Yes.

During prolonged fasts – over 24 hours – the body briefly breaks down muscle for ketones, then quickly crosses over to fat-based ketone production.

The transition to fat-based ketosis happens more quickly as you become adapted to fasting.

Even during shorter fasts or intermittent fasts, your body is being conditioned to use fat for fuel (ketones) when you don’t have access to carbs and other sources of energy – especially when you exercise.