...and that's the part I keep coming back to with the heat stuff, because the body doesn't distinguish between a fast and a heat wave. It just sees water going out and nothing coming in.
Right, and that's exactly the frame for today, because it's Yom Kippur.
It is. And Daniel sent us something for it. Not the usual. He says the week before the fast was rough on the family. Stomach bugs, back and forth to urgent care, Hannah ended up on an IV.
That's a rough run-up to a fast.
It is. And he's explicit about what he doesn't want. He doesn't want the Jewish legal questions. Who can fast, who shouldn't, when you're allowed to break it early. He says that's a recognised discussion in Jewish law and it belongs between people and their rabbinic authorities, not between us.
Fair.
What he wants is the pure physiology. He points out Judaism has two categories of fast, major and minor, and Yom Kippur is a major one, so it's twenty-five hours, no food and no water. He notes Israel is a hot climate, which changes the problem. And then he asks three things. What are the stages of dehydration to watch for. What are the danger signs, the point where someone is going from a bit dehydrated to significantly dehydrated. And what can you actually do to conserve hydration if you're going into one of these.
That's a good prompt, because he's already done the hard part of the framing. He's identified that dehydration is the risk, not hunger.
He says a healthy person can go twenty-five hours without eating without much difficulty beyond discomfort. But dehydration develops quickly and insidiously, even in healthy people.
He's right, and that's the whole episode. So where do we start?
With the asymmetry. Why hunger is the thing people fear and thirst is the thing that actually hurts them.
Start with the numbers, because they're stark. A healthy adult can go weeks without food. Weeks. The body has glycogen, then it shifts to fat metabolism, and there's a whole hormonal cascade that comes with it. Insulin drops. Growth hormone ticks up a little, which protects muscle mass. Cellular repair mechanisms switch on, the autophagy stuff everyone talks about.
And that kicks in around when?
Twelve to sixteen hours into a fast is when fat oxidation becomes a bigger contributor. So a twenty-five-hour fast absolutely runs through that window. But here's the thing, and I want to be honest about this because it gets oversold. A one-day fast produces temporary metabolic effects. It is not a lasting health intervention. You don't fast for twenty-five hours and bank the benefits for a year. The body resets.
So the metabolic story is real but modest.
Real but modest. Which is fine, because that's not why anyone's doing it. But it means the interesting physiology isn't the fasting metabolism. It's the water.
Because there's no reservoir.
There's no reservoir. That's the entire asymmetry in one sentence. Fat is stored energy. You can run a deficit for weeks. Water is not stored. You have a working volume, and when it drops, every system that depends on that volume starts to notice, and it notices within hours, not weeks.
And Israel makes it worse.
Israel makes it worse. A fast in a temperate climate in October is one problem. A fast in a hot climate is a different problem, because you're losing fluid through sweat on top of the normal losses, and you may not register that you're losing it. You're not exercising. You're sitting in a synagogue. But you're still losing it.
There's something almost cruel about that. You're doing the least physical thing you could do, and your body is quietly draining anyway.
That's the cruel part. The losses don't stop just because you've decided to sit still. You're still breathing, you're still sweating a little, your kidneys are still doing their thing. And in a hot room with a lot of people in it, the sweating is more than a little.
Which raises the question Daniel actually asked. If dehydration is the real risk, what does the progression look like, and how do you watch for it?
That's the clinical part, and it's useful, because it's staged. You can track this.
So let's track it.
The framework clinicians use is percentage of body weight lost as fluid. Mild is roughly one to two percent. Moderate is three to five. Severe is six percent and up. And the symptoms map onto those bands fairly cleanly.
Start at the bottom.
Mild, one to two percent. Thirst. And I want to flag that word, because in a healthy adult thirst is the earliest and most reliable signal you have. Dry mouth. Dry mucous membranes. Urine gets darker and more concentrated, and you produce less of it. Mild fatigue. Concentration drops off. That's it. That's the whole mild band.
Which sounds like a normal fast afternoon.
It sounds exactly like a normal fast afternoon, and that's the trap. Because that band is where the cultural script takes over. Everyone expects to feel bad, so everyone treats mild dehydration as the baseline state of fasting rather than as a signal.
