The human brain's preferential fuel is ketones, not glucose. The lower limit of carbohydrates in the human diet is zero. There is no such thing as an essential carbohydrate. Why chubby babies
Anthony Chaffee, MD: "This is something people talk about, that glucose is the brain's primary energy source. What do you say about that?"
Benjamin Bikman, PhD: Yeah. […] I just […] presented at […] the meeting of the American Association of Biological Anthropologists. […] I felt compelled to inform the person reaching out to me, […] 'Look, I'm a nutrient biochemist, mitochondrial physiologist. What do I have to say?' And he had said, 'I'm familiar with your work on brain energy use. I am putting together a session about the changes in […] our ancestor diets over these periods of evolution, and I want you to talk about the brain acting as a hybrid.' […]
"I […] shared with them a quote by the National Academy of Sciences in the US stating that the lower limit of carbohydrates in the human diet is zero. In other words, there is no such thing as an essential carbohydrate, and the idea that the human brain evolved because our ancestors ate a lot of carbs, that's utterly ridiculous. […] It's because they mistake dietary carbohydrates with blood glucose. What does appear to be the case is that the brain has some demand for some glucose. That appears to be accurate, although the lower limit is unknown. Early work by a fasting physiologist named George Cahill, he was putting people's glucose down to like 20 mg/dL, which most people would say you're unconscious, you're in a coma, and you're going to die. And these people, because they've been long-term fast adapted, which I would say ketone adapted, there appeared to be no deficit to cognition, and that's a pretty bloody low level of glucose.
"But nevertheless, let's kind of grant that side of it, that the brain has some requirement for some glucose. Well it is a minimal requirement, because if you take a body that has five millimolar (mM) glucose, then you start increasing the ketones to one or two or even 3 mM, which is still less than the 5 mM of glucose, so there's still less of the ketone in the blood than there is the glucose, by then the brain has already dramatically shifted its energy use. And even though the ketone may be less than half of what the glucose is in the blood, it's now providing twice as much of energy to the brain as the glucose is. So if the brain has any preferential fuel, it is absolutely for the ketone. […]
"You can take a newborn baby, and the baby can breastfeed or bottle feed, and then within an hour the baby is in a deeper state of ketosis than an adult would be after fasting for a full day. That baby will be at 2 mM ketones in an hour. And an adult, for me, if I want to get to 2 mM, I gotta fast for like 36 hours to get to that point. So if there's any natural state, […] it is clearly that a natural state is a state of ketosis. […]
"We are such totally unique creatures, where we are the only land-based mammals born obese, and the only animal who has a brain that is larger than the birth canal, much to a mother's chagrin. But that means we have these very big hungry brains, and all of this chubby, adorable baby fat that is just producing ketones like gangbusters to fuel the brain growth. And if you have a baby that is born premature and lacks sufficient adipose tissue, it is much more likely that they're going to develop neurological disorders, all the more reason to chubby up that baby as quickly as you can."
Benjamin Bikman, PhD with Anthony Chaffee, MD @ 35:22–40:57 (posted 2022-04-13)