The Truth About Fasting

The Jewish people begin fasting at sundown. What happens to the body when you fast?

yom kippur fastingYom Kippur image via Shutterstock

Tonight at sundown, the Jewish people begin fasting to observe Yom Kippur, the holiest day of the Jewish calender. Cantor Randall Schloss of Temple Ohabei Shalom, a reform synagogue in Brookline, says that fasting is part of the holiday because the day is supposed to be spent reflecting, repenting, and praying all day from sundown to sundown the following day. “We deprive our bodies from normal pleasures like working, eating, TV, sexual activity, and other regular life activities,” Schloss says.

Rabbi Barbara Penzner, of Temple Hillel B’nai Torah in West Roxbury, says that it is a day where we confront our mortality. “We are all souls on this day,” Penzner says. “It is possible to feel lifted up out of your body.” But Penzner also says that if you are frail, ill, or pregnant, you are commanded to eat or do what it takes for your survival. For the very religious that won’t do what’s commanded, they seem to have found a loophole by turning to intravenous drips to get through he day.

So is the feeling of an out-of-body experience due to heavy praying—or a lack of food and water? From a health perspective, what happens to the body when we fast? Is it healthy, or does it do damage?

The answers are mixed, but most agree that one day of fasting will not do any permanent damage. Glucose, which comes from he food we eat, is the body’s primary food source and critical for brain function, according to Dr. Caroline Apovian, professor of medicine and pediatrics at Boston University School of Medicine and director at the Center for Nutrition and Weight Management at Boston Medical Center whose book on fasting comes out next year. “When denied glucose for more than 4 to 8 hours the body triggers the liver for its glycogen stores, which is the stored form of glucose for fuel,” Apovian says. “The body can also use small amounts of protein from muscle as well.”

Apovian, who recommends intermittent liquid fasting one day a week (with the aide of protein shakes and other liquids), says that fasting brings about a change in metabolism due to the lack of glucose calories. “Less insulin will be produced by the pancreas. Insulin promotes water and salt retention,” Apovian says. “Without insulin, you lose water and salt quickly and that is why you can lose weight overnight.”

Our nutrition blogger, Ayla Withee, a registered dietician and the creator of, agrees. “Glycogen stores should last you 18 to 24 hours before the body moves into breaking down its own muscle and fat tissue for energy,” Withee says. “The physical effects of fasting can be headaches, nausea, fatigue, and a lack of mental clarity.”

For those who think that fasting for a day will aide in weight loss, Withee says that you may lose a few pounds of fluids, but they will return once you break the fast.

The bottom line: Before you start your fast tonight, make sure you drink plenty of water. If you feel ill during the day—you have permission, rather you are commanded—to eat or drink whatever you think will help you survive the day.