Bad breath when hungry is associated with factors such as H. pylori infection, indigestion, and oral diseases. H. pylori is a bacterium in the stomach that not only causes damage to the gastric mucosa, but is also a common cause of gastric disorders. The peristaltic function of the stomach is relatively slow when the stomach is infected with H. pylori, and the metabolites released by H. pylori can cause serious damage to the stomach and cause a foul smell. Especially when hungry, the smell will be emitted through the trachea, resulting in bad breath. Some people have digestive system indigestion, food emptying is relatively prolonged, and food stays in the stomach for a long time will produce a sour smell. The sour taste will enter the mouth through the esophagus when there is hunger, so it will cause bad breath. Some of the bad breath is caused by oral diseases, people with oral diseases are more likely to produce a foul smell when they are hungry because they secrete relatively more saliva.