Why Do You Need the Toilet More When It's Cold?
― The Water That Left as Sweat Now Takes a Different Exit
When the mornings and evenings turn chilly, doesn't it feel as if you suddenly visit the toilet more often? It isn't your imagination. The share of water leaving your body through each "exit" swaps around with the seasons. On top of that, a chilled body overestimates how much water it holds and moves to make more urine.
It's a October morning. You're waiting for a train on a slightly chilly station platform. You went to the toilet just before leaving home, yet you already need to go again.
In summer, even a lot of iced barley tea never did this to you. If anything, you're drinking less now.
You drink less, yet you pass more. There are two clear reasons for this odd mismatch.
Just two reasons
Your body loses water not only as urine but also as sweat and in your breath. When it gets cold, you hardly sweat at all. If you take in the same amount of water, the share that used to go as sweat has nowhere to go but out as urine.
When you get cold, the blood vessels near the skin narrow to keep heat in. The blood pushed out gathers in the core of the body. Sensors then read this as "there is a lot of water in the blood" and send an order to make more urine.
The first reason is about how the water "exits" are shared out. The second is about the body's "estimate of its water" going off. Let's take them in turn.
About the same amount of water goes in and out each day
An adult takes in roughly 2.5 litres of water a day. That is not just drinks. It also counts the water in food and the water made when the body burns food.
The amount of water in your body hardly changes from day to day. So about the same 2.5 litres that came in must leave by some route every day. There are four main exits: urine, stool, sweat, and "water that leaves without you noticing".
The last one means water vapour in your breath and water slowly evaporating from your skin. Even when you aren't sweating, about 0.9 litres a day is said to leave this way.
Look at Figure 1. The top bar is water coming in; the bottom bar is water going out. The two bars are always the same length. When the sweat section of the bottom bar grows, the urine section at the right end shrinks by the same amount. When sweat falls, urine stretches out.
In summer you sweat a little all the time, even sitting still. Even a light dampness can add up to around 0.5 litres a day. When the temperature drops, this sweat falls to almost zero. If you drink the same amount, that homeless 0.5 litres gets sent to urine.
The bladder is said to give a clear "I need to go" signal once about 300 millilitres of urine has collected. An extra 0.5 litres works out to one or two more toilet trips a day. This is why it feels like more trips even when you drink less in winter.
A chilled body pulls blood toward the "core"
The second reason is a side effect of the way the body protects its temperature. When it senses cold, the body narrows the blood vessels running near the skin. Warm blood flowing close to the skin would let heat escape rapidly.
Look at Figure 2. The left side shows warm conditions and the right side shows cold. On the right, blood pushed out from near the skin has gathered in the centre of the body, such as the chest and belly.
Near the heart there are sensors that watch how much blood flows in. When the blood in the core increases, these sensors take it to mean "the body has too much water". In fact, the water hasn't increased. It has only moved to a different place.
Even so, the body tries to cut its water as it estimates. Normally the kidneys squeeze water back out of the urine to make it concentrated. That "squeeze it back" order weakens, and a lot of dilute urine is made. This is why urine on a cold day looks nearly clear.
Have you ever felt a sudden urge during winter chores with water, or right after standing in a cold wind? Animal experiments have reported that merely chilling the skin can make the bladder contract more easily. Apart from the way urine increases, there seems to be a separate pathway that makes you feel "I need to go" more readily.
In cold weather you also feel thirst less. Add the water leaving as urine, and your body can quietly run short of water. Even if the number of trips bothers you, it is important not to cut how much you drink. If night-time trips are a problem, try drinking more by the evening rather than just before bed, which can make night trips easier to reduce.
Summary
There are two reasons you need the toilet more when it gets cold. One is that the water that left as sweat in summer loses its exit and is routed to urine. The other is that a chilled body gathers blood in the core, estimates that it has "too much water", and makes more urine. Both are perfectly normal workings of a body trying to keep its water level and temperature steady.
Same drinking, different exit.
Winter toilet trips are where the sweat went.
The body has other reactions to cold, too. Take a look at why we get goosebumps when it's cold, what happens when body temperature falls too far, and "heat shock", the sudden blood-pressure swings in cold places.
- For a week, note the morning temperature and how many times you went to the toilet that day. You will see whether colder days mean more trips.
- On a cold day, compare a day when you wear one extra layer and keep warm with a day in light clothes. If keeping warm reduces your trips, that is reason 2 at work.
- Check your urine colour too. If it is paler on cold days, that is a sign your kidneys are squeezing back less water.
Noting how much you drink makes comparison easier. If your trips are extremely frequent or painful, another health problem may be involved, so please consult a medical professional.
For those who want more ― terms, formulas and links to textbooksWe mark the level of each part, from middle-school science to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school biology
- High school+Advanced high-school content, or textbook sidebars
- UniversityUniversity specialist courses (physiology, nephrology) not taught in high school
- ResearchTopics still under study that even university courses don't teach as settled
Middle schoolTerms: this phenomenon has names
- Cold diuresis (kanrei-nyō): an increase in the amount of urine when the body is exposed to cold.
- Insensible water loss (fukan-jōsetsu): water that leaves without notice through breath and skin, apart from sweat.
- Antidiuretic hormone: the hormone that tells the kidneys to "squeeze water back out of the urine". When it falls, a lot of dilute urine is passed.
