Why is urine yellow?

Submitted by nateblackmt t3_11a9tvk in askscience

Liquids of various colors ranging from clear water to dark coffee are processed by the body and urine becomes a yellow color of different shades, how does the body do this?

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berliniam t1_j9qzfm0 wrote

Breakdown of red blood cells results in bilirubin which gets stored in your gallbladder. It then gets excreted into your intestines and converted a couple times to end up in two “exit” forms: stercobilin (which gives your stool it’s brown color) and urobilinogen which ends up in your bladder and peed out. This urobilinogen oxidizes with air once you pee and converts to urobilin which has a yellow color! People with gallbladder biliary blockages or liver disease which prevents the breakdown excretion of bilirubin may actually have near-pigmentless stool (acholic stool)

The other pigments in your food like from a strawberry or steak are more likely to be broken down in your intestines and excreted as poop (which is already pretty brown so color changes are less perceptible) or end up as colorless contributors to your urine. Beets are a strong example of a pigment that some folks can’t breakdown as well so it gets excreted in both stool and urine and giving a pink or red hue.

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hercola t1_j9rfpo2 wrote

Not quite correct in your explanation of biliary obstruction. If biliary outflow is obstructed, more soluble conjugated bilirubin is excreted into the urine (because it cannot go into the GI tract due to biliary obstruction), causing dark urine. The obstruction prevents excretion of conjugated bilirubin into the GI tract, so less urobilinogen (and later stercobilin) is formed and you get pale stool.

Edit: also, gallbladder obstruction would give you cholecystitis but would not give you symptoms of obstructive jaundice like dark urine or acholic stool

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berliniam t1_j9rp3s1 wrote

Awesome, thanks for the corrections. Over-layman-ized myself haha

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Practical_Cartoonist t1_j9sge1v wrote

Is urine colourless when it's still inside your bladder?

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sexualinnuend t1_j9tdxyw wrote

No. The bladder is just a container before it makes its exit. The production and filtering of sorts of the urine is done in kidneys and it's "tubes"

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lubacrisp t1_j9tm4zt wrote

Their question was almost certainly about the mixing with oxygen causing it to turn yellow. Not even sure what question you think you're answering

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mrshulgin t1_j9tptuh wrote

Bingo. I feel like I'm able to detect a yellowish tint in my urine stream if I'm dehydrated. Maybe I'm mistaken... but am I?

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Not_Pictured t1_j9trj7i wrote

Urine is definitely yellow in the bladder.

Source: catheters.

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mrshulgin t1_j9ttoq3 wrote

So... is the top answer incomplete?

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Edd1148 t1_j9tbwci wrote

Offshoot question: why does asparagus make urine so distinctly pungent?

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Dubanx t1_j9th5x6 wrote

Funnily enough, both the ability to smell asparagus pee AND whether or not your pee smells from asparagus is genetic.

Many people can't smell it, and many others don't have pee that smells. A lot of people don't produce a detectable smell after eating it, but the people who claim to smell it have been scientifically shown to agree on which samples smell and which do not. So it's not psychosomatic either.

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Zchwns t1_j9tou8g wrote

Do we know whether these genetic traits are dominant or recessive? I’m curious if it’s rarer to have both genetic traits expressed vs no expression of either.

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Dubanx t1_j9tqj7x wrote

People's sense of smell & taste tend are known to be heavily dependent on genetic factors, but are super complicated. It's my understanding that these aren't determined by a single gene but the interactions between multiple genes. So they're poorly understood as a result. In general, smell and taste aren't something that gets determined by a single recessive or dominant gene.

For what determines excretion, I just don't know.

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Zchwns t1_j9tr1a1 wrote

Interesting! Thank you!

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Dubanx t1_j9tst9o wrote

No problem. It's not just limited to asparagus pee too. Taste & Smell are exceptional in how much they vary from one person to another. A famous example is "phenylthiocarbamide". One person spilt some in a lab setting and was confused as to why everyone around him was talking about the smell.

Another example is how certain vegetables, such as spinach, taste terribly bitter to some people and perfectly fine to others. A common trope is a mother screaming at her son/daughter to eat their veggies while the dad sits quietly because he also hates spinach.

