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The Science of Fasting: A Short Yom Kippur Reflection

Yom Kippur, the holiest day on the Jewish calendar, has come and gone. It’s marked by a strict 25 hour fast during which nothing is to be ingested. No food and no water-- not even liquids to rinse your mouth after brushing your teeth. Yom Kippur translates to “Day of Atonement”, and fasting is an integral part of repenting. Abstaining from food is a sort of sign of respect, and contributes to the somber mood of the high holiday.

People fast nowadays for plenty more than just religious observance. It’s trendy to do intermittent fasts or to eat at extreme times in the day. Why do we do it? Does it make sense from a wellness perspective?

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A 24 hour fast has a pretty noteworthy effect on the human body. First, the mechanisms that produce energy in our cells get shaken up. Usually, our liver and muscles store and release glucose, a carbohydrate, into our bloodstream whenever we need it. However, once we fast for long enough, the liver runs out of glucose reserves, and the body transitions into gluconeogenesis-- a sort of failsafe mode where the body uses fat for glucose, and ramps up calories burn. Put simply, at a certain point fasting increases not just the amount of calories burned but the actual rate of the burn. 

This can get dangerous. Fast for too long and even fat won’t be enough to make glucose. Then your body will begin chewing away at your muscle tissue. At this point the body isn’t in fasting mode-- it’s in starvation mode.

Breaking a fast isn’t as simple as it seems. Doing so the right way requires some level of awareness. Drink plenty of water and eat smaller meals. You don’t want to overload the body with a massive influx of calories. Chew your food thoroughly to decrease strain on your digestive system, and cook everything thoroughly. Basically, regardless of how long the fast is, it’s best to play it safe.

So, with careful monitoring and a sound plan, certain types of fasting can promote weight loss. Do it wrong, however, and you can run into a litany of medical complications.

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