate oxidation to fatty acid oxidation, the Stephens Group undertook a study the reports of which are discussed in A threshold exists for the stimulatory effect of insulin on plasma L-carnitine clearance in humans.

They reasoned that while their previous studies with insulin infusion in an amount in the upper physiological range were successful, it would be difficult to achieve by dietary means alone.

They discovered that administered insulin will not stimulate muscle carnitine retention unless a serum insulin concentration greater than 90 mU/l is achieved during hypercarnitinemia. This level was substantially lower (and obtainable via dietary means) then the previous high concentrations used and stimulated muscle carnitine transport to a similar degree.

Extrapolating from data, skeletal muscle total carnitine content in this study with this threshold insulin amount would have been increased by about 10%.

Orally ingesting lower bioavailable L-carnitine together with high glycemic index carbohydrates

The Stephens Group in a study the results of which are reported in Carbohydrate ingestion augments L-carnitine retention in humans, investigated whether physiologically significant increases in skeletal muscle carnitine content can be achieved through the use of L-carnitine feeding in conjunction with a dietary-induced elevation in circulating insulin.

They examined serum insulin levels achieved from glucose ingestion, the plasma total carnitine level and the urinary total carnitine excretion levels in order to determine the amount of carnitine taken up in muscle by performing both a one day study and a 14 day study.

Both studies used oral ingestion of:

4.5 g L-carnitine L-tartrate (3 g L-carnitine) dissolved in 200 ml of water

followed by

94 g of simple sugars (CHO) either ingested twice at 1 hour & 4 hours after L-carnitine ingestion as in the 14 day study or as in the one day study four time across a 5 hour period.

Serum insulin concentrations during the period when simple sugars were ingested are graphed below. Surprisingly peak serum insulin concentrations of about 70mU/l proved to be sufficient.


The graph below indicates that the rise in insulin eliminated carnitine from plasma. The control subjects had more carnitine in plasma then those on the protocol. See below.


If the carnitine is not in plasma is it excreted? The graph below indicates that urinary excretion rates were lower over the measured 14 days in those following the protocol. See below.


“We suggest, therefore, that the lowering of plasma total carnitine (TC) concentration occurring immediately following CHO ingestion, and the lower urinary TC excretion during the CHO visit, collectively indicate that an increase in whole body carnitine retention occurred when L-carnitine feeding was accompanied by CHO ingestion. Given that skeletal muscle is the major site of carnitine storage within the body, and that maintaining hypercarnitinemia for 5h in the presence of hyperinsulinemia increases skeletal muscle TC accumulation (other Stephens Group studies), it is not unreasonable to suggest that this greater retention occurred mainly in this tissue.”

Extrapolating an accumulation strategy

Given that the increase in muscle carnitine content following a single dose, or 2 weeks, of L-carnitine feeding in the presence of elevated circulating insulin is likely to be small due to the poor bioavailability of orally administered L-carnitine (less then 20%), muscle carnitine accumulation was estimated indirectly from measurements of plasma and urinary carnitine concentration.

In this study 3 grams of carnitine results in at most 560 mg of absorbable plasma carnitine.

“Assuming all absorbed carnitine was either taken up into skeletal muscle tissue or excreted in the urine, it can be calculated that L-carnitine feeding in conjunction with CHO ingestion would have increased skeletal muscle total carnitine concentration by a further 0.1% (i.e., 60 mg) compared with L-carnitine ingestion alone.”

In fact “urinary total carnitine excretion was on average 70 mg/day lower in the CHO group over the 14 days of study. Consequently, if maintaining a daily L-carnitine feeding regime with CHO has an additive effect on muscle carnitine content, L-carnitine feeding for 100 days could increase muscle carnitine content by an additional 10%, which we believe could have a significant metabolic impact in contracting skeletal muscle.”

In the other comprehensive Stephens Group study they found that muscle total carnitine content was not reduced 24 h after a 15% increase, suggesting that a daily increase in muscle carnitine content can be maintained. In addition release of carnitine from skeletal muscle is a slow process, with skeletal muscle carnitine turnover time of 190 +/- 20 hours (Rebouche (1984)).

“Taken together with the maintained effect on whole body total carnitine retention observed in the 14 day study, these findings would suggest that daily L-carnitine and carbohydrate administration could well have an additive effect on skeletal muscle total carnitine accumulation. Importantly, if L-carnitine supplementation is to be used as a tool to modify skeletal muscle energy metabolism, the findings in the 14 day study also suggest that, at most, only two 500-ml CHO drinks (2 x 94 g CHO) are required to achieve the effect on L-carnitine retention.”

