In a calorie deficit, blood ketones may be low while breath ketones are high because tissues are rapidly utilizing ketones.

“I do notice that if I'm in a calorie deficit, my blood uh ketones are can be very low, but my breath ketones are high. And I think your ketone production that you're measuring is a consequence of ketone production and ketone utilization. And in the context of uh an energy deficit, your tissues are sucking up ketones out of your blood fast, but you're blowing off more uh acetone.”