L-carnitine is a popular supplement often touted for its potential to aid weight loss and enhance athletic performance. But what does the science say? This article dives deep into L-carnitine, exploring its functions, potential benefits, risks, and how it may impact weight loss, brain function, and overall health.
What is L-Carnitine?
L-carnitine is a naturally occurring amino acid derivative that plays a crucial role in energy production. It facilitates the transport of long-chain fatty acids into the mitochondria - the "engines" of your cells - where these fats are burned to produce energy. The body can synthesize L-carnitine from the amino acids lysine and methionine, requiring adequate vitamin C for this process.
Over 95% of the body's L-carnitine is stored in muscles, with smaller amounts found in the blood, liver, heart, and kidneys. Dietary sources include animal products like meat and dairy. Individuals following plant-based diets or those with specific genetic conditions may not produce or obtain sufficient L-carnitine, making it a conditionally essential nutrient.
L-Carnitine and Weight Loss: Exploring the Evidence
One of the most common reasons people take L-carnitine is for weight loss. The rationale is that by helping to move more fatty acids into the cells to be burned, L-carnitine can boost fat oxidation and reduce fat storage.
A meta-analysis of randomized controlled trials revealed that subjects who received carnitine lost significantly more weight. The studies included in the review were identified by a systematic search of PubMed, Embase, Cochrane Central Register of Controlled Trials, and reference lists of relevant marker studies. The meta-analysis showed that carnitine supplementation resulted in a statistically significant weight loss (MD: -1.33 kg; 95% CI: -2.09 to -0.57) and a decrease in body mass index (MD: -0.47 kg m(-2) ; 95% CI: -0.88 to -0.05) compared with the control group. However, the meta-regression analysis of the duration of consumption revealed that the magnitude of weight loss resulted by carnitine supplementation significantly decreased over time (p = 0.002).
Read also: The Role of L-Carnitine and Alpha-Lipoic Acid
Another 2021 review in 1,239 people with type 2 diabetes also found that taking 2 grams (g) of L-carnitine daily for at least 2 weeks helped promote weight loss.
It's important to note that while L-carnitine may aid in weight loss for some, it's not a magic bullet. A comprehensive approach involving a balanced diet and regular exercise is still crucial for achieving sustainable weight loss.
L-Carnitine and Brain Function
Beyond weight loss, L-carnitine, particularly acetyl-L-carnitine (ALCAR), may offer benefits for brain function. Some research suggests that ALCAR may help prevent age-related mental decline and improve markers of learning.
A 2018 study found that taking 1,500 milligrams (mg) of ALCAR daily for 28 weeks significantly improved brain function in people with dementia. However, other studies have yielded mixed results. For instance, a 2017 review of two studies showed that taking L-carnitine for 3 days had no effect on markers of brain function in young adults without cognitive impairment.
More research is needed to fully understand the potential benefits of L-carnitine supplementation on brain health.
Read also: The Power of L-Carnitine and B12
L-Carnitine and Athletic Performance
The impact of L-carnitine on sports performance is a subject of ongoing investigation. Some studies suggest that L-carnitine may improve exercise and muscle recovery, muscle oxygen supply, and exercise performance, endurance, and power. However, other studies have found no significant benefits.
The potential benefits of L-carnitine on athletic performance may be indirect and require weeks or months to manifest, unlike supplements like caffeine or creatine, which can provide a more immediate boost.
L-Carnitine and Type 2 Diabetes
L-carnitine may be beneficial for people with type 2 diabetes. A 2023 review concluded that L-carnitine supplementation may reduce fasting blood sugar and hemoglobin A1c, while also improving insulin sensitivity in people with diabetes, overweight, or obesity.
L-carnitine is believed to work by altering insulin receptors and changing the expression of specific genes that regulate sugar metabolism. It may also help improve the function of beta cells, which are responsible for producing insulin in the pancreas.
L-Carnitine and Depression
Some research suggests that L-carnitine could be beneficial for the treatment of depression. A 2018 review found that acetyl-L-carnitine significantly reduced symptoms of depression compared with placebo and was as effective as antidepressant medications but caused fewer adverse effects. However, more research is needed to confirm these findings.
Read also: L-Carnitine Injections for Weight Loss
Potential Side Effects and Risks
For most people, taking 2 g or less of L-carnitine per day is relatively safe and free from any serious side effects. However, some mild side effects like heartburn and indigestion are possible. Some research has also used doses of up to 4,500 mg per day.
It's important to note that L-carnitine supplements may raise blood levels of trimethylamine-N-oxide (TMAO) over time. High levels of TMAO are linked to an increased risk of atherosclerosis, a disease that clogs the arteries.
More studies on the long-term safety of L-carnitine supplements are needed.
Dietary Sources of L-Carnitine
Small amounts of L-carnitine can be obtained from the diet, primarily through meat and dairy products:
- Beef: 139 to 143 mg per 3.5 ounces (100 g)
- Pork: 25 to 61 mg per 3.5 ounces (100 g)
- Chicken: 13 to 34 mg per 3.5 ounces (100 g)
- Cow’s milk: 40 mg per 3.5 ounces (100 g)
- Hard cheese: 3 mg per 3.5 ounces (100 g)
Food sources of L-carnitine have a greater absorption rate than supplements. Approximately 57% to 84% of L-carnitine is absorbed when consumed from food, compared to only 14% to 18% when taken as a supplement.
Who Might Benefit from L-Carnitine Supplements?
L-carnitine supplements may be beneficial in specific cases:
- Vegans and Vegetarians: L-carnitine levels are often lower in individuals who restrict or avoid animal products. Supplements can help normalize carnitine levels in the blood and muscles.
- Older Adults: A 2016 study found that taking 1.5 g of L-carnitine per day for 10 weeks improved functional status and frailty in older adults.
- Individuals with Certain Chronic Conditions: The risk of deficiency is higher for those with diseases like cirrhosis and kidney disease.
Dosage and Forms of L-Carnitine
Studies on the potential benefits of L-carnitine have used a wide range of doses. Dosage may vary depending on the form of carnitine:
- Acetyl-L-carnitine (ALCAR): 500 to 3,000 mg per day (for brain health and function)
- L-carnitine L-tartrate: 1,000 to 4,500 mg per day (for exercise performance)
- Propionyl-L-carnitine: 2 g per day (for improving blood flow)
L-Carnitine: Additional Considerations
- Sleep: Some research suggests that taking 500 to 510 mg of L-carnitine daily may help treat narcolepsy.
- Pregnancy: Pregnant individuals may have a higher risk of carnitine deficiency and should consult with a doctor about their carnitine levels.
- Medication Interactions: People taking certain antibiotics and anticonvulsants should speak with a doctor before taking L-carnitine.
Prolonged L-Carnitine Supplementation: A Deeper Dive
While L-carnitine is often associated with short-term benefits, research has also explored the effects of prolonged supplementation on metabolism and metabolic changes.
A systematic review evaluated the effect of prolonged LC supplementation on metabolism and metabolic modifications. The review included studies on healthy human subjects, treated for at least 12 weeks with LC administered orally, with no drugs or any other multi-ingredient supplements co-ingestion.
The selected studies were conducted with healthy human subjects, with supplemented dose ranging from 1 g to 4 g per day for either 12 or 24 weeks. LC supplementation, in combination with carbohydrates (CHO) effectively elevated total carnitine content in skeletal muscle. Twenty-four-weeks of LC supplementation did not affect muscle strength in healthy aged women, but significantly increased muscle mass, improved physical effort tolerance and cognitive function in centenarians. LC supplementation was also noted to induce an increase of fasting plasma trimethylamine-N-oxide (TMAO) levels, which was not associated with modification of determined inflammatory nor oxidative stress markers.
L-Carnitine and Skeletal Muscle Protein Balance
Skeletal muscle mass depends on the rates of protein synthesis and degradation. Elevated protein synthesis and attenuated proteolysis are observed during muscle hypertrophy. Both of these processes are mainly regulated by the signaling pathway: insulin-like growth factor-1 (IGF-1) - phosphoinositide-3-kinase (PI3K) - protein kinase B (Akt) - mammalian target of rapamycin (mTOR). The activation of mTOR leads to phosphorylation and activation of S6 kinases (S6Ks) and hyperphosphorylation of 4E-binding proteins (4E-BPs), resulting in the acceleration of protein synthesis. At the same time, Akt phosphorylates and inactivates forkhead box O (FoxO), thereby inhibit the responsible for proteolysis ubiquitin ligases: muscle-specific RING finger-1 (MuRF-1) and muscle atrophy F-box protein (atrogin-1).
