L-carnitine is an amino acid derivative that your body produces naturally from lysine and methionine. Its primary job is shuttling long-chain fatty acids into the mitochondria, where they’re burned for energy. Without adequate carnitine, your cells can’t efficiently convert fat into usable fuel.

Most healthy adults make enough carnitine on their own, and can top up through diet. Red meat is the richest source (beef steak provides about 56-162 mg per 100g), while dairy, fish, and poultry contain smaller amounts 1. Vegetarians and vegans tend to have lower carnitine levels, though outright deficiency is rare unless there’s an underlying metabolic issue.

The synthesis process requires several nutrients working together: vitamin C, iron, vitamin B6, and niacin all serve as cofactors. A shortage in any of these can theoretically limit carnitine production, though this is more of a concern in people with poor overall nutrition or specific medical conditions.

What does the research actually show?

I should be upfront: L-carnitine research is a mixed bag. Some studies show promising results, others are underwhelming. The evidence is stronger for certain conditions than others, and much of the positive data comes from populations with specific health problems rather than healthy individuals looking for a performance boost.

1. Non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) affects somewhere between 20-30% of people in Western countries and 5-18% in Asia, making it one of the most common liver conditions worldwide 2. The condition ranges from simple fat accumulation to more serious inflammation and scarring.

A 2020 meta-analysis pooled data from 5 randomised controlled trials involving 334 NAFLD patients. Carnitine supplementation (300-2000 mg daily for 12-24 weeks) significantly reduced insulin resistance markers, liver enzymes (AST and ALT), and triglyceride levels 2.

The proposed mechanism involves carnitine’s effects on gene expression related to insulin signalling, plus its role in regulating the acyl-CoA balance within mitochondria.

My take: The results look encouraging, but 334 patients across 5 studies isn’t a huge evidence base. If you have NAFLD, carnitine might be worth discussing with your hepatologist, but it shouldn’t replace standard interventions like weight loss and dietary changes. For more on supplements that may support liver health, see our article on foods beneficial for fatty liver.

2. C-reactive protein (inflammation marker)

C-reactive protein (CRP) is an acute phase protein that rises in response to inflammation. Healthy people typically have levels below 1 mg/L, while anything above 10 mg/L suggests significant inflammation. People who smoke, have high blood pressure, or are overweight tend to have elevated CRP.

A 2017 meta-analysis of 6 randomised controlled trials (1,087 participants) found that oral carnitine significantly reduced CRP concentrations compared with placebo 3.

The catch: The included studies had substantial heterogeneity, meaning the patient populations and protocols varied quite a bit. This makes it harder to draw firm conclusions about who might benefit most.

3. Weight loss

Every supplement company on the planet markets L-carnitine as a fat burner. The logic is straightforward: carnitine helps transport fatty acids into mitochondria for burning, so more carnitine should mean more fat burning. Right?

A 2016 meta-analysis examined 9 studies with 911 participants and found that L-carnitine did produce statistically significant weight loss and BMI reduction compared with placebo 4. However, the effect was most pronounced in people with chronic conditions like diabetes or obesity rather than healthy individuals trying to lose a few kilos.

Reality check: Don’t expect miracles. The weight loss in these studies was modest, and carnitine won’t compensate for poor diet or lack of exercise. I wouldn’t bet on it as your primary weight loss strategy.

4. Type 2 diabetes

Diabetes prevalence continues to climb worldwide. In the US, roughly 35% of adults over 20 have prediabetes, rising to 50% after age 65 5. Untreated, over a third of prediabetic individuals progress to full diabetes within 4 years.

Two meta-analyses have looked at L-carnitine in type 2 diabetes. One analysis of 4 studies (284 patients) found improvements in fasting blood glucose and blood lipids including total cholesterol and LDL 6. Another review of 5 studies (631 patients) showed L-carnitine helped improve insulin resistance, with benefits increasing over longer treatment duration 7.

The mechanism appears to involve enhanced mitochondrial oxidation of fatty acids, induction of glycolytic enzymes, and regulation of insulin-related gene expression.

Worth noting: These are adjunctive benefits, not replacements for diabetes medications or lifestyle changes. If you have diabetes and are considering carnitine, speak with your endocrinologist first. Diabetes can also affect bladder function over time - see our article on diabetic bladder dysfunction for more information.

5. Cognitive function

Given carnitine’s role in cellular energy production, some researchers have wondered whether it might help with cognitive function, particularly in ageing populations.

A Cochrane review examined this question and found… not much. Two studies involving 418 healthy adults showed L-carnitine had no significant effect on reaction time, alertness, memory, or delayed recall 8.

In practice: If you’re healthy and hoping carnitine will sharpen your mind, the evidence doesn’t support that expectation. The picture may be different in people with certain neurological conditions, but for general cognitive enhancement, look elsewhere.

6. Cardiovascular protection after heart attack

This is where the evidence gets more interesting. A 2013 meta-analysis pooled data from 13 randomised controlled trials involving 3,629 patients who’d suffered acute myocardial infarction. Carnitine use was associated with a 27% reduction in all-cause mortality, 65% fewer ventricular arrhythmias, and 40% less angina 9.

The proposed mechanisms include better mitochondrial energy metabolism and removal of toxic fatty acid metabolites.

My assessment: These numbers are actually quite impressive. Given carnitine’s low cost and good safety profile, it may have a role in secondary prevention for people at high cardiovascular risk. However, more recent research has raised some concerns about carnitine metabolism and cardiovascular risk (more on this below), so the picture isn’t entirely straightforward. For other supplements that may support heart health, see our guides on fish oil and CoQ10.

