|Onions contain a compound which is toxic to the red blood cells of cats. The red blood cells are destroyed and this results in anemia. The specific type of anemia is called 'Heinz body anemia' because the red cells develop an abnormality called a 'Heinz body' which can be seen under the microscope.
In the past, when a cat would not eat, baby foods were sometimes used to coax the cat to eat. Unfortunately, the baby foods contained onion powder, and some cats developed Heinz body anemia from eating the onion-containing baby food. Caution should be used in feeding 'human' food to cats because it may contain ingredients that cats cannot tolerate, such as onion.
Garlic contains a compound similar to the one in onions that causes Heinz body anemia, however, the garlic compound is much less toxic.