The association between LC supplementation and the regulation of metabolic pathways involved in muscle protein balance have been shown in several animal studies. Four weeks of LC supplementation in rats increased plasma IGF-1 concentration. Elevated circulating IGF-1 led to an activation of the IGF-1-PI3K-Akt signalling pathway, causing augmented mTOR phosphorylation and higher phospho-FoxO/total FoxO ratio in skeletal muscle of LC supplemented rats. FoxO inactivation attenuated MURF-1 expression in quadriceps femoris muscle of supplemented rats (compared to control). Moreover, LC administrated for 2 weeks suppresses atrogin-1 messenger RNA (mRNA) level in suspended rats’ hindlimb, and only 7 days of LC administration downregulates MuRF-1 and atrogin-1 mRNAs reducing muscle wasting in a rat model of cancer cachexia. In fact, administration of acetyl-L-carnitine 3 g/day for 5 months in HIV-seropositive patients induced ten-fold increase in serum IGF-1 concentration. Conversely, neither 3 weeks LC supplementation in healthy, recreationally weight-trained men, nor 24 weeks LC supplementation in aged women did not affect circulating IGF-1 level concentration. Various effects might be due to different IGF-1 levels; significantly lower in the HIV-seropositive patients than in healthy subjects. Additionally, 8 weeks of LC supplementation in healthy older subjects, did not change total and phosphorylated mTOR, S6K and 4E-BP proteins level of vastus lateralis muscle. It must be highlighted that rat skeletal muscle TC increases ~ 50-70% following 4 weeks of LC supplementation, whereas comparable elevation has never been observed in human studies, even after 24 weeks of supplementation.
L-Carnitine and Body Composition: Obesity, Training, and Sarcopenia
The effect of L-carnitine supplementation on body composition has been investigated in various contexts, including obesity, training, and sarcopenia (age-related muscle loss).
A recent meta-analysis demonstrated a significant reduction in weight following LC supplementation, but the subgroups analysis revealed no significant effect of LC on body weight in subjects with body mass index (BMI) below 25 kg/m2. Therefore, authors suggested that LC supplementation may be effective in obese and overweight subjects. Surprisingly, intervention longer than 24 weeks showed no significant effect on BMI.
It has been assumed that a combination of LC supplementation with increased energy expenditure may positively affect body composition. However, either with aerobic or resistance training, LC supplementation has not achieved successful endpoint. Six weeks of endurance training (five times per week, 40 min on a bicycle ergometer at 60% maximal oxygen uptake) together with LC supplementation (4 g/day) does not induce a positive effect on fat metabolism in healthy male subjects (% body fat 17.9 ± 2.3 at the beginning of the study). Similarly, lack of LC effect has been reported in obese women. Eight weeks of supplementation (2 g/day) combined with aerobic training (3 sessions a week) had no significant effects on body weight, BMI and daily dietary intake in obese women.
In the recent study, LC supplementation 2 g/day has been applied in combination with a resistance training program (4 days/week) to healthy men (age range 18-40 y.o.), for 9 weeks. Body composition, determined by dual energy X-ray absorptiometry, indicated no significant effect in fat mass and fat-free mass due to supplementation. Moreover, LC administration did not influence bench press results. The number of leg press repetitions and the leg press third set lifting volume increased in the LC group compared to the placebo group. Different LC effect in the limbs may be associated with the higher rates of glycogenolysis during arm exercise at the same relative intensity as leg exercise.
Aged people have accelerated protein catabolism, which is associated with muscle wasting. LC could increase the amount.
User Reviews and Experiences
User reviews provide valuable insights into the real-world experiences of individuals taking L-carnitine supplements.
One user reported using NOW Foods L-Carnitine as part of their fitness routine and experienced a clean, natural boost in energy during workouts without any jitters or crash. They also praised the consistent quality and transparency of ingredients.
Another user shared their experience of losing 40 lbs in a few months by taking two tablespoons of L-carnitine in the morning and mid-afternoon.
However, it's important to remember that individual experiences may vary, and what works for one person may not work for another.