In an experiment, laboratory mice consumed low-fat diets that were identical except that they contained either soluble or insoluble fibre. After six weeks on the diet, the animals had distinctly different responses when the scientists induced illness by introducing a substance (lipopolysaccharide) that causes the body to mimic a bacterial infection.

"Two hours after lipopolysaccharide injection, the mice fed soluble fibre were only half as sick as the other group, and they recovered 50 per cent sooner. And the differences between the groups continued to be pronounced all the way out to 24 hours," said Christina Sherry, who worked on the study.

The new University of Illinois study touts the benefits of soluble fibre. It reduces the inflammation associated with obesity-related diseases and strengthens the immune system.

"Soluble fibre changes the personality of immune cells - they go from being pro-inflammatory, angry cells to anti-inflammatory, healing cells that help us recover faster from infection," said Gregory Freund, a professor in the University of Illinois' College of Medicine. This happens because soluble fibre causes increased production of an anti-inflammatory protein called interleukin-4, he said.

Now Freund has a new question: Could soluble fibre offset some of the negative effects of a high-fat diet, essentially immunizing obese persons against the harmful effects of fat?

Scientists have long known that obesity is linked to inflammatory conditions, such as diabetes and heart disease.

Yet, in a recent study, the University of Illinois scientists demonstrated that fat tissue produces hormones that appear to compensate for this inflammation. "There are significant anti-inflammatory components in fat tissue and, if they were strategically unleashed, they could potentially protect obese people from further inflammatory insults, such as a heart attack or stroke. In obese animals, you can see the body compensating in an effort to protect itself," he said.

Not all fat is bad, the researcher noted. The Mediterranean diet, which receives high marks for its health benefits, includes such foods as olive oil; salmon, tuna, sardines, and trout, which contain important omega-3 and -6 fatty acids; and plant sources of fat, such as flaxseed.

"Now we'd like to find a way to keep some of the anti-inflammatory, positive effects that develop over time with a high-fat diet while reducing that diet;s negative effects, such as high blood glucose and high triglycerides. It's possible that supplementing a high-fat diet with soluble fibre could do that, even delaying the onset of diabetes," he said.

This study is one of the first to provide two valuable lessons, said Sherry. The first, already noted, is that soluble fibre has direct anti-inflammatory effects and builds up the immune system. The second is that the amount of soluble fibre necessary to achieve these health benefits is a reasonable, not a pharmacological, amount, according to a University of Illinois press release.