Cats cannot eat onions (Allium cepa) or any other members of the Allium family. Allium spp., which includes onion, garlic, chives and leeks contain sulfur-containing oxidants (thiosulfinates and thiosulfonates). Sulfur-containing oxidants are metabolised into reactive oxidants which can lead to Heinz body hemolytic anemia.
Heinz bodies are abnormal structures that form as a result of the denaturation of hemoglobin, which is the protein that carries oxygen in red blood cells. Normally, hemoglobin is tightly folded and structured, but when exposed to sulfur-containing oxidants, becomes denatured and forms a clump, which is called a Heinz body. Heinz bodies appear as small, round or irregularly shaped inclusions within the red blood cell when viewed under a microscope. The presence of Heinz bodies can interfere with the normal function of red blood cells, such as oxygen transport, and may cause the cells to become more fragile and prone to hemolysis (rupture).