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A genetic variant in the enzyme that transports fats into your cells' energy factories, revealing whether your body can safely burn fat during exercise, fasting, or illness.
Your body has two main fuel sources: sugar and fat. When you exercise for more than a few minutes, fast overnight, fight an infection, or shiver in the cold, your muscles increasingly rely on burning fat. The CPT2 genotype test tells you whether you carry a specific change in the gene for CPT2 (carnitine palmitoyltransferase II), the enzyme responsible for the final step of shuttling long-chain fats into your mitochondria, the energy-producing compartments inside every cell. If this enzyme does not work well enough, your muscles can run out of fuel and break down during exactly the moments when fat burning matters most.
The variant this test focuses on is called S113L (also written as c.338C>T). It is the single most common disease-causing change in the CPT2 gene, accounting for roughly 60% of all pathogenic CPT2 alleles in the muscle form of the disease. Your result will come back as one of three genotypes: C/C, C/T, or T/T, reflecting how many copies of the variant you carry.
Knowing your genotype matters because CPT2 deficiency is dramatically underdiagnosed. The average delay from first symptoms to diagnosis is 26.7 years. Many people with unexplained muscle pain, exercise intolerance, or episodes of dark urine after exertion go years without an answer. A simple genetic test can change that.
Think of your mitochondria as furnaces that burn fuel to produce energy. Long-chain fatty acids, the main fat molecules in your blood, cannot simply walk into the furnace. They need a shuttle system. CPT2 sits on the inner membrane of the mitochondrion and performs the last handoff: it takes the fatty acid off its carrier molecule (carnitine) and passes it to coenzyme A so it can enter the fat-burning cycle (called beta-oxidation).
The S113L variant causes the CPT2 protein to be made normally but to degrade more quickly than usual, resulting in markedly reduced protein levels inside cells. The enzyme that remains works, but there is not enough of it. Homozygous individuals (T/T) retain only about 15 to 20% of normal enzyme activity. That residual activity is enough to handle everyday fat metabolism at rest. But when your body ramps up its demand for fat fuel, during prolonged exercise, fasting, cold exposure, or fever, the system hits a bottleneck.
There is an additional vulnerability specific to this variant. Normal CPT2 gets a performance boost from a molecule called cardiolipin, a fat found in the inner mitochondrial membrane. The S113L version of the enzyme does not respond to cardiolipin. Worse, at body temperature (37 degrees Celsius), and especially at fever temperatures, the variant enzyme loses activity more sharply than the normal version. This helps explain why infections and fevers are such potent triggers for muscle breakdown episodes in people with this genotype.
Your result will be reported as C/C, C/T, or T/T. Each has a different level of clinical significance, and the distinction matters for how you manage your health.
| Genotype | What It Means | Enzyme Activity | What to Expect |
|---|---|---|---|
| C/C | No copies of the S113L variant | Normal (~100%) | No increased risk of CPT2 deficiency from this variant |
| C/T | One copy (carrier) | About 50% of normal | Usually no symptoms; fat burning during prolonged exercise may be mildly impaired; important for family planning |
| T/T | Two copies (affected) | About 15 to 20% of normal | Myopathic CPT2 deficiency: risk of muscle breakdown with exercise, fasting, cold, or fever |
What this means for you: if your result is C/C, this particular variant is not a concern. If you are C/T, you are a carrier. You are unlikely to have symptoms, but your fat oxidation during exercise is measurably lower than someone with two normal copies. Carrier status matters most for family planning: if your partner is also a carrier, each child has a 25% chance of being T/T. If your result is T/T, you have the genetic basis for the muscle form of CPT2 deficiency, and the management strategies below apply directly to you.
One important caveat: this test screens for one variant, and there are more than 90 known disease-causing changes in the CPT2 gene. A C/C result on this specific test does not completely rule out CPT2 deficiency if you have suspicious symptoms. However, because S113L is present in at least one allele in 95% of affected individuals with the muscle form, testing for this variant alone would identify 97% of myopathic CPT2 deficiency cases.
The muscle form of CPT2 deficiency is the most common inherited disorder of fat metabolism affecting skeletal muscle and the most frequent cause of hereditary muscle breakdown with dark urine (myoglobinuria). Despite being historically called the "adult form," 60% of those affected actually have their first symptoms in childhood, between ages 1 and 12.
The hallmark is recurrent episodes of rhabdomyolysis, a condition where muscle fibers break apart and release their contents into the bloodstream. This occurs in about 86% of affected individuals. The triggers form a recognizable pattern:
Episode frequency varies enormously, from once a year to as many as 85 times per year. While the overall prognosis is favorable with proper management, serious complications can occur. In one clinical series of 13 individuals, two required dialysis for acute kidney injury caused by the muscle breakdown products overwhelming the kidneys.
A detailed metabolic study of a woman homozygous for S113L uncovered a broader metabolic picture beyond muscle symptoms. She had virtually absent fat oxidation, severe insulin resistance despite normal blood sugar, and a compensatory shift toward burning almost exclusively carbohydrates even at rest. Her body had also reduced its baseline rate of fat release from storage, a built-in adaptation to the fact that released fat could not be properly burned.
Because this is a genetic variant, you cannot change your genotype. But if you are T/T or a symptomatic carrier, several interventions can reduce the frequency and severity of episodes by working around the enzyme bottleneck.
Dietary modification: The cornerstone of management is shifting your fuel mix away from the fats your body cannot efficiently burn. A high-carbohydrate (about 70% of calories), low-fat diet reduces the demand on the impaired enzyme by keeping carbohydrate stores topped up and minimizing the need for long-chain fat oxidation. Some clinicians also recommend supplementing with medium-chain triglycerides, which bypass the CPT2 shuttle entirely and enter mitochondria through a different route. An anaplerotic diet approach, which supplies alternative fuel molecules that feed into the energy cycle downstream of the block, has shown benefit in clinical reports.
Trigger avoidance: The most effective prevention is learning your personal trigger threshold and staying within it. For most affected individuals, exercise lasting longer than 15 to 60 minutes is sufficient to provoke symptoms. Avoiding prolonged fasting, dressing warmly in cold weather, and aggressively managing fevers and infections (including with intravenous glucose in clinical settings to prevent the body from shifting to fat burning) are all established strategies.
Supplements and medications: L-carnitine supplementation helps convert potentially toxic long-chain fat intermediates into a form the body can excrete more safely. Bezafibrate, a lipid-lowering drug, has shown benefit in some individuals by increasing the expression of whatever residual CPT2 enzyme the body can produce. During acute episodes of rhabdomyolysis, aggressive intravenous hydration is critical to protect the kidneys from damage caused by muscle breakdown products.
Medications to avoid: Certain drugs can worsen the condition or trigger episodes. These include valproic acid (an antiseizure medication), general anesthesia, high-dose ibuprofen, and high-dose diazepam. If you carry this genotype, make sure any prescribing clinician is aware.
Family screening: If you are T/T, genetic testing of blood relatives is strongly recommended. In one Austrian family study, screening 24 blood relatives of an affected individual identified 5 homozygotes and 15 heterozygotes, including 3 people with previously undiagnosed CPT2 deficiency. Early identification allows preventive management before a serious episode occurs.