Carotenoids appear to work synergistically. In a study on humans, beta-carotene deficiency caused suppressed immunity. Beta-carotene alone did not restore immunity. However, when a mixture of carotenoids was taken, immunity was restored. They can sicken a dog as well.
Onions contain substances that destroy red blood cells.
The condition, known as hemolytic anemia, is caused by molecules in onions that create high levels of oxidative stress in animals' blood cells. Glutathione is quickly depleted, and the cells fall apart. Don't feed your animal baby food without checking the label first. Some brands contain onion powder. Chocolate has similar effects, so steer clear of chocolate and onions.