7. Male fertility (idiopathic oligospermia)

Idiopathic oligoasthenoteratozoospermia is a mouthful that basically means “unexplained poor sperm quality”. It’s characterised by low sperm count (<15 million cells/ml), reduced motility (<40%), and abnormal morphology. Oxidative stress appears to play a role in many cases.

A 2019 meta-analysis of 7 studies (693 patients) found that combination therapy with L-carnitine and acetyl-L-carnitine improved several parameters: forward sperm motility, total motile sperm count, and pregnancy rates 10.

The upside: This is one of the more promising applications for carnitine supplementation. The combination with acetyl-L-carnitine seems to matter. For a broader overview of supplements that may support male fertility, see our article on health foods beneficial for male fertility.

8. Renal anaemia in haemodialysis patients

Chronic kidney disease affects 8-16% of the population, and end-stage renal disease requiring haemodialysis creates additional complications including anaemia. Patients typically need erythropoietin (EPO) injections to stimulate red blood cell production, but response varies.

A 2021 meta-analysis of 18 randomised controlled trials (1,090 haemodialysis patients) found that L-carnitine supplementation reduced the amount of EPO needed to maintain adequate blood counts 11. However, carnitine didn’t actually raise haemoglobin or haematocrit levels on its own.

In practice: Reducing EPO requirements could be beneficial given that drug’s cost and potential side effects. This seems to be an established use of carnitine in nephrology, though individual responses vary.

Types of carnitine supplements

Not all carnitine supplements are the same. Here’s what’s available:

L-carnitine: The standard form found in most supplements. Well-absorbed and widely studied.

Acetyl-L-carnitine (ALCAR): Has an acetyl group attached, which allows it to cross the blood-brain barrier more readily. Often marketed for cognitive benefits, though as noted above, evidence in healthy people is limited. May cause agitation in Alzheimer’s patients and increase seizure frequency in epilepsy.

L-carnitine L-tartrate: This form absorbs faster, which is why sports supplements favour it. May help with exercise recovery and muscle soreness, though the marketing tends to outpace the evidence.

Propionyl-L-carnitine: Best studied for circulation issues like peripheral vascular disease. May enhance nitric oxide production.

D-carnitine: Avoid this form entirely. It’s biologically inactive and can actually interfere with L-carnitine absorption, potentially causing deficiency.

Dosage

Dosages vary considerably across studies:

L-carnitine: 500-2,000 mg daily
Acetyl-L-carnitine: 600-2,500 mg daily
L-carnitine L-tartrate: 1,000-4,000 mg daily
Propionyl-L-carnitine: 400-1,000 mg daily

I’d suggest starting at the lower end of these ranges and working up if needed. There’s no strong evidence that higher doses provide proportionally greater benefits.

Side effects

L-carnitine is generally well tolerated. Serious adverse effects are uncommon at standard doses, but you may experience:

Nausea and stomach upset
Vomiting
Abdominal cramps
Heartburn
Diarrhoea

At doses above 3,000 mg daily, a more distinctive problem emerges: fishy body odour affecting breath, urine, and sweat. This is caused by trimethylamine, a metabolite of carnitine produced by gut bacteria 12.

The TMAO controversy

Here’s something worth knowing: when gut bacteria metabolise carnitine, they produce trimethylamine, which the liver then converts to trimethylamine N-oxide (TMAO). Some research has linked elevated TMAO levels to increased cardiovascular risk 12. This is somewhat ironic given the positive cardiovascular findings mentioned earlier.

The clinical significance of this remains debated. Red meat eaters may be at higher risk because their gut bacteria are better adapted to metabolise carnitine into TMAO. Whether carnitine supplements at moderate doses materially increase cardiovascular risk in people with otherwise healthy diets is unclear.

Who should avoid L-carnitine?

Contraindications

Hypothyroidism: Carnitine may interfere with thyroid hormone activity in peripheral tissues. There’s evidence it can worsen hypothyroid symptoms, so people with underactive thyroid should avoid supplementation unless supervised by their doctor 13.

Anticoagulant therapy: Carnitine may enhance the effects of warfarin and similar medications, increasing bleeding risk. If you’re on blood thinners, avoid carnitine or use only under medical supervision.

Seizure disorders: Acetyl-L-carnitine specifically may increase seizure frequency in people with epilepsy.

Thyroid hormone therapy: Carnitine can interfere with thyroid hormone action, potentially requiring dose adjustments in people taking levothyroxine or similar medications.

General precautions

The following groups should consult a healthcare provider before using carnitine:

Pregnant or breastfeeding women
People with liver disease
People with kidney disease (other than those already on dialysis where it’s sometimes used therapeutically)
People with peripheral vascular disease
People with diabetes (due to potential blood sugar effects)
People with high blood pressure

Conclusion

L-carnitine occupies an interesting middle ground in the supplement world. It’s not the miracle fat-burner that marketing would have you believe, but it’s not useless either. The evidence is strongest for specific clinical populations: NAFLD patients, people recovering from heart attacks, haemodialysis patients, and men with unexplained infertility.

For healthy individuals seeking general performance or cognitive benefits, the evidence is considerably weaker. If you’re considering carnitine supplementation, I’d encourage having a clear reason for taking it rather than hoping it might do something vaguely beneficial.

As with any supplement, quality matters. Choose products from reputable manufacturers, start with lower doses, and pay attention to how your body responds. And avoid D-carnitine or mixed D,L-carnitine products entirely.