The effect of vitamin C supplementation in healthy dogs on antioxidative capacity and immune parameters

Hesta, M., Ottermans, C., Krammer-Lukas, S., Zentek, J., Hellweg, P., Buyse, J., & Janssens, G. P. J. (2009). The effect of vitamin C supplementation in healthy dogs on antioxidative capacity and immune parameters. Journal of Animal Physiology and Animal Nutrition, 93(1), 26–34. https://doi.org/10.1111/j.1439-0396.2007.00774.x

Setting the Stage

In their 2009 paper published in the Journal of Animal Physiology and Animal Nutrition, Hesta and colleagues set out to answer a simple but important question: Does extra vitamin C improve health in dogs that are already well?

Dogs, unlike humans, make their own vitamin C in the liver from glucose. Because of this, vitamin C is not considered an “essential” nutrient for dogs. But the authors highlight some important caveats:

• Dogs produce vitamin C at lower rates than many other species.
• In times of stress, illness, or heavy exercise, their bodies may not be able to keep up with demand.
• Vitamin C acts as an antioxidant, helping to protect cells from oxidative stress, and it also plays a role in recycling vitamin E, another antioxidant.
• For these reasons, vitamin C has often been added to pet diets and supplements, even though it isn’t technically required.

This background led the researchers to test whether adding vitamin C to the diet of healthy dogs would improve their antioxidant capacity (the body’s ability to neutralize damaging free radicals) or immune system markers.

Study Design

The researchers used 15 healthy adult Beagle dogs in a crossover study, meaning every dog tried every treatment. Dogs were supplemented with capsules containing:

• 0 mg vitamin C + 60 mg vitamin E
• 30 mg vitamin C + 60 mg vitamin E
• 60 mg vitamin C + 60 mg vitamin E

Each treatment lasted 36 days, with wash-out periods in between to reset vitamin levels. During the trial, researchers measured:

• Blood vitamin levels (C, E, and A)
• Antioxidant markers (TBARS, a marker of oxidative stress, and uric acid)
• Immune parameters, including:
  • Subpopulations of lymphocytes (CD4+, CD5+, CD8+, CD21+)
  • Levels of IgA and IgG antibodies
  • Immune cell proliferation (how strongly immune cells responded to stimulation in the lab)

Key Findings

• Vitamin C status: Plasma vitamin C rose slightly with supplementation, but the change was small.
• Antioxidant activity: No meaningful changes were observed in oxidative stress markers, even when dogs received extra vitamin C.
• Immune effects:
  • CD4+ lymphocytes (a type of “helper” T-cell) increased modestly.
  • At 30 mg, vitamin C boosted immune cell proliferation; at 60 mg, it reduced it, suggesting that more is not always better.
  • Antibody levels (IgA and IgG) were not affected.
• General health: Body weight and condition stayed stable, showing no adverse effects of supplementation.

Relevance to Canine Health

The authors conclude that vitamin C supplementation does not substantially change antioxidant or immune measures in healthy, resting dogs, especially when diets already provide ample vitamin E.

However, they also emphasize a key takeaway: in dogs under stress, illness, or heavy physical activity, where the body’s natural vitamin C production may be insufficient, supplementation could become more important.

This helps explain why vitamin C continues to appear in some canine health supplements, as a potential extra layer of support when the body is under pressure.