CORVALLIS - A new animal study on the biological impact of omega-3 fatty acids has confirmed they can suppress the immune response of healthy dogs.

However, these findings don't suggest that such dietary supplementation is necessarily good or bad, Oregon State University researchers say - either for dogs that increasingly find such nutrients in their dog food or for humans who take "fish oils" for their alleged health benefits.

The actions of these essential nutrients are powerful, complex and might be either positive or negative depending on the situation, scientists say.

"It's increasingly clear that nutrition can play a major role in our response to the environment, especially affecting the immune and nervous systems," said Jean Hall, an OSU assistant professor of veterinary medicine. "At least with pets and possibly with humans, what we may be moving toward is a prescription diet based on an individual's particular health needs."

The latest research findings on omega-3 fatty acids will be published soon in the Journal of Nutrition, in an article co-authored by Hall and Rosemary Wander, an OSU associate professor of nutrition and food management.

The study shows a cell-mediated depression of immune response in otherwise healthy beagles that were given dietary supplements of omega-3 oils, which are often touted as beneficial for health problems ranging from atherosclerosis in humans to skin allergies in dogs.

In fact, immune suppression may well be the reason that this type of dietary oil can sometimes have positive effects, Hall said.

In humans, such diseases as arthritis, psoriasis, asthma, ulcerative colitis and allergies can sometimes be linked to an overactive immune response. A dietary therapy that reduced the immune reaction may help.

In one study of 93 dogs with skin and other allergies, treatment with a diet high in omega-3 fatty acids provided good to excellent results in at least one-third of the animals.

And there's evidence such diets may also alleviate pain associated with hip dysplasia, a fairly common problem in dogs, and inhibit tumor growth.

"One thing people need to understand is that different types of fatty acids are essential nutrients," Hall said. "We need at least some of them for normal health. But the proper amounts and ratio is less clear, as is the mechanism of action of these nutrients on the cell, the immune response and other biological functions. That's what we're trying to determine."

So far, studies on immune suppression point towards an impact on monocytes, a type of white blood cell that recognizes and "eats" foreign bacteria and viruses, helps to destroy these foreign invaders, and tells other cells to help mount a response.

Research so far, Hall said, does not suggest that the amount of immune suppression caused by higher levels of omega-3 fatty acids is sufficient to cause major health problems, at least not in animals of normal health.

"It's a concern and one we need to learn more about," she said, "but supplementation with these oils is not shutting down the immune system. It's just reducing the response. But in normal animals or those with already compromised systems, there could be greater risk of infection."

Other concerns about use of these nutrients, Hall said, are also being studied - including their effect on antioxidant levels in the body, such as vitamin E; greater risk of bleeding; retinal damage; or increases in LDL cholesterol, the "bad" type of cholesterol that can cause atherosclerosis.

Despite any benefits of these dietary oils, Hall said, studies in dogs and other small animals should be continued to monitor long-term effects. Some adverse side effects were reported in one study of dogs. And supplementation of pet diets with vitamin E or other antioxidants seems to be indicated when higher levels of omega-3 oils are being used, Hall said.

These studies have been supported by a $124,000 grant from Hill's Pet Nutrition, Inc., a private pet food products company.

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Jean Hall, 541-737-6537