Antioxidants do more than neutralize free radicals—they also help combat inflammation. Horse owners might turn to antioxidants as nutritional supplements to manage oxidative stress. In emerging research, scientists have shown these compounds also help reduce inflammation and support a process called mitophagy—the targeted removal of damaged mitochondria in the cells.

What Are Free Radicals?

Free radicals are highly reactive, unstable molecules that can damage DNA, lipids, proteins, and other key cellular structures. While not all free radicals harm the body—some aid the immune system and improve adaptation to exercise—excessive levels can overwhelm the body’s antioxidant defenses. When this balance tips, oxidative stress occurs.

A prime example of free radical production in horses comes from skeletal muscle mitochondria (the cells’ power plants) during intense exercise. Prolonged aerobic activity, such as galloping cross country, floods the body with free radicals, disrupting antioxidant systems and contributing to muscle dysfunction, fatigue, slow recovery, and reduced performance, reported researchers on a recent study (L. Stucchi et al., 2025).

“In addition, intense exercise may also activate inflammatory pathways,” says Christoph Marycz, PhD, visiting professor at the University of California, Davis, and general director of the International Institute of Translational Medicine, in Wrocław, Poland.

Mitochondria, a major source of free radicals, can trigger inflammation when overworked or damaged. This damage leads to the release of inflammatory mediators and molecules called cytokines.

“Proinflammatory cytokines such as interleukin-1 and -6 and tumor necrosis factor-alpha stimulate free radical production by activating various cellular pathways,” explains Asim Duttaroy, PhD, professor of nutrition at the University of Oslo, in Norway.

“It’s a vicious cycle. Free radicals fuel inflammation, and inflammation increases the production of free radicals,” says Marycz. “This cycle can damage muscle tissue, cause fatigue, and often lead to microinjuries or even serious injuries in sport horses.”

Duttaroy and colleagues have written that reactive oxygen species (ROS, a subset of reactive molecules that contain oxygen and can cause oxidative damage—not all ROS are free radicals, but all can contribute to oxidative stress) amplify inflammation by triggering signaling pathways that stimulate cytokine production (Bohl et al., 2024).

“Oxidative stress and inflammation are closely linked processes that can detrimentally affect the health and performance of horses, which experience oxidative stress if not managed properly,” says Marycz. “Even horses that aren’t in regular training can generate excessive oxidative stress if their workload doesn’t match their condition or if they’re pushed beyond their limits. Poorly planned training, regardless of discipline, can lead to a buildup of oxidative stress, trigger inflammation, and ultimately increase the risk of injury.”

Antioxidants as a Defense for Horse Health

Antioxidants neutralize free radicals, protecting key cellular components such as DNA and membranes. The body produces antioxidants—considered endogenous, because they come from within the body—such as superoxide dismutase and glutathione peroxidase. Horses also receive antioxidants—via feed or supplements, so exogenous antioxidants—including vitamins A, C, and E, selenium, zinc, copper, omega-3 fatty acids such as DHA, coenzyme Q10, and alpha-lipoic acid.

In another recent study Marycz’s team found that antioxidants not only control oxidative stress but also reduce inflammation through mitophagy (Giercuszkiewicz-Hecold et al., 2025). Again, “mitophagy is the process of selective degradation of damaged mitochondria that produce excessive free radicals,” he says. “Removing damaged mitochondria through mitophagy doesn’t just reduce oxidative stress, but it also helps reduce inflammation because these damaged mitochondria act like alarm beacons within the cell. They release so-called danger signals (known scientifically as DAMPs—damage-associated molecular patterns), which alert the immune system and trigger inflammation.”

Therefore, mitophagy clears out the dysfunctional mitochondria and prevents the body from releasing inflammatory signals, he says. “As a result, inflammation decreases—not just because free radicals are gone, but because the entire source of cellular distress has been removed.”

Astaxanthin: A Dual-Action Antioxidant for Horses?

Astaxanthin, found in microalgae, shrimp, and salmon, offers both antioxidant and anti-inflammatory effects. Marycz identifies this compound as especially useful for performance horses. “High-level horses face chronic low-grade inflammation and oxidative stress,” he says. “Astaxanthin supports cellular health, recovery, and long-term performance.”

To explore astaxanthin’s benefits Marycz’s team studied 12 healthy Arabian racehorses. Six received 250 milligrams of astaxanthin daily for 10 weeks. Researchers collected blood and isolated white blood cells—specifically peripheral blood mononuclear cells (PBMCs)—at baseline, Week 5, and Week 10.

In the lab, researchers exposed these PBMCs to hydrogen peroxide to induce oxidative stress. Then they added astaxanthin and measured oxidative markers, mitophagy-related gene expression, and cytokine levels. Although the researchers took PBMCs from both astaxanthin-supplemented and control horses, they treated all cells the same in the in vitro experiment (i.e., adding astaxanthin after inducing oxidative stress). “This approach let us isolate the direct cellular effects of astaxanthin, demonstrating its ability to reduce oxidative stress and inflammatory signaling and enhance mitochondrial maintenance—all key components in supporting recovery and performance in sport horses,” says Marycz.

“Importantly, the results showed that adding astaxanthin to PMBCs reduced the hydrogen-peroxide-induced inflammatory response by decreasing the expression levels of all tested proinflammatory cytokines,” he says.

Astaxanthin also increased the expression of antioxidant enzymes such as SOD2 and glutathione peroxidase. In horses that received the supplement, the researchers noticed an increased expression of mitophagy genes such as PINK1 and PARKIN.

“Astaxanthin provides both antioxidative and anti-inflammatory benefits and enhances mitochondrial quality control mechanisms, achieving a comprehensive approach to improving equine performance,” says Marycz.

Take-Home Message

Duttaroy emphasizes that understanding the relationship between cytokines, reactive oxygen species, and antioxidants can help veterinarians and horse owners address inflammation and oxidative stress in horses.

For horses undergoing intense training, nutritional strategies that target both oxidative stress and inflammation—such as antioxidant supplementation—can help maintain cellular balance and support long-term health and performance.