The Science of Yom Kippur Fasting

Our bodies can handle 24 hours without food perfectly well, though our minds may find it taxing. There are ways to make it all easier.

Ilan Assayag

The Jewish fast on Yom Kippur, the Day of Atonement, is different from most other fasting practices around the world because not only do we not eat, we don’t drink.

Our bodies are built to handle a 24-hour fast easily, at least if we're healthy, says says Mariana Urbach, senior dietitian at Clalit Health Services and division head at Clalit Dan Petah Tikva. “It’s considered very short-time duration. It may feel long to us, but not to our bodies. People like certain sadhus in India can even fast for months.”

It’s the absence of liquid intake that’s the problem.

Humans didn’t evolve near grocery stores, and if we couldn’t survive periods of starvation, we’d have gone extinct. Our bodies store energy for lean times in the form of glycogen, a polysaccharide, or long chain of sugar, in the liver and muscles.

When we don’t eat, our bodies tap the glycogen for energy, explains Urbach. It helps to make sure to fill that “storage room” in order to fast for 24 hours without physiological distress.

Clalit Health Services

But forgoing drink, as Judaism prescribes for Yom Kippur, is another matter. Eschewing liquids for 24 hours does cause physiological stress and can cause real damage. “We aren’t camels,” Urbach points out.

This is all the more true in hot, dry countries like Israel, but you Alaskan Jews out there, don’t think you’re off the hook.

Water is the biggest component of our bodies by weight, and it can’t be stored like glycogen. We can live without food for a long time, but not without liquid intake. Moreover, we are constantly losing water, even if we don’t realize it, even by breathing.

“Most people live in a perennial state of borderline partial dehydration,” Urbach claims. “They may pay a price for this, for instance, in the form of kidney stones. If we begin the Kippur fast without being saturated with water, we’re more likely to suffer from headaches through to dehydration.”

Why you don’t lose weight

In addition, since our physiology is water-based, when we dehydrate our bodies become stressed and our metabolism slows down. This helps explain, physiologically speaking, why people don’t lose weight during that 24-hour fast, though logic would suggest otherwise.

“Our bodies are smart,” Urbach explains.” When our body senses a deficiency, it reacts cleverly and shuts down systems that aren’t immediately needed, in order to conserve its resources.” So even though you’re not eating and ingesting calories, your body will conserve energy and not break down your fat reserves.

Another effect is that your body temperature will drop. “Maintaining a constant body temperature is very expensive in terms of resources,” says Urbach. So the water-stressed body will lower the “thermostat” a little.

Orthodox Jews take part in a Tashlich prayer, the Jewish New Year ritual, at Ashdod beach, September 25, 2014. Photo by Reuters

This wisdom applies to dieting in general: Don’t stint on water when trying to lose weight. It will have the opposite effect.

Yet after the 24-hour fast, if you weigh yourself, you see a decrease. Why? Because you have lost water, not fat. “To lose a single kilo of real weight, you have to forgo about 9,000 calories and that doesn’t happen in a day,” Urbach observes.

How to make the fast go easier

Got it. We can do without food but not liquid. Yet that’s what observance requires. But you can still make the fast easier on yourself and your demanding metabolism.

Start preparing your body a week before Yom Kippur by cutting down on coffee and other high-caffeine drinks, such as colas and "energy" drinks.

Caffeine addicts who go cold turkey on coffee (or caffeinated drinks) on Kippur itself are asking for a massive headache, followed by nausea, around 14 hours after your last cup. Black tea is weaker on its stimulative impact, but if you’re a heavy user, cut down on that, too, well in advance. “Normally you’d take an aspirin for the headache, but you can’t do that on Yom Kippur,” Urbach points out. “Start drinking green tea or herbal teas instead.”

Orthodox Jews take part in a Tashlich prayer, the Jewish New Year ritual, at Ashdod, September 25, 2014. Photo by Reuters

Secondly, the low-carb-high-protein Atkins diet is terrible for Kippur observers. So is the gluten-free fad (leaving aside people with gluten intolerance, who don’t have a choice).

“Before the fast, you want to avoid loading your body with superfluous toxins,” says Urbach. “People who don’t eat enough carbohydrates and eat a lot of protein are producing a lot of urea, which the body has to secrete, and that requires a lot of water.”

In other words, to get rid of the toxic urea, you need to urinate a lot and yet you’re not drinking for 24 hours, which is a recipe for trouble.

What to eat before and after the fast

An Orthodox Jewish woman takes part in a Tashlich prayer, the Jewish New Year ritual, at Ashdod beach, September 25, 2014. Photo by Reuters

We have reached the morning itself. The fast will start that night. Go heavy on carbs to fill your glycogen storehouse, Urbach urges.

“You could start with a breakfast of a nice healthy whole-grain toast, or porridge, as long as it has carbohydrates,” she says. Then have a mid-morning snack of fruit or honey cake, for instance, and a cup of green tea on the side. Leggo that coffee!

For lunch she suggests pasta, rice or potato, and throughout the day, drink more than usual, mainly water. Saturate your body. You know you’ve drunk enough if your urine is clear and doesn’t smell, she says. So you go to the bathroom more. Deal with it.

The last meal before the fast — this is important — keep it light. You spent the day storing glycogen. Now, don’t blow it all by distending your stomach in a pre-fast panic. A distended stomach sends hunger signals sooner.

Keep the spicing down, again to prevent thirst. Urbach recommends a clear soup with kneidlach, kubbe or kreplach, if one likes. The meal itself may consist of a modest portion of fish or meat, up to say 150 grams (5 1/2 ounces) is plenty (think: one chicken leg), with vegetables and up to one cup of a carbohydrate food like rice or pasta.

Alternatively, a cold pasta salad with vegetables is great, she suggests, adding: Do not eat some giant lettuce salad, lest you distend your stomach.

Not convinced about stomach distension and hunger? Try this: If we overdo that meal, we increase our insulin level in the blood and that will make us hungry sooner.

And don’t have dessert. Save the compote or baked apple in honey for the post-fast meal.

After the fast: Don’t dive into the well

We have finished our fast. We ate plenty of carbs before and cut down the coffee (shudder) for a week before. But we are hungry and, mainly, thirsty.

Do not stand by the sink and guzzle cup after cup of water. You will simply and suddenly dilute your blood and not give your body a chance to adjust, says Urbach. That will make you dizzy to the point of possible unconsciousness.

Slowly sip one cup of water, sweetened tea or other drink. One cup. Slowly. Then nibble on something. Some grapes, a pomegranate, a piece of honey cake (where on earth did that tradition come from? Urbach wonders as an aside) or a baked apple.

No, you won’t do yourself any harm if you gorge on food, but you will feel uncomfortable, so why go there?

The time for a real meal is an hour after that tea and snack, Urbach recommends.

Who shouldn’t fast?

That’s a minefield. Anybody in doubt, consult your doctor. If you take prescription medications, or have any chronic condition, ask your doctor. The same applies to the pregnant and the elderly — don’t be a hero. Ask.

Yom Kippur-type fasting, in which we forgo liquid, is harder when you have a fever because your hotter body is losing more water. So if you're in doubt, ask your doctor whether you can fast.

As for when to allow (require?) children to start fasting, that’s also an individual matter that depends on the child’s physiology, Urbach sums up. A general rule of thumb is to wait until after the bar mitzvah, but even then, it depends on the child’s physique: Smaller, thinner children will have a harder time of it. In any case, before bringing any child into the fasting fold, she recommends: ask the doctor.

Tzom kal. Have an easy fast.

This article was originally published in September 2014