In conclusion:

“…muscle free carnitine availability becomes limiting to carnitine palmitoyltransferase I (CPT1) at a concentration of about 6 mmol/kg dry muscle….

Thus, assuming the average 70 mg/day retention in the present studies resided within skeletal muscle and that daily L-carnitine/carbohydrate feeding for 100 days would have an additive effect, then muscle carnitine content would increase by about 2 mmol/kg dry muscle, which could alleviate the decline in fat oxidation rates routinely observed at exercise intensities above 70% VO2 max, which could be of major relevance to exercise performance due to the sparing of muscle glycogen.

In line with this theory, increasing skeletal muscle carnitine availability has been reported to delay fatigue development by 25% in rat soleus muscle strips in vitro (Brass (1993)).”

Further more it is worth reiterating that the Stephens Group has demonstrated in the study involving intravenous L-carnitine administration that a 15% increase in skeletal muscle carnitine content, achieved during hyperinsulinemia, resulted in a 30% decrease in muscle PDC activity and 40% decrease in muscle lactate content compared with control. Furthermore, following an overnight fast, muscle glycogen and long-chain acyl-CoA content was 30% and 40% greater than control, respectively, despite carbohydrate administration over the previous 24 hours being exactly the same.

This is the first study to demonstrate that the retention of orally supplemented L-carnitine can be increased if accompanied by carbohydrate ingestion and that this retention is likely to reside in skeletal muscle, because insulin is known to stimulate muscle total carnitine accumulation. “These findings could have a significant effect on the integration of fat and carbohydrate oxidation in contracting skeletal muscle.”

Final Note

An immediate threshold amount of increase in muscle carnitine concentration can be had with administration of highly bioavailable Synthetine™ (sterile L-Carnitine) with insulin or oral ingestion of two high glycemic index drinks such as SyntheDEXTRIN™ (Maltodextrin Pure Carbohydrate).

An accumulation strategy of daily oral ingestion of low bioavailable l-carnitine with oral ingestion of two high glycemic index drinks such as SyntheDEXTRIN™ (Maltodextrin Pure Carbohydrate) will lead to a threshold amount of muscle carnitine concentration within 100 days.

These strategies should enable reversing the switch – Turning up the rate of fat oxidation & turning down the rate of carbohydrate oxidation.

Click Here to Purchase Synthetine!

To discuss this article click HERE!

Stephen’s Group (Centre for Integrated Systems Biology and Medicine) research:

Insulin stimulates L-carnitine accumulation in human skeletal muscle, Stephens FB, Constantin-Teodosiu D, Laithwaite D, Simpson EJ & Greenhaff PL, FASEB J 20, 2006 377–379

An Acute Increase in Skeletal Muscle Carnitine Content Alters Fuel Metabolism in Resting Human Skeletal Muscle, Stephens FB, Constantin-Teodosiu D, Laithwaite D, Simpson EJ & Greenhaff PL, J Clin Endocrinol Metab 91, 2006 5013–5018

A threshold exists or the stimulatory effect of insulin on plasma L-carnitine clearance in humans, Stephens FB, Constantin-Teodosiu D, Laithwaite D, Simpson EJ & Greenhaff PL, Am J Physiol Endocrinol Metab, Feb 2007; 292: E637 – E641

Carbohydrate ingestion augments L-carnitine retention in humans, Stephens FB, Evans CE, Constantin-Teodosiu D & Greenhaff PL, J Appl Physiol 102, 2007 1065–1070

Other References:

Achten J& Jeukendrup AE (2004), Relation between plasma lactate concentration and fat oxidation rates over a wide range of exercise intensities, Int J Sports Med 25, 32–37

Barnett C, Costill DL, Vukovich MD, Cole KJ, Goodpaster BH, Trappe SW& FinkWJ (1994), Effect of L-carnitine supplementation on muscle and blood carnitine content and lactate accumulation during high-intensity sprint cycling, Int J SportNutr 4, 280–288.

