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" Myth #3: Fasting Causes Low Blood Sugar Sometimes people worry that blood sugar will fall very low during fasting and they will become shaky and sweaty. Luckily, this does not actually happen. Blood sugar level is tightly monitored by the body, and there are multiple mechanisms to keep it in the proper range. During fasting, our body begins by breaking down glycogen (remember, that’s the glucose in short-term storage) in the liver to provide glucose. This happens every night as you sleep to keep blood sugars normal as you fast overnight. FASTING ALL-STARS AMY BERGER People who engage in fasting for religious or spiritual purposes often report feelings of extreme clear-headedness and physical and emotional well-being. Some even feel a sense of euphoria. They usually attribute this to achieving some kind of spiritual enlightenment, but the truth is much more down-to-earth and scientific than that: it’s the ketones! Ketones are a “superfood” for the brain. When the body and brain are fueled primarily by fatty acids and ketones, respectively, the “brain fog,” mood swings, and emotional instability that are caused by wild fluctuations in blood sugar become a thing of the past and clear thinking is the new normal. If you fast for longer than twenty-four to thirty-six hours, glycogen stores become depleted. The liver now can manufacture new glucose in a process called gluconeogenesis, using the glycerol that’s a by-product of the breakdown of fat. This means that we do not need to eat glucose for our blood glucose levels to remain normal. A related myth is that brain cells can only use glucose for energy. This is incorrect. Human brains, unique amongst animals, can also use ketone bodies—particles that are produced when fat is metabolized—as a fuel source. This allows us to function optimally even when food is not readily available. Ketones provide the majority of the energy we need. Consider the consequences if glucose were absolutely necessary for brain function. After twenty-four hours without food, glucose stored in our bodies in the form of glycogen is depleted. At that point, we’d become blubbering idiots as our brains shut down. In the Paleolithic era, our intellect was our only advantage against wild animals with their sharp claws, sharp fangs, and bulging muscles. Without it, humans would have become extinct long ago. When glucose is not available, the body begins to burn fat and produce ketone bodies, which are able to cross the blood-brain barrier to feed the brain cells. Up to 75 percent of the brain’s energy requirements can be met by ketones. Of course, that means that glucose still provides 25 percent of the brain’s energy requirements. So does this mean that we have to eat for our brains to function? "
― Jason Fung , The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting
