The benefits of an exercise regimen in building resistance to disease may depend on attitudes toward the exercise, not just on whether people put themselves through the paces.
While regular, moderate exercise has been shown effective in reducing the incidence and severity of many illnesses including heart disease, diabetes, bacterial infections and the common cold, a new study by University of Colorado faculty member Monika Fleshner suggests that forced exercise can be stressful and can actually suppress immunity to disease. But if individuals are able to exert control over the type, intensity and duration of their physical activity, exercise can strengthen the immune system.
Fleshner, an assistant professor in the department of kinesiology on the CU-Boulder campus, is presenting her research findings today at the annual meeting of the Society for Neuroscience in New Orleans.
The study, which used laboratory rats to measure stress and immune responses to an exercise regimen, highlights the importance of a previously unexplored factor in the research on physical exercise -- that of an individual's control over the level of his or her activity.
"The real hope of this work is to understand what aspect of voluntary physical activity is promoting or optimizing immune system function,'' Fleshner said.
The study confirmed previous research showing that forced treadmill training, in which rats or mice are required to run at the speed and for the duration determined by a researcher, can trigger physiological responses indicative of stress, including immune system suppression. It went on to show that voluntary freewheel exercise enhanced the animals' immune system functioning without their experiencing the stress response.
For the study, two groups of adult male rats (10 to 12 per group) were assigned to either forced treadmill exercise or voluntary freewheel exercise. The forced treadmill exercise consisted of the standard training regimen, five days per week for eight weeks. The voluntary freewheel exercise rats lived in cages with an attached running wheel and voluntarily chose to run. Two other groups of rats were kept sedentary to control for handling of the animals and the environmental effects of living in one type of cage as opposed to the other.
The animals were injected with a foreign protein, keyhole limpet hemocyanin (KLH), at the start of the exercise training so that the status of their immune systems could be assessed. The KLH injection stimulates an immune response similar to that produced by a vaccination, Fleshner said.
The level of the immune response then was determined through a variety of measures. Antibodies in the blood were measured in samples taken once a week for five weeks after the KLH injection. Spleen lymphocytes and lymph node cells also were sampled to determine the cellular immune response. In addition, the adrenal and thymus glands were weighed after eight weeks of exercise as indicators of stress.
While voluntary and forced exercise were found to be equally effective in producing weight loss in the animals, their immune systems were affected differently according to the type of exercise. Forced exercise resulted in a suppression of the antibody and cellular proliferation responses, whereas voluntary exercise resulted in an increase in those responses.
The measures of stress also were affected differently by the type of exercise. Forced exercise resulted in increased adrenal weight and decreased thymus weight, which are indicative of chronic stress, while voluntary exercise produced no changes in the weight of either gland.
Further research is needed to confirm the dimension of psychological control in the health benefits of exercise and to determine if it affects humans in the same way as laboratory animals, Fleshner said.
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