Under what circumstance might ketones be present in urine?

Ketones may be present in urine primarily as a result of fasting or uncontrolled diabetes. When the body does not have sufficient glucose for energy—due to fasting, prolonged exercise, or insufficient insulin in diabetes—it begins to break down fatty acids for fuel. This process produces ketones as a byproduct. The accumulation of ketones in the blood can lead to a condition known as ketonuria, where these substances are then excreted in urine. High levels of ketones can indicate inadequate carbohydrate availability, often associated with metabolic states like diabetic ketoacidosis.

The other circumstances mentioned do not generally lead to ketonuria. Drinking water typically dilutes urine without contributing to the production of ketones. High carbohydrate intake usually prevents the body from entering a state of ketogenesis, as glucose becomes the primary energy source. Caffeine consumption may influence metabolism, but it does not directly cause ketone production and therefore is not a relevant factor for ketonuria.