Ketones are molecules produced by the liver when fat is broken down—your body’s backup fuel system. Production begins when glycogen depletes (typically 12-24 hours into a fast) and ramps up significantly by 24-48 hours. Research by Cahill documented how ketones can fuel most tissues including, crucially, the brain—providing up to 75% of its energy needs during extended fasting.
This companion covers what ketones are, the hour-by-hour timeline of ketone production, what they can and cannot fuel, and why mental clarity often improves once ketosis is established. (4 min read)