Ketones are a more efficient energy source than glucose

Grace "Doctor — you said earlier that during fasting, ketones supply about 70% of the brain's energy. So as lipid metabolism shifts into high gear, some of the lipid moves to the liver and is converted to ketones, and the brain begins to receive ketones — is that the picture?"

Dr. Lee "Yes. Ketones dissolve in blood and travel readily, and they cross the brain's lipid membrane easily. Glucose, by contrast, is admitted into the brain through glucose transporters only when needed. The transport efficiency of glucose and ketones is often described in 'pull-push' terms (Cunnane et al., 2016). Glucose has to be 'pulled' into the brain from inside; ketones are 'pushed' from outside the brain into it. Ketones cross into the brain far more readily. When blood ketones rise, the amount entering the brain rises in step."

Grace "If more energy substrate reaches the brain, brain activity should improve too. What do you think?"

Dr. Lee "A very important question. Today's adults suffer chronically from many metabolism-related diseases. Most of these arise from an excess of nutrients — and paradoxically, that excess clogs the vessels and blocks nutrient delivery to the brain. Imagine too many supply trucks on the road: they jam the lanes and gridlock follows. The 'cars' tangling and blocking the road are like blood clots blocking vessels — atherosclerosis is the prime example. The brain in particular is only 2% of body weight but consumes about 20% of the body's nutrients and oxygen — a high-energy organ. Research shows that brain nutrient utilization drops sharply with old age, easily leading to cognitive decline, depression, and mood swings."