Middle schoolHigh schoolCheck with a formula: how much more urine when sweat falls?
Assuming the body's water stays about the same over a day, let's estimate urine in summer and in the cold season. We leave out the "make more urine" order of reason 2 and calculate only the swap of exits from reason 1.
| In symbols | U ＝ W − （ I ＋ F ＋ S ） |
| In words | Urine = water in − (water from breath and skin + water in stool + sweat) |
| Where it comes from | It comes from the water balance: body water stays nearly constant over a day, so water in = water out. If one exit shrinks, the others must grow. |
| Symbol | Meaning and unit |
| U | Daily urine (litres) |
| W | Water in per day (total of drinks, food and body-made water, litres) |
| I | Insensible water loss (water from breath and skin, litres) |
| F | Water leaving in stool (litres) |
| S | Sweat (litres) |
| Water in per day W (adult guide) | about 2.5 L |
| Insensible water loss I | about 0.9 L |
| Water in stool F | about 0.1 L |
| Summer sweat S (assuming light dampness) | about 0.5 L |
| Cold-season sweat S | almost 0 L |
| Amount that gives a clear urge (one trip) | about 0.3 L |
| Water leaving other than as sweat | 0.9 ＋ 0.1 ＝ 1.0 L |
| Water leaving other than as urine in summer | 1.0 ＋ 0.5 ＝ 1.5 L |
| Summer urine | 2.5 − 1.5 ＝ 1.0 L |
| Cold-season urine | 2.5 − 1.0 ＝ 1.5 L |
| Summer toilet trips | 1.0 ÷ 0.3 ≒ 3.3 trips |
| Cold-season toilet trips | 1.5 ÷ 0.3 ＝ 5 trips |
| Increase in urine | 1.5 ÷ 1.0 ＝ 1.5 times |
Without changing how much you drink, sweat vanishing alone makes urine 1.5 times larger and adds one or two toilet trips a day. In reality you drink more in summer, so the gap is smaller, but the "make more urine" order of reason 2 is added on top.
High schoolHigh school+How do the kidneys decide how much urine to make?
High schoolThe kidneys filter blood and are said to make more than 150 litres of "primary urine" a day. About 99% of it is returned to the body, and the rest becomes urine. How much water is returned is decided by antidiuretic hormone (vasopressin) released from the pituitary gland in the brain. When this hormone falls, less water is reabsorbed in the collecting ducts, and more dilute urine is made.
High school+The walls of the heart's atria contain sensors that detect stretch (stretch receptors). When blood in the core increases and the atria are stretched, a reflex lowers antidiuretic hormone. The atria themselves also release a hormone that increases urine (atrial natriuretic peptide).
UniversityHow is cold diuresis explained?
Diuresis from cold exposure is thought to start with peripheral vasoconstriction raising central blood volume. From there, three routes are thought to be involved: suppression of vasopressin secretion through atrial stretch receptors (the Gauer–Henry reflex), secretion of atrial natriuretic peptide, and pressure diuresis from raised blood pressure. Reports differ on which route matters how much under which conditions.
📖 Derivations and further reading: Wikipedia: Vasopressin (Japanese) ／ Wikipedia: Atrial natriuretic peptide (Japanese)
ResearchWhat is still not clear
- How much does each of the three routes contribute? Studies disagree on how much the fall in antidiuretic hormone, the atrial hormone and pressure diuresis each contribute to cold diuresis.
- How skin chill drives the bladder Animal experiments suggest that the skin's "cold sensors" may take part in a bladder reflex. How far the same holds in people is still being checked.
- Season and night-time urination It is known that night-time toilet trips tend to rise in winter, but how much is due to cold itself and how much to changes in sleep and lifestyle has not been fully sorted out.
So the content of this article, too, is "an explanation within what is known today". Research on the details of how the body shares out water and heat is still going on.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science, year 2: "Structure and function of animal bodies (excretion)" | Water exits and the role of the kidneys |
| High school | Biology: "Maintaining the internal environment (kidneys, hormones)" | Reabsorption of primary urine and antidiuretic hormone |
| High school+ | Advanced biology: "Regulation of body fluid volume" | Atrial sensors and atrial natriuretic peptide |
| University | Physiology, nephrology | The three routes of cold diuresis |
| Research | Thermal physiology, urology | Route proportions; skin chill and the bladder reflex |
| ― | Links to daily life | Staying hydrated in cold weather; handling night-time trips |
- Wikipedia, "Insensible water loss" (ウィキペディア「不感蒸泄」)
- Wikipedia, "Vasopressin" (ウィキペディア「バソプレシン」)
- Wikipedia, "Atrial natriuretic peptide" (ウィキペディア「心房性ナトリウム利尿ペプチド」)
- Honma Ken'ichi et al. (eds.), 『標準生理学』 (Standard Physiology), Igaku-Shoin
- Hall, J. E. "Guyton and Hall Textbook of Medical Physiology", Elsevier
- Japanese Continence Society and Japanese Urological Association (eds.), 『夜間頻尿診療ガイドライン』 (Clinical Guidelines for Nocturia)
* This article is a popular-science explanation. The figures given are rough guides to help you understand the mechanism and vary greatly with build, activity and diet. If your toilet trips suddenly increase, or come with pain or fever, please consult a medical professional.