Smile While You Sweat

NEW ORLEANS--Like to exercise? That may be key to how much good your workout does you. A neuroscientist reported today at the annual meeting of the Society for Neuroscience here that rats forced to exercise suffered weakened immune systems, while those that exercised the same amount without coercion showed healthier immune responses.

Exercise clearly has direct health effects, such as building muscle mass and improving cardiovascular health, but research has also shown that it can have indirect benefits, such as enhancing the immune system and reducing stress-related illnesses. On the other hand, many animal studies on exercise have come to the opposite conclusion, suggesting that exercise suppresses the immune system. Monika Fleshner of the University of Colorado, Boulder, suspected that the blame might not lie with the workout itself, but the enforcement of the regimen.

Fleshner studied two groups of male rats. Those in one group were forced to run on a treadmill once a day. The others had exercise wheels in their cages and were allowed to run whenever they wanted to. At the beginning of the 8-week study, Fleshner challenged the rats' immune systems with a foreign protein, and toward the end of the study they received a booster of the same protein. Fleshner sampled the antibodies the rats made in response to the protein, and also checked the animal's spleens and lymph nodes for reactive immune cells.

The two groups exercised comparable amounts of time over the course of the study, Fleshner said, and they lost comparable amounts of weight. But the animals that were forced to exercise "showed classic signs of chronic stress," including the suppressed antibody responses, as well as enlarged adrenal glands. The rats that were forced to exercise had immune responses 20% to 30% weaker than those of control animals that didn't exercise at all, while the immune responses of the animals that exercised by choice were enhanced. The forced exercise is probably causing the animals stress because they feel out of control, says Fleshner.

The study could have important clinical implications, says exercise researcher Judy Cameron of the University of Pittsburgh, because it points out that "the perception of what exercise is like may have health effects." Indeed, says Fleshner, military cadets, who are forced into exercise for training as well as punishment, have been shown to be immunosuppressed, although it hasn't been linked to the exercise. The study may spur the military to reconsider how it uses exercise, says Fleshner. Likewise, cardiac physicians might want to consider what their patients enjoy when designing exercise regimens.