Ketones Are More Nuanced Than I Thought

Yesterday I wrote about the three types of ketones and how they appear in a kind of natural sequence — urine first, then blood, then breath. I described it as a progression from “waste” to “transport” to “combustion,” and it felt like a clean story.

Today I learned that while the general direction is right, parts of that explanation weren’t accurate.

The biggest correction is this: ketones don’t stay out of the blood. Both acetoacetate (AcAc) and beta-hydroxybutyrate (BHB) are released into the bloodstream from the liver. They’re water-soluble, so they travel easily.

When urine strips turn positive, it’s not because the ketones aren’t in the blood — it’s because the liver is producing more acetoacetate than the body is currently using, so some of it spills into the urine.

I also learned that breath acetone isn’t direct proof that ketones are being “burned” for energy. Acetone is simply a byproduct that forms when acetoacetate spontaneously breaks down. It increases as ketone production rises, but it doesn’t necessarily mean your cells are using the ketones more efficiently at that exact moment.

The More Accurate Explanation

Acetoacetate (AcAc) is the first ketone produced in the liver. The lower your insulin level, the more of AcAc is converted into BHB (the stable transport form).

• Urine strips turn positive when there is enough Acetoacetate in the blood that some of it spills into the urine. This usually happens first because AcAc is produced early.
• Blood meters show rising levels mainly of BHB, which becomes the dominant ketone as ketosis deepens.
• Breath analyzers only rise when enough Acetoacetate is breaking down into acetone. This tends to happen later, when total ketone production is higher.

This is why many people see urine strips turn positive first, followed by rising blood BHB levels, and only later do they get a clear reading on a breath analyzer. It’s not really a clean three-phase “evolution” of understanding in the body — it’s more about the different ways we measure the same underlying process at different stages of production and breakdown.