Endurance and Stamina Supplements: Carnitine and Rhodiola Examined

Carnitine and rhodiola rosea are two of the most widely marketed supplements for energy and stamina. Before spending money on either, it helps to know what the research actually supports, and what it does not.

Carnitine: What It Is and What It Does

Carnitine is a compound the body makes from the amino acids lysine and methionine. According to the NIH Office of Dietary Supplements (ODS), it acts as an essential cofactor that transports long-chain fatty acids into the mitochondria so they can be oxidized to produce energy as adenosine triphosphate (ATP). It also helps transport some toxic compounds out of the mitochondria, and it is concentrated in tissues that rely heavily on fatty acids for fuel, mainly the heart and skeletal muscle.

Because the body synthesizes carnitine and most people obtain additional amounts from meat and dairy, the NIH has not established a daily adult target or a tolerable upper intake level for carnitine from food or supplements.

What the Evidence Says About Carnitine

The ODS grades evidence for carnitine across several health areas. Ratings below reflect how the ODS characterizes the current research:

• Energy production (ATP): Strong evidence supports carnitine's role as a cofactor in fatty acid transport for energy production. This is a well-established physiological function.
• Primary and secondary carnitine deficiency: Moderate evidence supports that high-dose supplemental carnitine resolves these deficiency conditions.
• Athletic performance: The evidence is rated insufficient. Research has not consistently shown that carnitine supplements enhance exercise performance in people who are not deficient.
• Weight loss: Evidence is limited. Some studies suggest a possible modest effect, but findings are not strong enough to draw firm conclusions.
• Cardiovascular outcomes: Evidence is limited for improvements in mortality and heart function.
• Glycemic markers in insulin resistance and diabetes: Evidence is limited.
• Alzheimer's disease or dementia: Evidence is insufficient.
• Osteoarthritis symptoms: Evidence is insufficient.

In short, carnitine has a clearly understood and important role in normal metabolism, but that physiological role does not automatically mean that taking extra carnitine will improve performance or stamina in a healthy person who is not deficient.

Carnitine Safety and Side Effects

The ODS notes that doses of about 3 grams per day can cause nausea, vomiting, abdominal cramps, diarrhea, and a fishy body odor. Muscle weakness can occur in people with uremia, and seizures have been reported in people with seizure disorders who take carnitine supplements.

An additional concern flagged by the ODS involves gut bacteria. Unabsorbed carnitine can be converted by gut bacteria into trimethylamine N-oxide (TMAO), a compound that some research links to higher cardiovascular disease risk, though this relationship is still being studied.

Carnitine Drug Interactions

The ODS identifies several clinically relevant interactions. Always speak with a clinician or pharmacist before combining carnitine with any medication:

• Pivalate-conjugated antibiotics such as pivampicillin: chronic use can deplete carnitine by reducing absorption or increasing excretion.
• Valproic acid: reduces blood carnitine levels. The ODS notes that intravenous L-carnitine may help treat valproic acid toxicity in combination with the risk of hepatotoxicity and elevated plasma ammonia.
• Other anticonvulsants including phenobarbital, phenytoin, and carbamazepine: reduce blood levels of carnitine.

Rhodiola Rosea: What It Is and What It Does

Rhodiola rosea is a flowering plant that grows in cold, mountainous regions of Europe and Asia. The National Center for Complementary and Integrative Health (NCCIH) describes it as traditionally used to support work performance, endurance, and physical stamina. It has also been historically taken for fatigue and low energy, and it is commonly promoted as an adaptogen, a term used to describe substances said to help the body handle stress.

No daily adult target and no upper intake level have been established for rhodiola by any NIH body.

What the Evidence Says About Rhodiola

The NCCIH is direct about the current state of evidence. Research to date is described as low to moderate quality, and the NCCIH states it is not sufficient to confirm a benefit for any of rhodiola's common uses. Evidence grades across all areas are rated insufficient:

• Athletic or physical performance: Insufficient evidence to confirm a benefit.
• Stress reduction or adaptogen effects: Insufficient evidence. The NIH has not established that rhodiola produces an adaptogenic effect.
• Fatigue relief or energy increase: Insufficient evidence.
• Mood improvement or depression: Insufficient evidence.
• Cognition: Insufficient evidence.

This does not mean rhodiola has been proven ineffective. It means the quality and consistency of available studies have not yet reached the bar needed to confirm these uses.

Rhodiola Safety and Side Effects

The NCCIH describes rhodiola as possibly safe when taken by mouth for up to 12 weeks. Safety beyond that period has not been established. Reported side effects include dizziness, headache, insomnia, and either dry mouth or excessive saliva production.

Very little is known about whether rhodiola is safe during pregnancy or breastfeeding. The NCCIH advises avoiding it in those situations unless a clinician specifically recommends otherwise.

Rhodiola Drug Interactions

The NCCIH reports that interactions between rhodiola and losartan, a medication used for high blood pressure, have been documented. Because herbs and medicines can interact in harmful ways, the NCCIH advises talking with a health care provider before using rhodiola alongside any prescription medication.

Key Takeaways for Anyone Considering These Supplements

• Carnitine has a strong, well-understood physiological role in energy metabolism, but evidence that supplements enhance athletic performance in healthy, non-deficient adults is rated insufficient by the ODS.
• Rhodiola has a long history of traditional use, but the NCCIH currently rates evidence for all its promoted benefits as insufficient.
• Neither supplement has an established daily target or upper limit for healthy adults.
• Both carry real safety considerations and potential drug interactions that deserve attention.
• These statements have not been evaluated by the FDA, and supplements are not intended to diagnose, treat, cure, or prevent any disease.
• For personal dosing decisions and to check for interactions with any medications you take, consult a clinician or pharmacist before starting either supplement.

Sources: NIH Office of Dietary Supplements Carnitine fact sheet (ods.od.nih.gov), National Academies Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids, and NCCIH Rhodiola fact sheet (nccih.nih.gov).