fasting -> not enough carbohydrate in diet -> burn fat stored in adipose tissue -> beta oxidation in mitochondrial matrix -> produce acetyl COA
normally acetyl COA enters citric acid cycle: acetyl COA + oxaloacetate ------------> citrate
however since not enough oxaloacetate due to oxaloacetate is used in gluconeogenesis (in response to glucagon) to generate glucose, acetyl COA cannot enter citric acid cycle hence generate ketone body (acetoacetete, beta hydroxybutyrate, acetone)
result: ketoacidosis, increased breath rate to expel bicarbonate in an attempt to counter blood acidity, fruit smell breath due to acetone. In extreme case this can be fatal without medical intervene.