Hi Geoff, this is a great quetion.

The short answer is no.

We don’t have to be in ketosis to burn fat. Measurable ketosis is an indicator of intense fat burning as an energy source in the absence of dietary glucose and empty glycogen stores. The body always creates glucose even if we don’t ingest any.

When the body detects low glucose levels (e.g., emptying glycogen stores), it can convert triglycerides into acetyl CoA molecules and use them to generate ATP.

Fat oxidation starts when fatty acids are converted into fatty Acyl-CoA molecules.

If the body creates excessive acetyl CoA from the oxidation of fatty acids and cannot handle it, the acetyl CoA is diverted to generate ketone bodies.

So ketosis shows that fat loss happens in an accelerated mode for the body to provide the required energy to several organs, including the brain.

Simply put, it is possible to burn fat slowly without entering ketosis. However, our bloodstream will always have a tiny amount of ketones for different metabolic reasons.

Your question inspired me to write an article about fat metabolism to inform my readers.