A camel's hump is not a water tank. It is fat. Which makes it the most misunderstood lunchbox in the world. Yet a camel crossing a desert does get water out of it. When fat is burned for energy, it turns into carbon dioxide and water, and the water weighs more than the fat did: about 107 to 110 grams of water for every 100 grams of fat.
Your body does the same thing, every day, in a smaller way. Some of the water in you never came out of a glass.
That matters, because the usual advice treats hydration as a matter of drinking more. Drink eight glasses. Carry a bottle. Sip all day. And then you can still feel thirsty, tired and headachy with a full bottle on the desk. The bottle is full. You are not.
Water is not only about how much goes in. It is about where it comes from, and what keeps it in. Ahead: the three sources of the water in you, the 180 litres your kidneys handle every day, the sailor surrounded by water he could not drink, what coffee really does, why a drink stops thirst before it could possibly have reached your blood, when plain water is not enough, the "structured water" that falls apart in less time than light takes to cross the width of a hair, and why your fingers wrinkle in the bath.
Where your water comes from
You get water three ways.
From drinks. The obvious one, and the biggest. Not all drinks hold on to water equally. What to Drink Instead of Plain Water, linked below, has the 13-drink test, and the three drinks that kept more fluid in the body than water did.
From food. A lot of food is mostly water. An egg is about 75% water. Meat and fish carry plenty too, and a bowl of soup or a mug of bone broth is a drink that happens to come in a bowl. 7 Fun and Tasty Ways to Stay Hydrated Without the Boredom, linked below, has how food counts towards the total.
From your own metabolism. Burning food for energy makes water. Every 100 grams of fat burned yields about 107 to 110 grams of water, 100 grams of protein about 41 to 42, and 100 grams of carbohydrate about 60. Some desert animals live on this water alone, and so do migrating birds on their long non-stop flights. You will not, but it is part of your daily total.
So hydration does not start and end with the glass. A plate of eggs and meat with a mug of broth puts water into you too. Breakfast counts.
How your body keeps it
Getting water in is half of it. Keeping it is the other half, and that job belongs to your kidneys.
Your kidneys filter your blood all day. In an adult, they make about 180 litres of filtered fluid a day, roughly a bathtub's worth, and then take almost all of it back. What leaves as urine is usually somewhere between 0.8 and 2 litres. Your kidneys are less a drain than a very picky recycling plant.
Two signals tell them how much to keep. One is a hormone, vasopressin, which tells them to hold water back. Alcohol works against it, which is why a night of drinking sends you to the bathroom more than the drinks alone would explain. What to Drink Instead of Plain Water, linked below, has what that does to your fluid.
The other is salt. Water follows sodium. When there is too little salt in the body, the kidneys let water go to keep the balance. That is the grain of truth in the idea that minerals help you "hold" water, and it is why a drink with salt in it stays in the body longer than plain water. James DiNicolantonio and Siim Land's goes through the minerals that do this work. The same article has the oral rehydration drink, salt and sugar in clean water, that was worked out for cholera and still saves lives.
Salt can tip the other way, too. Your kidneys hold the salt in your blood close to 0.9%. Seawater is about 3.5%. Drink it, and your kidneys need more water to get rid of the extra salt than the seawater brought in. Coleridge's becalmed sailor had it right in 1798: "Water, water, every where, / Nor any drop to drink." It is usually misquoted as "not a drop". The sea does not care which.
Sleep plays a part as well. In more than 11,000 healthy American adults, those who usually slept six hours a night had 59% higher odds of urine concentrated enough to count as inadequate hydration than those who slept eight. Chinese adults showed the same pattern. A short night leaves you drier in the morning, whatever you drank.
Coffee gets blamed for drying you out, and that charge mostly fails. Best Coffee Alternatives: What's Really in Each Cup has the 50 men who drank either four mugs of coffee a day or the same amount of water, three days of each. Their body water, blood and urine came out much the same. For an everyday drinker, coffee counts as fluid. Its real cost is somewhere else. Adults told at random to drink coffee on some days and avoid caffeine on others slept 36 minutes less on coffee nights, and a short night is the one that dries you out.
Thirst runs ahead
Thirst is quicker than it has any right to be.
You drink a glass of water, and within moments you are no longer thirsty. But the water has barely reached your stomach, let alone your blood. So what switched the thirst off? Not the water. The water is still on its way.
In mice, the answer turned out to be prediction. The nerve cells that drive thirst listen to signals from the mouth and throat while you eat and drink, and they combine those with what the blood is telling them. They work out how the drink is going to change your fluid balance and switch the thirst off ahead of time. It explains three old observations: we drink most during meals, thirst fades fast, and cooling the mouth quenches thirst.
The prediction works well in a healthy young body. It works less well with age. 7 Fun and Tasty Ways to Stay Hydrated Without the Boredom, linked below, has the older men who were less thirsty than younger men after a full day without water, though their blood showed they needed it more. For an older parent, thirst is a poor alarm. Children sit at the other end. The average adult is about 60% water and the average child about 65%, and the same article has why babies and young children are at higher risk of running short.
When water alone is not enough
For most days, water is enough, and food does the rest. There are a few times when plain water is not.
