Carnitine carries fat into the part of the cell that turns it into energy. In PCD, the door that lets carnitine into cells does not work well, so carnitine leaks away. Taking extra carnitine by mouth gets enough of it in through a different door, which is why the daily dose works.
You do not need a biology degree to take care of a child with PCD. But knowing roughly how carnitine works makes the rules make sense. It explains why the dose matters every day, why fasting is the thing we watch, and why the treatment is so reliable.
What is the carnitine shuttle system?
The carnitine shuttle is the system that moves fat into the part of the cell that burns it for energy. Fat is a major fuel for the heart and the muscles, and a backup fuel for almost everything else when you have not eaten for a while. But fat cannot get into the cell's power plant (the mitochondria) on its own. Carnitine grabs it and carries it across. Your body gets carnitine two ways: it makes some, and it absorbs more from food, mostly meat and dairy. Either way, carnitine has to be pulled into cells and held on to by the kidneys to do its job. One specific protein does that pulling. It is called OCTN2.
Where does PCD interrupt this process?
PCD breaks the OCTN2 door. OCTN2 is built from instructions in a gene called SLC22A5. In PCD, a change in that gene means the door does not work well. Carnitine cannot get into cells efficiently, and the kidneys let it wash out in urine instead of saving it. The result is low carnitine everywhere in the body. That is why doctors call it "systemic." Without enough carnitine reaching the mitochondria, fat piles up instead of becoming energy. The liver, the muscles, and the heart feel it most, because they lean on fat the hardest. That is especially true during fasting, illness, or other physical stress.
Why does carnitine supplementation work?
Taking carnitine by mouth works because, in a high enough amount, carnitine can get into cells through a second door the body does not normally rely on much. The broken OCTN2 door gets bypassed. The supplement does not cure PCD, and it does not fix the gene. It replaces what the body keeps losing, every day, for life. That is the whole idea behind the treatment you will read about on the Treatment and Lifestyle page.
I'm a person living with PCD, not your doctor. Everything here is to help you understand and ask good questions; your child's metabolic team makes the medical decisions.
Last updated October 2026.
Where this comes from
I wrote this page from the study below and from living with PCD. Your care team is the final word on your child.
- Longo N, Frigeni M, Pasquali M. "Carnitine transport and fatty acid oxidation." Biochimica et Biophysica Acta, 2016. https://doi.org/10.1016/j.bbamcr.2016.01.023