Vitamin C (Ascorbic acid)

It is a water-soluble vitamin and a strong antioxidant. It supports the immune system and is essential for collagen synthesis (the building material of joints, bones, and blood vessels). Unlike humans, dogs and cats produce it themselves in the liver from glucose, in sufficient amounts — which is why it is not an essential dietary vitamin for them and NRC standards do not establish a requirement for it [1] [2].

Where it's found: Organ meats (liver, kidneys, adrenal glands) contain small amounts. When needed (joint problems, growth of giant breeds, convalescence, severe stress), a natural supplement in the form of rosehip meal is sometimes used — but that is an option, not a necessity.

Deficiency: Does not occur in healthy animals, because the body synthesizes vitamin C on its own [1]. Theoretically, the requirement may locally exceed production in severe liver disease, heavy exertion (sled, sporting dogs), or strong infections, when antioxidant consumption rises — this is an area without unambiguous clinical data, so without overstating it.

Excess: Large supplementation with synthetic vitamin C can raise the risk of calcium oxalate urolithiasis (ascorbic acid is metabolized partly to oxalates) and cause osmotic diarrhea [2]. Therefore do not supplement vitamin C routinely "for immunity" — in a healthy dog and cat it is unnecessary, and in large doses potentially harmful.

Sources

1. National Research Council (NRC) (2006). Nutrient Requirements of Dogs and Cats — vitamin C is not essential in the diet of dogs and cats (endogenous synthesis). National Academies Press. https://nap.nationalacademies.org/catalog/10668/nutrient-requirements-of-dogs-and-cats
2. Hand MS, Thatcher CD, Remillard RL, Roudebush P, Novotny BJ (2010). Small Animal Clinical Nutrition, 5th Edition. https://www.markmorrisinstitute.org/sacn5.html