Upset stomach? Diarrhea? Unexplained abdominal pain? These symptoms may be telling of a larger problem—a Functional Gastrointestinal Disorder (FGID). FGID are estimated to affect 25-40% of people living in the United States. This group of disorders is characterized not by structural or biochemical (i.e. tumors or masses) abnormalities, but rather abnormal functioning of the GI tract. Consequently, diagnosing FGID in the clinic with x-rays, CT scans, blood tests, or endoscopic exams can often produce negative results. Therefore, there is a need to identify better diagnostic methods for individuals suffering from FGID.
FGID are increased in both incidence and severity in women as compared to men, but the mechanisms behind this difference are not well understood by scientists. One hypothesis suggests estrogen, or related hormones, my cause changes in the way the vagus nerve (which connects the gut to the brain) signals to the GI tract. This theory is supported by reported decreases in gastric motility, or contractions of the stomach, during high-estrogen stages of the estrogen cycle in rats. However, identifying methods measuring changes in gastric motility non-invasively is challenging.
In our lab, we are able to measure vagal tone (or vagal action) non-invasively using the high-frequency spectrum of heart rate variability (HF-HRV). Heart rate variability is the fluctuation in the time that exists between each heartbeat. Our lab has demonstrated that HF-HRV can be used as a surrogate marker for vagal tone. We (and others) have shown that a decrease in HF-HRV is representative of decreases in vagal tone and is associated with increased risk of all-cause mortality and adverse pathologies (including sepsis and pulmonary artery disease). The aim of our study was to test the hypothesis that HF-HRV is positively correlated with vagal tone, and that estrogen disrupts this relationship.
We found that gastric motility and HF-HRV are indeed positively correlated in adult male and low- but not high-estrogen female rats at baseline. This positive correlation is maintained in male rats following interventions known to affect GI motility in a vagally dependent manner. These data support the careful use of HF-HRV as a non-invasive measure of vagally mediated changes in gastric motility. Understanding how HF-HRV correlates with gastric motility may aid in diagnosing and identifying the cause of FGID. Our lab is currently investigating the role estrogen plays in the relationship between HF-HRV and gastric motility to better understand the mechanisms behind sex differences in vagally mediated gastric motility.
By Alissa Meister, Neuroscience PhD Candidate