Certain biological pathways in the brain cannot utilize ketones and can only use glucose because ketone molecules are too small.
"there are certain pathways in the brain that can't use ketones, they can only use glucose because ketones are too small." (said at 0:41:20)
While certain biological pathways in the brain (such as the pentose phosphate pathway) require glucose rather than ketone bodies to provide specific metabolites like NADPH and ribose-5-phosphate, this functional requirement is not because ketone bodies are "too small." Ketone bodies (beta-hydroxybutyrate, 104 Da; acetoacetate, 102 Da) are smaller molecules than glucose (180 Da), but substrate specificity in metabolic pathways is governed by enzymatic binding site structure and chemical properties, not molecular size. PMID 14769485 notes that glucose is required for specific pathways like the pentose phosphate pathway that cannot be fulfilled by ketones, but this is due to chemical structure and pathway stoichiometry rather than molecular size.
- contradicts: Brain uptake and metabolism of ketone bodies in animal models. (Prostaglandins, leukotrienes, and essential fatty acids 2004)
"Thus, the use of ketone bodies by the immature rodent brain serves to spare glucose for metabolic pathways that cannot be fulfilled by ketones such as the pentose phosphate pathway mainly. The latter pathway leads to the biosynthesis of ribose mandatory for DNA synthesis and NADPH which is not formed during ketone body metabolism and is a key cofactor in lipid biosynthesis." (abstract, results, passage verified)
pubmedfull study (doi)