Under physiological conditions, blood ketones below 10 mM are a safe nutrient

Dr. Lee "Yes. Research shows that around the 10th day of fasting, blood beta-hydroxybutyrate reaches about 5 mM. By day 20 it climbs to about 5.5 mM, and by day 30 it's close to 6 mM. Levels below 10 mM are considered a healthy physiological ketosis. When I myself fasted for about 10 days, I crossed 5 mM after only 3 days. Lean people seem to rise even faster. Generally, when the brain weighs more relative to body mass, ketones rise faster — which is why women rise faster than men, and children faster than adults."

Zoe "Doctor — animals that hibernate for months — do they survive on stored ketones?"

Dr. Lee "Yes. The state in which an animal lowers its body temperature, metabolism, and activity to extreme levels in order to survive without food is called torpor. In hibernating ground squirrels in torpor, blood glucose is low and ketones are elevated. The paper 'An adaptive mechanism regulates the preferential use of ketones in heart and brain of hibernating mammals during arousal' (Andrews et al., 2009) reports findings of this kind. The ground squirrel, which hibernates for five months, enters torpor in September; from that point blood beta-hydroxybutyrate rises above 1.0 mM, and by December it reaches about 2.2 mM. Meanwhile, blood glucose, which exceeded 8 mM during the active phase, falls to under 7 mM in September torpor, and to roughly 3 mM in March. We see that the animal uses glucose as the main fuel during activity, then — when food is unavailable and activity ceases — uses stored fat, which the liver converts to ketones for use. And this is not unique to ground squirrels: polar bears and bats show similar patterns. Humans don't hibernate, but during fasting our blood ketones rise in the same way and become the brain's energy source — and autophagy gets to work cleaning the cells."

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