Ketones are produced normally by the liver as part of fatty acid metabolism. In normal states these ketones will be completely metabolised so that very few, if any at all, will appear in the urine. If for any reason the body cannot get enough glucose for energy it will switch to using body fats, resulting in an increase in ketone production making them detectable in the blood and urine.
This is a companion discussion topic for the original article at https://patient.info/doctor/urine-ketones-meanings-and-false-positives-pro