Isoflavones in soybean help protect pigs against viral infections

Pigs that regularly consume soybeans in their diets may be better protected against viral pathogens, according to a new study from the University of Illinois. Researchers attribute this effect to isoflavones, a naturally occurring compound found in soybeans.

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) is a widespread disease that costs U.S. swine producers about $650 million a year. There is evidence that feeding soybeans helps protect pigs from the disease, but it is not clear why or how it works, says Ryan Dilger, co-author of the study and associate professor in the Department of Animal Science, Division of Nutritional Sciences and Neuroscience Program at the University of Ireland.

Dr. Dilger and his collaborators have previously pointed to dietary soy isoflavones as an active ingredient and wanted to investigate this hypothesis further.

"In this study, we are specifically looking at isoflavones and their beneficial effect on the immune response," says Dr. Dilger. "We wanted to understand how we can take a primary protein source from a diet already used for pigs and provide a practical way for producers to combat endemic VRSRP.

Isoflavones are a flavonoid compound that occurs naturally in plants, with a particularly high concentration in soybeans. It has well-known health benefits and is used as a dietary supplement for humans," says Brooke Smith, lead author of the study and a graduate researcher from the University of Ireland's Veterinary Medicine Programme.

"When incorporated into the diet of infected pigs, these isoflavones appear to support them by helping pigs eliminate secondary infections or by preparing them for a more effective immune response so that they eliminate the infection and do not succumb to it," Smith says.

Dilger adds that the research is unique in its focus on nutrition. "We're using something that goes through the digestive tract to try to modify the immune response to a virus that's actually in the lungs. So the virus never comes into direct contact with the isoflavones. They're two different systems," he says.

The study involved 96 pigs, divided into three groups. Two groups were infected with PRRSV; one group received isoflavone supplementation while the other did not. A control group of uninfected pigs received an isoflavone-free diet.

Infected pigs that did not consume isoflavones had an infection-related mortality rate about 50% higher than those that received the supplement. Therefore, isoflavones in the diet could have a significant economic effect on producers, the researchers conclude.

In a second part of the study, the researchers looked more specifically at whether isoflavones could benefit the immune system indirectly by altering the profiles of bacteria in the pig's large intestine.

"We didn't know if there was a direct effect of isoflavones on the immune system or if it was because the isoflavones benefited the resident bacteria, which then had an indirect effect on the host," Smith says.

They were able to rule out the indirect effect of the isoflavones through the bacteria. However, although the researchers were unable to explain the biological mechanisms, it is clear that the isoflavones are beneficial.

Soybeans are generally fed to pigs, and the researchers recommend that producers keep it that way. They say more studies are needed to determine the ideal amount for optimal benefits.

While isoflavones have a wide variety of anti-inflammatory and antioxidant cellular activities, they also have estrogen-like components that can affect breeding females and alter the reproductive cycle. Pig farmers need to balance antiviral effects and estrogenic activity when determining isoflavone levels in gilt diets.

The researchers point out that their findings may also have implications for human health. Their study was inspired by human research on the antiviral properties of isoflavones.

"We introduced the human context in pork, and put it into the scenario of production farming. We've learned something that could benefit pig farmers, but it certainly goes the other way, potentially helping human health," says Dilger.