Mutated genes, increased levels of trimethylamine-producing bacteria and excess dietary proteins can cause trimethylaminuria, a condition in which a person's sweat, breath and urine smell like fish, according to MedicineNet. Kidney or liver disease also cause the condition. The condition is also common in women beginning menstruation and premature infants.
During digestion, the FMO3 enzyme typically converts trimethylamine, which has a fishy odor, into an odorless molecule, explains MedicineNet. If the FMO3 gene mutates and decreases the enzyme's function, or if the enzyme is missing, the body's trimethylamine level increases. The body releases the excess through urine, sweat and breath, resulting in the strong odor.