Objective: Abnormalities of the biological stress response (hypothalamic-pituitary-adrenal axis and the autonomic nervous system) have been identified in both fibromyalgia (FM) and chronic fatigue syndrome (CFS). Although these changes have been considered to be partly responsible for symptom expression, we examine an alternative hypothesis that these HPA and autonomic changes can be found in subsets of healthy individuals in the general population who may be at risk of developing these conditions. Exposure to "stressors" (e.g., infections, trauma, etc.) may lead to symptom expression (pain, fatigue, and other somatic symptoms) in part by precipitating lifestyle changes. In particular, we focus on the effect of deprivation of routine aerobic exercise on the development of somatic symptoms.
Methods: Eighteen regularly exercising (>/=4 h/week) asymptomatic, healthy adults refrained from physical activity for 1 week. We predicted that a subset of these individuals would develop symptoms of FM/CFS with exercise deprivation, and this manuscript focuses on the baseline HPA axis, immune, and autonomic function measures that may predict the development of symptoms.
Results: Eight of the subjects reported a 10% increase in one or more symptoms (pain, fatigue, mood) after 1 week of exercise deprivation. These symptomatic subjects had lower HPA axis (baseline cortisol prior to VO2max testing), immune (NK cell responsiveness to venipuncture), and autonomic function (measured by heart rate variability) at baseline (prior to cessation of exercise) when compared to the subjects who did not develop symptoms.
Conclusions: A subset of subjects developed symptoms of pain, fatigue, or mood changes after exercise deprivation. This cohort was different from the individuals who did not develop symptoms in baseline measures of HPA axis, immune, and autonomic function. We speculate that a subset of healthy individuals who have hypoactive function of the biological stress response systems unknowingly exercise regularly to augment the function of these systems and thus suppress symptoms. These individuals may be at risk for developing chronic multisymptom illnesses (CMIs) (e.g., FM or CFS among others) when a "stressor" leads to lifestyle changes that disrupt regular exercise.