Millions across the world will forgo food and drink from before sundown on September 24 until nightfall on September 25 in observance of Yom Kippur (The Day of Atonement), the Jewish holiday devoted to prayer, introspection and forgiveness.

Fasting has gotten popular, especially intermittent fasting, which can include alternate-day fasting, periodic fasting or eating only at certain times of the day. There’s also therapeutic fasting, a supervised cleansing and detox practice when people only drink water.

Whatever bodily benefits those types of fasts may or may not have, it’s not the same as fasting for religious reasons. 

Because Jews have been observing mandated fast days for millennia (and Muslims since the seventh century), we asked two Israeli physicians for their insights and advice ahead of Yom Kippur.

“Judaism’s approach to fasting is to emphasize the spiritual over the physical during certain periods,” says Dr. Zev Wimpheimer, a senior physician in the emergency department of Shaare Zedek Medical Center in Jerusalem and medical director of an urgent care center in Beit Shemesh.

“My understanding is that the goal is to take away the physical enjoyment of food and drink to spiritually enhance ourselves but not specifically to suffer. Therefore, it would be reasonable to take measures to reduce the physical effects of fasting.”

What happens when you don’t eat

Dr. Miriam Maisel, a certified lifestyle physician and family practitioner in Tel Aviv who lectures on nutrition and health as well as stress management for doctors, is trained in supervising therapeutic fasting. 

“When you eat a meal, your blood sugar goes up and your body releases insulin and other hormones to deal with the sugar. At some point the sugar level drops and you get hungry. If you don’t eat, the body takes measures to maintain blood sugar levels,” she explains.

“Our body’s first response is to use starch stored in the liver and muscles, which can last about a day, after which it switches to sugar from another source, first by breaking down muscle protein to create new sugar. Later, fat is broken down to ketones as the main energy source,” says Maisel.

Fasting for 24 hours isn’t enough to trigger ketosis, the body’s “emergency system” in which fat is burned for energy. You won’t lose more than a few ounces from a Yom Kippur fast, assuming a deficit of about 2,000 calories.

What to eat before 

The Israeli Ministry of Health recommendations for the day before Yom Kippur include eating a few small meals based on the Mediterranean diet, avoiding salty foods, drinking plenty of water and tapering off caffeine.

“People who drink caffeinated beverages are likely to get a headache when fasting so it might be helpful to stop or reduce consumption a few days in advance,” says Maisel.

Wimpfheimer adds that it’s not advisable to skip meals on the day before fast to “get used to it.” In fact, rabbinic tradition encourages eating a lot on the day before Yom Kippur.

For the pre-fast meal, avoid salty or spicy foods to keep from feeling too thirsty on the fast day. Wimpfheimer also says to avoid sugary foods because “they cause a surge of insulin after the meal, which will make you hungry,” he cautions. 

“Choose high-fiber foods – the body breaks down fiber slowly – and foods that retain water such as rice, brown rice or cooked vegetables. Drink a lot of water at the pre-fast meal, and fruit is a good option for dessert.”

Maisel advises going easy on white flour products, like bread, that are binding. And she doesn’t think it’s necessary to carbo-load. 

“I don’t know if there’s any objective research proving that carbohydrate loading is helpful when fasting. My impression is that most of us don’t require extra carbs to have enough glycogen in the liver and muscles to make into sugar for a 24-hour period,” she says.

What to eat after

When breaking the fast, the key is to reintroduce food slowly and gradually.

“If you eat a heavy meal after concluding a fast, you may feel uncomfortable,” says Maisel, “so even though it’s tempting to eat a lot right away, take your time and check in with yourself how you feel. That’s more important than the content of what it is you’re eating.”

“What’s most important is how you eat the other 364 days of the year,” Dr. Miriam Maisel.

There’s actually a recognized medical condition, Dumping Syndrome, characterized by pain, bloating, nausea or vomiting when too much food is introduced to the first portion of the GI tract too quickly, says Wimpfheimer. 

“Therefore, start by drinking. Water or tea are ideal. Then wait a few minutes and slowly eat a meal that’s high in protein. This approach will help you get back into a normal diet easier and will make you feel better over the subsequent 24 hours,” he says.

Muslims traditionally break the Ramadan fast on water and dates, a practice endorsed by the Health Ministry.

“You certainly need water after a fast. And dates are an example of a nice way to get a bit of sugar very quickly so you’ll feel better. They don’t have any downside,” says Maisel. 

“Any fruit, as opposed to a piece of cake, has fiber so the sugar isn’t completely liberated into your bloodstream right away and won’t cause a dramatic sugar spike.”

Ask your doctor

The Yom Kippur fast is a religious obligation for females over age 12 and males over age 13. People with certain health conditions are exempt, and in many cases the decision is based on a physician’s recommendation.

Wimpfheimer says nursing mothers and pregnant women – particularly in the second and third trimester when they are more prone to dehydration and have naturally lower blood pressure — should consult a physician before fasting, as should seniors. 

“Certain medications often prescribed to the elderly for blood pressure contain diuretics and fasting could potentiate dangerous dehydration.”

People with diabetes who take medication to lower blood sugar may not be able to fast safely, he adds. 

Can a pill help?

Various supplements are marketed as beneficial to take before a fast to minimize discomfort.

Wimpfheimer coauthored a study published in 2010 in Headache: The Journal of Head and Face Pain about the effect of the drug etoricoxib (similar to ibuprofen) on a fasting-related headache commonly called “Yom Kippur Headache” or “First-of-Ramadan Headache.” 

Study subjects who took etoricoxib before starting their fast “demonstrated a marked statistical decrease in the incidence of fasting headache and reported generally feeling better and tolerating the fast better than control subjects who were given a placebo,” says Wimpfheimer. 

“Anecdotally, patients who took the medication reported that their spiritual wellbeing, and ability to concentrate and get meaning out of the Yom Kippur day, was enhanced.”

Eat well all year round

Maisel concludes with the most critical piece of advice.

“What’s most important for health is how you eat the other 364 days of the year. Fasting is not a solution to having a bad diet the rest of the time.”