Period 3. Principles of the Plant-Based Ketogenic Diet

Dr. Lee "All right, shall we begin the third period? Last time I explained that the main energy reserve in the body isn't carbohydrate but lipid. Lipids have 5 to 6 times the energy-storage efficiency of carbohydrates. That's why carbohydrate is stored as glycogen — about 100 g in the liver and 200–300 g in the muscles. By contrast, about 13.5 kg of lipid is stored. These figures are based on a man who is 175 cm tall and weighs 70 kg (Figure 1-2)."

"About 4 hours after we eat, the carbohydrate supplied by that meal begins to be depleted. The body's stored glycogen is then broken down and released into the bloodstream as glucose. If the fasting state continues for about 18 hours, protein and lipid in the body are converted into carbohydrate form to supply the energy that's needed. At the same time, fat in the subcutaneous adipose cells is broken down into fatty acids and glycerol, which are then used as nutrients."

Stored energy in a 70-kg man:

Figure 1-2. Comparison of energy reserves in the human body.

Source: Cahill, G. F. (1970). Starvation in man. New England Journal of Medicine, 282(12), 668–675.

Emily "So if we fast for about a day, we use glucose and fatty acids as energy at the same time?"

Dr. Lee "Yes. Postprandially, mostly glucose — a carbohydrate — is used. In the fasting state, both glucose and fatty acids are used. As the fast lengthens, glucose use gradually declines and fatty-acid use rises."

Zoe "But Doctor, I thought the brain can only use glucose."