The autonomic nervous system is comprised of two main parts: The sympathetic system is responsible for the short-term energized activity known as the fight-or-flight response, while the parasympathetic system conserves energy in a resting phase.
Fagundes and his colleagues found that breast cancer survivors experiencing chronic exhaustion -- which occurs in one-third of patients -- had an imbalance between the two systems, with higher activity in the sympathetic system, which prior research suggests is a signal for systemic inflammation.
This finding was pivotal, since the researchers were searching for reliable biomarkers for cancer-related fatigue. Earlier research indicated that body-wide systemic inflammation may be such a signal.
The researchers also observed that fatigued participants experienced lower heart rate variability (HRV), which has been linked to high blood pressure, heart attacks, strokes and diabetes.
While inflammation has been linked to fatigue in other forms of cancer, Fagundes' study only gathered data on breast cancer patients.
The study authors pointed out that more research is needed. "We cannot say with certainty that lower HRV or higher norepinephrine leads to great fatigue, or vice versa, a limitation of the study. It is possible that fatigued cancer survivors have [these levels] due to inactivity and deconditioning," they wrote in the report.
"Fatigue is something we've known about for quite some time but we really don't know that much about it, so I think this adds to the literature," said Lee Jones, scientific director of Duke Cancer Institute's Center for Cancer Survivorship in Durham, N.C.
"We don't see this [fatigue] as much with other cancer survivors," Jones added, noting that some breast cancer drugs may have a negative impact on long-term energy levels because they can be toxic to the heart.
Fagundes said the ongoing exhaustion in some women may be a sign of ac
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