Brass EP, Scarrow AM, Ruff LJ, Masterson KA, Van Lunteren E., Carnitine delays rat skeletal muscle fatigue in vitro, J Appl Physiol 75: 1595–1600, 1993

Brass EP, Hoppel CL & Hiatt WR (1994), Effect of intravenous L-carnitine on carnitine homeostasis and fuel metabolism during exercise in humans, Clin Pharmacol Ther 55, 681–692

Clausen T (1986), Regulation of active Na+/K+ transport in skeletal muscle, Physiol Rev 66, 542–580

Clausen T (2003), Na+/K+ pump regulation and skeletal muscle contractility, Physiol Rev 83, 1269–1324

Ewart HS & Klip A (1995), Hormonal regulation of the Na+/K+ ATPase: mechanisms underlying rapid and sustained changes in pump activity, Am J Physiol Cell Physiol 269, C295–C311

Garland PB & Randle PJ (1963), Effects of alloxan diabetes and adrenaline on concentrations of free fatty acids in rat heart and diaphragm muscles, Nature 199, 381–382

Garland PB, Randle PJ & Newsholme EA (1963), Citrate as an intermediary in the inhibition of phosphofructokinase in rat heart muscle by fatty acids, ketone bodies, pyruvate, diabetes, and starvation, Nature 200, 169–170

Green AL, Hultman E, Macdonald IA, Sewell DA & Greenhaff PL (1996), Carbohydrate ingestion augments skeletal muscle creatine accumulation during creatine supplementation in humans, Am J Physiol Endocrinol Metab 271, E821–E826

Putman CT, Spriet LL, Hultman E, Lindinger MI, Lands LC, McKelvie RS, Cederblad G, Jones NL & Heigenhauser GJ (1993), Pyruvate dehydrogenase activity and acetyl group accumulation during exercise after different diets, Am J Physiol Endocrinol Metab 265, E752–E760

Randle PJ, Garland PJ, Hales CJ & Newsholme EJ (1963), The glucose fatty-acid cycle. Its role in insulin sensitivity and the metabolic disturbances of diabetes mellitus, Lancet 1, 785–789

Randle PJ, Newsholme EJ & Garland PJ (1964), Effects of fatty acids, ketone bodies and pyruvate, and of alloxan-diabetes and starvation, on the uptake and metabolic fate of glucose in rat heart and diaphragm muscles, Biochem J 93, 652–665

Rebouche CJ, Engel AG. Kinetic compartmental analysis of carnitine metabolism in the human carnitine deficiency syndromes , Evidence for alterations in tissue carnitine transport, J Clin Invest 73: 857–867, 1984

Roberts PA, Loxham SJG, Poucher SM, Constantin-Teodosiu D & Greenhaff PL (2002), The acetyl group deficit at the onset of ischaemic contraction in canine skeletal muscle, J Physiol 544, 591–602

Roepstorff C, Halberg N, Hillig T, Saha AK, Ruderman NB, Wojtaszewski JF, Richter EA & Kiens B (2005), Malonyl-CoA and carnitine in regulation of fat oxidation in human skeletal muscle during exercise, Am J Physiol Endocrinol Metab 288, E133–E142

Steenge GR, Lambourne J, Casey A,Macdonald IA & Greenhaff PL (1998), Stimulatory effect of insulin on creatine accumulation in human skeletal muscle, Am J Physiol Endocrinol Metab 275, E974–E979

Sweeney G & Klip A (1998), Regulation of the Na+/K+-ATPase by insulin: why and how?, Mol Cell Biochem 182, 121–133

Tamai I, Ohashi R, Nezu J, Yabuuchi H, Oku A, Shimane M, Sai Y & Tsuji A (1998), Molecular and functional identi?cation of sodium ion-dependent, high affinity human carnitine transporter OCTN2, J Biol Chem 273, 20378–20382

van Loon LJ, Greenhaff PL, Constantin-Teodosiu D, Saris WH & Wagenmakers AJ (2001), The effects of increasing exercise intensity on muscle fuel utilisation in humans, J Physiol 536, 295–304

Vukovich MD, Costill DL & FinkWJ (1994), Carnitine supplementation: effect on muscle carnitine and glycogen content during exercise, Med Sci Sports Exerc 26, 1122–1129

Wachter S, Vogt M, Kreis R, Boesch C, Bigler P, Hoppeler H & Krahenbuhl S (2002), Long-term administration of L-carnitine to humans: effect on skeletal muscle carnitine content and physical performance, Clin Chim Acta 318, 51–61

Zorzano A, Fandos C & Palacin M (2000), Role of plasma membrane transporters in muscle metabolism, Biochem J 349, 667–688

Written by: M.M. a/k/a DatBtrue
Copyright 2009 by M.M. a/k/a DatBtrue
Licensed in perpetuity to Synthetek Industries Pty Ltd.
All rights reserved.

No part of this article may be reproduced in any form without the written permission of the copyright owners and licensee.