The Complete Guide To Fasting

by Jason Fung

Of the three major macronutrients, there are no essential carbohydrates that the body needs to function, so it is impossible to become carbohydrate deficient. However, there are certain proteins and fats that we have to get in our diet. These are called the essential amino acids (the building blocks of proteins) and essential fatty acids. These cannot be manufactured by the body and must be obtained in the diet. The body normally loses both essential amino acids and essential fatty acids in urine and stool. During fasting, it reduces these losses to hang onto much of the necessary nutrients. Bowel movements usually decrease during fasting—since no food is going into the stomach, there is less stool formation—and this helps to prevent loss of protein in the stool. Essential nutrients, particularly nitrogen, can be lost through the urine. Nitrogen in the urine is a sign of protein metabolism, and as protein metabolism decreases during fasting, nitrogen in the urine decreases significantly, to almost negligible levels. To further preserve proteins, the body breaks old proteins down into their component amino acids and recycles these into new proteins. By keeping essential nutrients in the body instead of excreting them, the body is able to recycle many of them during fasting. Of course, no matter how good the body is at compensating, fasting means we’re not consuming essential fatty acids and amino acids. Before and after fasting, it can be helpful to follow a low-carbohydrat...