Heavy sweating. Sweat carries salt out of you. Heat Survival Guide: How to Stay Cool and Safe in Hot Weather has why salt matters as much as water when you are sweating a lot, and what to do when heat turns dangerous. Your body learns, too. With repeated time in the heat, you start sweating sooner and more freely, your core temperature runs lower, and you hold on to fluid better. A week or less of exposures was already enough to improve how people coped.
Illness. Diarrhoea and vomiting take water and salt out together. This is the case the rehydration drink was built for.
The first weeks of low-carb or carnivore eating. The kidneys let more sodium go, and water goes with it. Why Processed Food Is Hard to Leave — and When It Gets Easier has the other half: each part of the carbohydrate stored as glycogen is held with three to four parts water, and as low-carb eating uses it up, the water goes, and minerals go with it. That is why the scales drop fast at first. The headaches and tiredness that follow are often a salt problem, not a water problem. You were not short of water. You were short of salt.
In each case, the fix is the same: a pinch of salt in your water, or a mug of salted bone broth. An unrefined salt such as does the job, and when there is no time to make broth, stir into hot water. Salt it to taste, not to a formula. If you have high blood pressure, or heart or kidney disease, salt carefully and check first.
Too much water. Water can be overdone. 7 Fun and Tasty Ways to Stay Hydrated Without the Boredom, linked below, has the marathon runners whose blood salt fell too low from drinking more than they sweated out, and the kidneys' limit of about a litre an hour.
What does not hydrate
"Structured water". The claim is that water in plants, or water put through a special device, is arranged into a shape your cells absorb better. It is sold under several names: structured, hexagonal, gel, cluster water. The trouble is physics. Water molecules do group together, but those groups break and re-form in less than 200 femtoseconds, two ten-trillionths of a second. No structure survives the trip from the bottle to your stomach, let alone into a cell.
The "harmonic frequency of the earth". There is no such thing in your glass. There is, however, a price on the label.
Juice, aloe and coconut water. Juice is sugar with water in it. What to Drink Instead of Plain Water, linked below, has aloe juice's laxative ingredient and coconut water's sugar. Neither is a hydration plan.
The skin keeps water out
The idea that you can hydrate through your skin gets the skin backwards. Its whole job is to keep water where it belongs: in you, and the rest of the world outside.
Your fingers prove it. Soak them in the bath and they wrinkle. That is not water seeping in. The wrinkles are made by your nervous system, on purpose. And they seem to have a use. In one test, people moved objects out of water faster with wrinkled fingers than with smooth ones. With dry objects, the wrinkles made no difference. Your fingers grow treads for the wet.
The Epsom salt bath has the same problem. Why Most Stress Supplements Fail (And the 9 Worth a Look) has the reviewers who found nothing solid behind the claims for magnesium through the skin, from flakes, sprays or salt baths. Enjoy the bath. Do not count it as a drink.
Oil on the skin does help the skin itself. It slows the water the skin loses to the air, which is why a dry patch feels better with it. That is skin care. It does nothing for the water in the rest of you.
How to tell
You do not need to count glasses. Your body keeps its own records. It even colour-codes them.
Look at your urine. 5 Ways to Harness Your Energy and Beat the Afternoon Slump has the tests where its colour tracked lab measures of hydration closely. Pale straw is good. Dark yellow means it is time for a glass. Two things can fool the check. Extra riboflavin, vitamin B2, found in B-complex and multivitamin pills, leaves in the urine and turns it a bright yellow that has nothing to do with water. And beetroot turns it pink or red in about 10 to 14% of people, more often in those short of iron.
Notice the signs: tiredness, a headache, cramps, a dry mouth. They have other causes too, but a glass of water, or water with a pinch of salt, is the cheapest thing to try first.
It takes less than you might think. 7 Fun and Tasty Ways to Stay Hydrated Without the Boredom, linked below, has the 25 young women who lost just over 1% of their body weight in water: their mood dropped, tasks felt harder, their concentration slipped and headaches came on. Twenty-six young men at about 1.6% made more mistakes on a test of attention.
And for anyone older in your life, do not wait for them to say they are thirsty. Offer a drink with every meal.
Back to the camel
The camel crosses the desert on fat, not on a hidden tank. You run on the same chemistry, in a gentler climate. Water comes from what you drink, what you eat and what you burn, and it stays because your kidneys and a little salt keep it.
Health is a sum of habits, not quick fixes. Hydration is one of the simplest. It is also one of the few that comes out of a tap.
Next time you feel dry and tired with a full bottle beside you, put a pinch of salt in the glass.
Wondering which drinks hold water best? What to Drink Instead of Plain Water — and What the Popular Alternatives Do to Your Body — the 13-drink test, why salt helps water stay in the body, and the drinks that cost more than they give.
Bored of plain water? 7 Fun and Tasty Ways to Stay Hydrated Without the Boredom — seven ways to drink more without forcing it, and why thirst is a poor alarm.
Know someone who carries a two-litre bottle everywhere and still feels flat? Send them this, and a pinch of salt.
Disclaimer: This article is general information. For advice about your own health, talk to a qualified health professional. If you have high blood pressure, heart or kidney disease, or take medicine that affects your fluid or salt, check before adding salt to your water or food.
Affiliate Disclosure: This article contains affiliate links. As an Amazon Associate, All For Life earns from qualifying purchases, at no extra cost to you. We only link to products we would recommend without the commission.
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