There may be some truth in the old saying, "an apple a day keeps the doctor away." Researchers from the University of Illinois have found that soluble fiber can help strengthen the immune system.

In the experiment, laboratory mice consumed low-fat diets that were identical except that they contained either soluble or insoluble fiber. 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 fiber were only half as sick as the other group, and they recovered 50 percent sooner. And the differences between the groups continued to be pronounced all the way out to 24 hours," said researcher Christina Sherry.

The recommended daily dietary intake of total fiber is 28 to 35 grams.

Dietary sources of soluble fiber include oat bran, barley, nuts, seeds, lentils, citrus fruits, apples, strawberries and carrots. Nutritional supplements such as psyllium, methylcellulose and polycarbophil all provide soluble fiber as well.

Researchers say insoluble fiber, found in whole wheat and whole-grain products, wheat bran and green, leafy vegetables, is valuable for providing bulk and helping food move through the digestive system, but it doesn’t provide the boost to the immune system that soluble fiber provides.