Starfruit (Averrhoa carambola) is a popular tropical fruit in numerous countries of Asia and South America, and is increasing found in Western countries as well. Why am I writing about this on the Renal Fellow Network?
It turns out that the starfruit contains an as-yet unidentified substance which is renally cleared which may act as a neurotoxin in individuals with severely reduced renal clearance. There are numerous case series (Neto et al, Nephrol Dial Transplant 2003, 18(1):120-125) which describe ESRD or advanced CKD patients which, after ingesting as little as one starfruit, went on to develop neurologic symptoms ranging from hiccups to seizures to coma to even death. Injection of purified starfruit toxin into uremic rat brain can also induce neurologic problems. Interestingly, the toxin appears to be dialyzable, as altered mental status can be rapidly improved with dialysis.
In addition to this unidentified neurotoxin which can cause major neurologic problems, starfruit contains another renally-relevant substance: it is rich in oxalate. In patients with CKD, oxalate crystal deposition may accelerate renal deterioration.