Exogenous Ketones

Exogenous ketones break a fast. Whether taken as a ketone ester, ketone salt, or ready-to-drink ketone product, they contain calories, so by a strict definition they end the fasted state.

Exogenous ketones are the most counterintuitive item in this category, because they raise the number on a ketone meter while doing little to create a true fasting state. The beta-hydroxybutyrate in these products is the same molecule your body makes, so it will not pull you out of ketosis, and a reading taken afterward looks higher, sometimes reaching the 2 to 3 millimole range a ketogenic diet produces. But a high ketone reading from a supplement is not the metabolic state a fast is meant to produce. Ketones taken from outside actually lower the free fatty acids and glucose circulating in your blood, which means they suppress the fat breakdown a fast is working to drive rather than accelerate it, and the calories still break the fast. So exogenous ketones can raise your ketone level and work against your own fat-burning at the same time.

For fasting, that makes them a poor fit on a clean or autophagy fast: the calories break it, and a borrowed ketone reading does not reflect the cellular state you are after. People mainly use exogenous ketones to feel the effects of ketones quickly, for energy or appetite control, or to ease the transition onto a ketogenic diet, which are eating-window or fat-focused uses rather than ways to extend a true fast. To keep a fast intact, the dependable options remain water, black coffee, plain tea, electrolytes, and certain non-caloric supplements.

Source: On the Metabolism of Exogenous Ketones in Humans (Frontiers in Physiology, 2017)