The gag reflex, also known as the pharyngeal reflex or laryngeal spasm, is a contraction of the back of the throat triggered by an object touching the roof of your mouth, the back of your tongue, the area around your tonsils, or the back of your throat. The reflex helps prevent choking, as well as helping to moderate the transition from liquid to solid foods during infancy.
By thrusting objects in the throat toward the opening of the mouth, the gag reflex expels substances that the brain has deemed harmful. In the first few months of a baby's life, this reflex is triggered by any food that a region of the brain stem called the "nucleus tractus solitaries" (which is wired to nerve endings in the mouth) judges to be too chunky for a baby's stomach to digest. Starting around the 6- or 7-month mark in babies, the gag reflex diminishes, allowing a baby to swallow chunky or solid foods.
In children and adults, the reflex is usually only triggered by the presence of an unusually large object in the back of the throat. However, 10-15 percent of people have a hypersensitive gag reflex (HGR), which continues to get activated by substances in the mouth. Most often, sufferers of HGR gag while eating sticky foods that tend to get stuck in the mouth, such as bananas and mashed potatoes; in extreme cases, oversensitive gagging can cause picky eating or even malnourishment.
The reasons why some adults gag while eating are not yet fully understood, but research shows that HGR usually happens in people who did not have solid foods introduced into their diets until after the age of 7 months. Experts recommend starting this process sooner, between the fifth and sixth month of an infant's life, in order to let the baby's gag reflex develop properly.