Polyphenols are a diverse group of compounds plants produce as part of their natural defense mechanisms against environmental stressors, including ultraviolet light, insects, and disease. They also contribute to the color, bitterness, and dry, puckering sensation associated with foods such as fruits, berries, herbs, and tea.

Researchers have identified more than 8,000 polyphenols and classified them based on their chemical structures. These include flavonoids, such as quercetin and green tea catechins; phenolic acids, such as ferulic acid; stilbenes, such as resveratrol; lignans; and tannins.

“Horses evolved eating polyphenols as forages and other plants containing a variety of phenolic acids, flavonoids, and isoflavones,” says Kelly Vineyard, PhD, of EquiVine PhD Nutrition. The amount they consume depends on the plant species, maturity, daily intake, and how the forage is harvested and stored.

Polyphenols, Antioxidants, and Oxidative Stress

The horse’s body naturally produces reactive oxygen species (ROS, unstable oxygen-containing molecules that react with other molecules in a cell) during normal metabolic processes. These molecules also contribute to cell signaling and immune defense. However, when the body’s production of ROS exceeds its ability to neutralize them, oxidative stress can occur, potentially damaging cell membranes, proteins, and DNA.

Strenuous exercise, metabolic disease, transportation stress, and aging can contribute to oxidative stress. Horses rely on their own antioxidant enzymes, along with dietary antioxidants such as vitamin E, to help maintain a balance between ROS production and antioxidant defenses.

In laboratory settings polyphenols show antioxidant activity to combat oxidative stress, but their effects inside the body are more complicated. Horses absorb most polyphenols in relatively small amounts and metabolize them quickly, meaning their direct antioxidant effects might be modest outside the digestive tract, says Vineyard.

Instead, these compounds might help activate the body’s own antioxidant defenses and regulate inflammatory pathways. “I would describe polyphenols as potentially useful modulators of these systems in the horse’s body,” she says.

Equine Polyphenol Research

Researchers have investigated several polyphenols, including resveratrol and curcumin, for their potential effects on inflammation, joint health, respiratory disease, and metabolic function. Results have varied depending on the compound, formulation, dosage, and outcome measured.

In a 2016 study, researchers examined immune cells collected from six senior horses. Several polyphenols reduced the production of inflammatory cytokines (signaling proteins involved in the inflammatory response). However, because researchers conducted the work in a laboratory rather than feeding supplements to horses, the findings did not establish whether oral supplementation produces the same effects in living animals.

Vineyard noted other studies in which researchers have evaluated polyphenols in horses with specific health conditions such as:

• Equine asthma: In a 2025 study, researchers investigated mixed-polyphenol supplementation in horses with equine asthma. They found reduced airway neutrophils and the inflammatory marker interleukin-6 (IL-6) in treated supplemented horses. Because the randomized portion of the study included only 10 asthmatic horses, larger controlled trials are still needed.
• Joint health: In a randomized, blinded trial involving 45 horses with lameness localized to the lower hock joints, researchers evaluated resveratrol supplementation alongside corticosteroid joint injections. Horses receiving the supplement had improved owner-reported performance and one objective movement measure, but researchers found no differences between groups in subjective lameness scores. These mixed findings suggest resveratrol might offer supportive benefits in some circumstances, but they do not establish it as a stand-alone treatment for lameness associated with joint pain.
• Broodmare health: Researchers on a 2026 study involving six mares susceptible to persistent breeding-induced endometritis found that resveratrol modified uterine inflammatory responses. The researchers noted improvements in embryo recovery and 14-day pregnancy rates.
• Metabolic health: Researchers on a preliminary study involving 15 horses with equine metabolic syndrome found that six weeks of resveratrol supplementation reduced insulin response to an oral sugar test, along with serum leptin, triglycerides, and the inflammatory marker TNF-α. Researchers did not observe significant changes in body weight, body condition, neck crest score, or body fat. However, the small sample size and short study duration do not establish resveratrol as an effective treatment for insulin dysregulation or demonstrate that it reduces laminitis risk.

Polyphenol Supplementation for Horses

Currently, no established nutritional requirement for polyphenols in horses exists, and evidence does not support routine supplementation for healthy animals. Prioritize adequate forage and a balanced diet that meets the horse’s essential nutrient requirements before considering additional bioactive compounds, says Vineyard. “I would not use a polyphenol supplement to compensate for inadequate vitamin E, an unbalanced ration, or a management problem.”

Senior horses, performance horses, horses with limited access to fresh pasture, and those with metabolic or inflammatory conditions might be candidates for targeted supplementation, she says, but owners should work with their veterinarians or equine nutritionists to determine whether a particular compound is appropriate for their horse and intended purpose. As researchers learn more about specific polyphenols, their doses, and which horses might benefit, supplementation could become an option in some cases.

“More is not always better, and natural does not automatically mean safe,” says Vineyard. “Tannins are one example of why we should avoid treating all polyphenols as universally beneficial. Tannins can bind dietary protein and minerals like iron, which can reduce digestibility, and they can make feeds bitter and less palatable.”

Choosing a Polyphenol Supplement

Because polyphenols represent such a large group of compounds, findings from one ingredient or formulation do not automatically apply to another. Researchers have not established broad feeding recommendations for horses.

When evaluating a product, Vineyard recommends looking for manufacturers that identify the specific polyphenol and amount provided per daily serving, offer research relevant to the intended use, and maintain transparent quality-control practices. The National Animal Supplement Council (NASC) Quality Seal can help owners identify companies that have passed its quality audit.

Take-Home Message

Polyphenols naturally exist in forage and might help regulate inflammation in horses and support the body’s antioxidant defenses. Although early equine research shows promise, evidence does not yet support routine supplementation. Prioritize balanced nutrition and consult your veterinarian or an equine nutritionist before adding a supplement.