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16. Intermittent Fasting - How Many Carbs a Day to Survive?

Updated: Jun 9, 2023

Glucose - the energy of life. How do we survive without getting any glucose from diet during fasting? Can our brain function normally without any glucose intake? Dr. Sten Ekberg is a holistic doctor and a former Olympic decathlete. His analysis on how many carbohydrates or glucose our body needs to function well blows my mind away. Basically, our body is so optimized that it will generate exactly the amount of calories the whole body needs with the exact amount of glucose to support brain function during fasting. I am so fascinated by the fact how delicate our body is built. Here is the summary of his presentation.

He started with debunking three of the myths we have always heard of. 1. Carbs are our preferred fuel. 2. Carbs are essential for us. 3. Our brain can only use glucose. Here comes his arguments. 1. Our body can use any fuel that is available. Carbs or glucose is just one of them. The reason our body takes care of it first (not necessarily burn it) is because (high) blood sugar is toxic to the brain, it destroys tissues. So It is essential and critical that our body brings it down first. The way to take care of it is to convert it into glycogen or fat. 2. Essential means you cannot live without. Our body cannot manufacture it by itself and it has to take it from outside. If not, the body will die. Two examples are essential amino acids (EAAs) and essential fatty acids (EFAs). But there is no essential carbohydrates. 3. It is true that glucose is necessary to the brain and it is true the brain has limited fuels to choose from because of the blood-brain barrier blocks proteins and long FA to come across it. But this does not mean glucose is the only fuel for the brain. When carb supply is limited and the body is fat adapted, a byproduct of fat metabolism - ketone body, is also used by the brain as fuel. This happens during ketosis and fasting, as much of 75% of energy comes from ketones and the rest 25% comes from glucose. He said many researches say that the brain works better when using the mixture of ketones and glucose than when using glucose alone.

Next, he talks about how many carbs the brain needs. Research shows that the brain used 20% of the total calories* we burn each day. Assume we burn 2000 calories per day, then the brain uses 400 calories. Given that only 25% of the energy comes from glucose and one gram of glucose can produce 4 calories, the brain needs only 25g of glucose each day. However, this 25g of carbs does not have to come from diet on regular basis. The process, "Glyconeogenesis", can generate glucose from amino acids, lactate and glycerol that are already exist in our body.

What is glycerol? It is the backbone of triglycerides. Each triglyceride has three fatty acids connecting to one glycerol. Glycerol (C3H8O3) is a molecule with three carbons . Glucose (C6H12O6) has six carbons. The figures below show the structure of a glucose molecule from different angles. The gray dot represents carbon, the red dot represents oxygen and the white dot represents hydrogen. You can consider a glycerol is half of a glucose molecule with four hydrogen added. The calorie generated by one glycerol is about the same as the calorie generated by half of a glucose molecule.

Why do our bodies store energy as triglycerides? When we eat carbs, carbs are broken down into glucose, and insulin sends the glucose into the cells. The cells now have three choices, using it as energy, storing them as glycogen, or storing them as triglycerides. Our body usually can burn 80-100 calories per hour, and can usually store up to 1500 calories of glycogen. and almost unlimited amount of triglycerides, from 100k to 1million calories, depending on whether you are lean or not. Triglycerides (one glycerol combines with three fatty acids) are 6.3x more efficient than glycogen in energy storage. Below is his analysis. Given 1 mol of triglycerides, it contains 92g of glycerol and 769f of fatty acids, the energy generated is 368 calories from glycerol and 6924 calories from fatty acids, respectively. The total is 7292 calories, in which glycerol contributes 5% and fatty acids contribute 95%. Now let's compare triglycerides and glycogen in equal weight. Triglycerides can store 3840 calories per pound, but glycogen can only store 605 calories per pound, because one glycogen molecule has to bind with two water molecules. So here comes the 6.3x difference.

Now the fascinating part starts. If we burn 2000 calories per day, during fasting we can only burn triglycerides. How many triglycerides do we need? Based on the previous analysis, we need 0.52 pounds, which is 236g. How many carbs or glycerols are there? 25.3g. Back to the beginning, how many carbs do our brain need to function if we need 2000 calorie per day? 25g. It is just right. How amazing our human body is designed and built. In conclusion, you don't need to worry about not having enough energy or glucose for your brain during intermittent fasting. All is well.

Note: * one calorie in nutrition is one kilocalorie in energy. They are usually used interchangeably.


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