Moderate.
Three to five percent. Now thirst is marked, not just present. Mouth and tongue are very dry. Urine is amber, and you're urinating infrequently. Headache. Dizziness, lightheadedness, and specifically on standing, which is the orthostatic thing. Weakness, lethargy, irritability. Skin turgor drops, which means if you pinch the skin on the back of the hand it stays tented instead of snapping back. Sunken eyes.
The standing test.
The standing test is the one I'd actually teach people, because it's free and it's specific. Stand up from a chair. If you go grey at the edges, if the room tilts, if you have to grab something, that's falling blood pressure, and falling blood pressure is an early warning that you've moved past mild.
And it's better than thirst, because thirst is subjective and this is observable.
It's better than thirst for exactly that reason. You can argue with yourself about whether you're thirsty. You can't argue with the room tilting.
And severe.
Six percent and up. Confusion. Disorientation. Difficulty speaking. Rapid heartbeat, rapid breathing. Very low urine output or none at all. Cold, clammy skin. Fainting. Inability to stand or walk steadily.
That's an emergency.
That's an emergency. That's not a fast anymore. That's a medical event.
So the bands are clean. The problem is the signal.
The problem is the signal, and this is the part of the research that surprised me. There's a study of nursing home residents, Johnson and Hahn, published in JAMDA in 2018. They measured a fluid retention index, which is a marker of dehydration, and they asked the residents how thirsty they were. And the residents reported less thirst as their dehydration marker got worse.
Less thirst.
Less thirst. The signal inverted. And it wasn't just thirst. Clinical signs like dry mucous membranes and skin turgor also correlated poorly with the objective markers in that population.
So in older people, the instrument is broken.
The instrument is broken. And that's the single most important practical takeaway in this whole episode. An older person fasting cannot rely on thirst. They can be at three percent and feel fine, or feel less thirsty than they did at one percent. Which means the monitoring has to be external.
Urine color.
Urine color. Standing safely. Cognitive clarity. Is grandma tracking the conversation, or is she drifting? Those are the instruments, because the internal one has failed.
Is this an aging thing specifically, or is it a general thing that just shows up earlier in the elderly?
I think it's aging-specific, and I don't know the full mechanism. Some of it is thought to be a blunted thirst response, the osmoreceptors get less sensitive with age. Some of it may be medication. But I'd be guessing past that, and I don't want to guess.
Then there's the group Daniel flags.
The pre-existing conditions. Hypoglycemia, diabetes, kidney disease, heart conditions. People on diuretics or certain other medications. Pregnant and breastfeeding women. And the elderly. Those are the people where the normal margin is thin to begin with.
And Daniel's family is coming into this off a stomach bug.
Which is the real-world illustration of the whole thing. Gastrointestinal illness depletes fluid and electrolytes before the fast even starts. So you're not beginning at baseline. You're beginning in deficit, and then you add twenty-five hours on top of it. That's a different physiological situation than a healthy person starting hydrated.
It's the difference between running a race on a full tank and running it on fumes.
That's exactly the analogy. And the awful part is you can't tell from the outside. Hannah on an IV on Tuesday and Hannah feeling fine on Friday look the same on Friday. But the tank isn't full yet.
Baseline matters more than anything that happens during the fast.
Baseline matters more than anything that happens during the fast. Which is a good bridge, actually.
To what you do about it.
To what you do about it. And the headline is that the leverage is almost entirely before the fast, not during it.
Which is counterintuitive, because people think of the fast as the event.
The fast is the event, but the preparation is the intervention. And the first thing to get right is how you drink, not how much. There's a line from Dr. Michael Segal's guide that I think is the best single piece of advice in the whole literature. Don't attack the water right before the fast. Drinking a liter at once doesn't stay in the body.
Where does it go?
You excrete it. Your kidneys handle a bolus load by dumping the excess. So you drink a liter at four in the afternoon, and by six you've urinated most of it out, and you've gained nothing. What actually works is a glass of water every half hour from morning until the fast begins. Same total volume, spread out, and the body retains far more of it.
So it's not a volume problem, it's a rate problem.
It's a rate problem. Your kidneys can only hold onto so much at once. Give them a slow trickle and they keep it. Give them a flood and they open the gates.
