Why do we hiccup?

A hiccup is a reflex. The diaphragm, the main breathing muscle at the base of the lungs, contracts suddenly and involuntarily, and the vocal cords snap shut, which makes the "hic" sound. Common triggers include eating too fast or too much, fizzy drinks, alcohol, excitement and stress. Why the reflex exists at all is not settled. [1] [2]

National Geographic describes a reflex arc that carries nerve signals from the diaphragm to the brain and back again. Something triggers the diaphragm to contract and move downward, as in a normal in-breath. Partway through that breath, the reflex tells the epiglottis, the flap at the top of the throat, to close. That closure produces the sound, and the cycle repeats until something interrupts the arc. [3]

The initial trigger often involves the phrenic or vagus nerves, which supply the diaphragm. Cleveland Clinic describes irritation of these nerve pathways as what makes the diaphragm contract involuntarily and draw in air. [3] [4]

Hiccups can start and stop for no obvious reason, but they often follow something that irritates the diaphragm. Mayo Clinic lists the usual triggers for hiccups lasting under 48 hours: carbonated drinks, too much alcohol, overeating, excitement or emotional stress, sudden temperature changes, and swallowing air, for example while chewing gum or smoking. [1] [2]

Hiccups that last longer than 48 hours are usually linked to something else. Mayo Clinic names nerve damage or irritation, central nervous system disorders such as stroke or meningitis, metabolic problems such as diabetes or kidney disease, and certain drugs, including the steroid dexamethasone. The NICE clinical knowledge summary gives a longer list, including some surgeries and conditions affecting the chest and abdomen. [2] [5]

Scientific American says scientists have tried to pin down a clear reason for hiccups but still do not know it. Two ideas appear in the sources, and neither is established. [6]

The first is an evolutionary leftover. National Geographic reports the view that the reflex traces back to fish and amphibian ancestors, which switched between gill and lung breathing. Closing the ancestral epiglottis could have sent water to the gills without filling the lungs. The phrenic nerve also began as a short path to the gills, and it is now a long one running to the diaphragm, which leaves more nerve to be irritated. [3]

The second concerns infants. Emergency doctor Dan Howes suggests that babies, who hiccup more than adults, swallow air while suckling, and hiccups may help clear it from the stomach like a self-initiated burp. He notes some evidence that babies can take in 20 to 30 percent more milk after burping, which could be a survival advantage. These are proposals from the sources, not consensus findings. [3]