Ketones are acidic molecules the liver makes from fat when cells can’t get enough Glucose for fuel. For example when Insulin is absent or very low.
They are one of the Fuels in blood plasma - a normal backup fuel in small amounts (e.g. fasting). Unlike Glucose, you don’t eat them; the liver makes them, so they’re a fuel but not a nutrient.
The problem is quantity: when ketones build up too high they make the blood acidic, causing ketoacidosis, which is dangerous. This is a key risk in Diabetes type 1.
The main ketone measured in blood is beta-hydroxybutyrate.