“Straining really hard during a bowel movement can activate the vagus [nerve],” says Ellen M. Stein, MD, associate professor of medicine in the division of gastroenterology and hepatology at Rutgers Medical School in New Brunswick, New Jersey.
The vagus nerve is one of the main nerves in your parasympathetic nervous system, which controls functions like digestion, heart rate, and immune system function. For some people, vagus nerve activation can slow heart rate and lower blood pressure, leading to something called vasovagal syncope, which can cause sudden lightheadedness and fatigue, says Dr. Stein.
“If you’re straining or pushing hard, your stomach muscles are contracting, and that will decrease blood flow into the vagus nerve,” says Rudolph Bedford, MD, a gastroenterologist at Providence Saint John’s Health Center in Santa Monica, California. “That can make you feel dizzy when you get up after a bowel movement, or tired. Some people will even faint or lose consciousness if they’re straining enough.”
Holding your breath when you push to poop can also cause you to feel weak afterward, says Stein.
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