Steady, not flooding.
Steady, not flooding. And start earlier than the day of. Forty-eight hours out is the window people talk about, aiming for two to three liters a day in the days before.
There's a figure floating around about that.
There is, and I want to be careful with it. There's a claim that people drinking two and a half liters or more the day before report something like sixty percent fewer dehydration symptoms. I'd treat that number with real caution. It's a secondary citation, it's not from a controlled trial I can point you to, and the direction of the effect is more trustworthy than the size of it.
So hydrate well, don't quote the sixty percent.
Hydrate well, don't quote the sixty percent. And don't overdo it either, which is the part people miss. Drinking too much water can wash out your electrolytes. That's hyponatremia, and it's dangerous. Dr. Tzvi Dwolatzky at Rambam makes exactly this point. You can hurt yourself preparing for a fast by drinking too much, not too little.
So there's a window. Too little is bad, too much is bad.
There's a window, and it's wider than people think, but it's not infinite. Two to three liters a day is the target. Three liters in an hour is a problem.
The salt thing.
The salt thing is the next lever, and it's the one people get backwards. Salt makes you thirsty because your body needs extra water to dilute the extra salt. So a salty pre-fast meal doesn't help you retain water. It makes you need more of it. Segal's list of things to avoid is basically a list of the traditional pre-fast table. Pickles. Cheese. Tomato sauce. Processed and sliced meats. Salted fish. Soy sauce. MSG.
That's a lot of the menu.
It's a lot of the menu, and I'm not going to pretend it isn't. But the mechanism is clear. Cut the salt and you need less water to feel fine.
I want to push on that, because the folk wisdom is the opposite. People eat salty food before a fast because they think it helps them hold water.
And that folk wisdom is half right and half wrong. Salt does help you hold water, in the sense that water follows sodium. But the price is that you need more water to hold, and you're about to stop drinking. So you've raised your requirement right when your supply is about to hit zero. It's borrowing against a paycheck that isn't coming.
That's a clean way to put it.
It's the cleanest way I know. The salt pulls water into the wrong compartment and then you can't top it up.
Caffeine.
Caffeine and alcohol both promote water loss, so both are worth cutting. But caffeine has a second problem, which is withdrawal. The withdrawal headache peaks one to two days after you stop and can last up to nine days. So if you're a heavy coffee drinker and you quit cold the morning of the fast, you're going to spend the fast with a withdrawal headache on top of everything else.
Taper.
Taper over several days. It's a free intervention and almost nobody does it.
Because nobody thinks of the headache as a caffeine problem. They think of it as a fasting problem.
They think of it as a fasting problem, and so they never fix it. They just endure it, and they blame the fast, and the fast didn't do it. The coffee did.
What about the meal itself.
Slow-digesting food. Fats and oils delay stomach emptying, so you feel full longer. Complex carbs, whole wheat pasta, sweet potato, legumes, they sustain satiety. Fruit carries water in a form that releases slowly, which is useful here. And the counterintuitive one, at least for the pre-fast meal specifically, is to go easy on the giant fiber salad.
The thing everyone thinks is the healthy choice.
The thing everyone thinks is the healthy choice. High fiber moves things through faster. For the pre-fast meal you want slow, not fast.
So the salad is great on a normal Tuesday and wrong on the eve of a fast.
Context changes the answer. The salad isn't bad food. It's bad timing.
Then you're in it.
Then you're in it. And during the fast the strategy is boring, which is the point. Avoid sweating. Don't overdress. Stay out of the heat. Minimize physical exertion. Rest. Don't push through a workout because you feel like you should. Every drop you sweat is a drop you can't replace.
And then the break.
The break matters too, and this is where people hurt themselves after surviving the hard part. Go slow. Start with a big glass of fluid, but not too much at once, because the same hyponatremia risk applies on the way back in. You'll drink a normal day's fluid, but spread it over several hours, not twenty minutes. Water or tea first. Then something easy, a banana, a date, after about ten minutes. Then a light meal after an hour. And go easy on the salt at the break-fast, for the same reason as before.
And the weight.
