My 16 months son has ethyl malonic aciduria, which kind of fats that harm his health (saturated or monoun saturated fat) ?

His doctor can help: With a young child who has a metabolic disorder, the clinical genetics doctor and the pediatrician can help with advice on dietary restrictions. The cause of ethylmalonic aciduria may be a mutation that causes abnormal handling of certain Amino Acids (which come from digesting dietary protein), which means the type of fats in the diet shouldn't matter much.

Not fat problem: Methyl malonyl acidemia (aciduria) is a disorder arising from inability to metabolize certain amino acids.