The weight is water. Whatever you lose in twenty-five hours is overwhelmingly water, and it comes back fast once you rehydrate. It is not fat loss. Anyone treating a Yom Kippur fast as a diet intervention is measuring the wrong thing.
There's a quote from Segal I want to put in here, because it's the cleanest statement of the whole episode. Most people think the difficulty of fasting is feeling hungry. Actually, avoiding thirst is much more important for how you feel.
And Dwolatzky's version of it, which is the medical line. There's a marked difference between the intended feeling of being uncomfortable with not eating or drinking for twenty-five hours and actually becoming ill as a result of not getting enough liquids and nutrients.
That's the line between discomfort and illness.
That's the line, and it's a real line. Discomfort is expected. Illness is not, and it doesn't announce itself the way people think.
Which is where the cultural script comes in.
It is.
Because everyone knows they're supposed to feel bad.
Everyone knows they're supposed to feel bad, and so they do, and they don't ask whether the bad feeling is the expected kind or the other kind.
Hilbert: Can I ask you something about the word insidious.
Sure.
Hilbert: You both used it. And I think people hear it wrong. They hear it as, it sneaks up on you. Like it's hiding.
That's the usual reading.
Hilbert: That's not what I saw. I did intake on nights at a clinic out in the desert, and we got the heat cases, and I was the one taking vitals and asking the questions before the doctor came in. And the people who came in bad, the ones already confused, they didn't get there because it snuck up on them. I'd ask when the symptoms started and they'd say, oh, I felt dizzy hours ago. I thought that was normal.
They knew.
Hilbert: They knew. They told me the time. They'd been dizzy since two in the afternoon and they came in at nine at night. It wasn't hiding. They were ignoring it on purpose, because they thought feeling bad was the deal. That's the thing I'd correct. It's not that the body doesn't tell you. It's that people have been told the alarm is supposed to be ringing.
So the failure is interpretive, not sensory.
Hilbert: The failure is interpretive. In the older ones it's sensory too, I'm not arguing with your study. But for the ones who could feel it fine, the problem was they'd decided the feeling was the point.
And you saw this specifically around fasts.
Hilbert: We staffed up for it. That was the one night of the year where the schedule changed. Extra oral rehydration solution on the cart, extra nurse on, because we knew. The nurses called it the Yom Kippur rush. Same thing every year. People who were fine at the start of the day and came in at eight at night because they'd been dizzy since two and thought that was normal.
The clinic knew before the patients did.
Hilbert: The clinic always knows before the patients do. Anyway. Someone's waiting for me in the car park, so.
Go.
Hilbert: I'm going.
So the thing I keep coming back to is the older fasters. Because if dehydration is the real risk, and the elderly are the most vulnerable, and they are also the least able to detect it in themselves, then the monitoring can't be self-monitoring. It has to be somebody else watching.
Which reframes it as a communal thing rather than an individual one. You're not asking grandma to check her own urine color. You're asking the person sitting next to her to notice that she's stopped tracking the conversation.
And that's a different kind of care than most people think they're providing.
It is. It's not the spiritual version and it's not the halachic version. It's just the medical version, and it's the one that's easiest to forget because it feels like fussing.
And I think there's something worth saying about why it feels like fussing. Because in a lot of these communities, the fast is treated as a test. You're supposed to endure it. So the person who checks on you is implicitly saying you might not be able to. And nobody wants to be that person.
Right. The monitoring carries a social cost, which is exactly why it doesn't happen.
So the reframe has to be that the monitoring is the practice. It's not an interruption of the fast. It's part of how the community keeps everyone in it.
Which is the same logic as the heat wave stuff we did, actually. The neighbour who knocks on the door of the old man living alone. Nobody thinks that knock is an insult.
Except the old man, sometimes.
Except the old man, sometimes. But we do it anyway, because the alternative is worse.
If you found this useful, send it to someone who's fasting. And if you're the one fasting, listen to your body. But know that your body might not be telling you the truth.
Especially if you're older. Then borrow somebody else's ears.
Thanks to our producer, Hilbert Flumingtop, who is currently walking to a car park. This has been My Weird Prompts.
The human-AI collaboration podcast. If you want to send us something, email us at show at my weird prompts dot com. We read everything.
We'll be back soon.
See you tomorrow.