Mom lacks the genes which make them taste bad while the children and dad do. The kids aren't just being picky, they literally taste significantly differently between mother and child.

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BizzarduousTask t1_j9tuhf1 wrote

And cilantro! For a not-insignificant number of people it tastes like soap!

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Elegyjay t1_j9ublnv wrote

And broccoli, cauliflower and Brussels sprouts... the very smell of them cooking makes me gag and I have tried to eat them but can't keep them down.

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GypsyV3nom t1_j9tsldx wrote

Taste and smell have some pretty insane under-the-hood processing that occurs in the brain. There are only about 400 different receptors in the human nose, but the different responses of those 400 receptors to odorants allows humans to detect over one trillion distinct scents. And that's just for humans, who have a relatively poor sense of smell compared to other mammals

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adhocflamingo t1_j9u3f3q wrote

Do I understand correctly that there are people who can smell asparagus pee but don’t produce it themselves?

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CharlesDuck t1_j9uvse7 wrote

There are two separate superpowers. The ability to smell asparagus pee, and the ability to produce asparagus smelling pee. You can have both, one, or neither power.

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Dubanx t1_j9u8xsk wrote

And vice versa, yes. It's hard to distinguish these people, though, since they often don't know what asparagus pee smells like and have nothing to compare it to if they did smell it.

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oneAUaway t1_j9tlti0 wrote

Asparagus contains a disulfide compound called asparagusic acid, whose metabolites produce the strong odor. Other foods with sulfur-containing compounds (such as cruciferous vegetables like cabbage and broccoli) can produce foul-smelling urine as well, though as u/Dubanx notes, there is a lot of variability in the production and experience of those odors.

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lubacrisp t1_j9tmhnr wrote

The breakdown of aspargusic acid into sulphur products that readily evaporate. Sone people can't smell the smell and some people can't make the smell. Both genetic markers

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Kirmes1 t1_j9vjkk9 wrote

> some people can't make the smell

So ... does that mean that their body breaks down aspargusic acid in a different way or do they capture the sulphur products immediately or ...?

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TutorStriking9419 t1_j9tk1e8 wrote

Your example of beets reminded of one time when I hadn’t eaten beets in a while. Apparently my body doesn’t break them down well and I nearly ran to emergency.

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hellraisinhardass t1_j9uw6ga wrote

Yep been there. Took a massive rock climbing fall the day before, then had beets with diner. Went pee the next morning and was very concerned I had a kidney bleed. Luckily I remembered about the beets before I finished dressing and scheduling a Dr. Apt.

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Whako4 t1_j9rdquz wrote

Off topic but kinda adjacent, why when you mix a bunch of colors the corresponding mix always goes toward the colored brown?

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KeyofE t1_j9rmvyd wrote

Mixing colors from the opposite side of the color wheel (blue and orange, green and red for example) makes brown,so if you mix a bunch of colors together, you are likely going to mix some that are across from each other.

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Ausoge t1_j9sqkp9 wrote

The colour of an object is defined by the wavelengths of light it absorbs. When you add lots of colours together, the absorbtive characteristics of each are combined together and, generally speaking, the darker it gets the more colours you add. If you were to perfectly combine an equal ratio of cyan, magenta, and yellow (incidentally, CMY is the negative/inverse of Red/Green/Blue), you'd get black.

This is in contrast to coloured light, which selectively emits specific wavelengths, rather than selectively absorbing them. In the case of equally combining red, green, and blue light emissions, you end up with white.

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viridiformica t1_j9tg13o wrote

In part because 'brown' is a huge range of colours. Everything from dark red to dark green will be called 'brown' if it isn't highly colour saturated. Just about the only colour that won't be is blue, which is a fairly rare pigment in nature

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[deleted] t1_j9tfxzq wrote

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tailuptaxi t1_j9tkuxo wrote

I also have no gallbladder. As I understand, its function was a reservoir for excess and it does not actually produce anything (except painful spasms when it has stones.)

The rest of the biliary system continues to function normally and the liver produces bile.

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CoffeeWanderer t1_j9tn0x0 wrote

Wait, the pigment in urine caused by beetroot depends on individual? I thought it happened to everyone!

Should I be concerned that my urine is dark red each time I eat beets?

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BizzarduousTask t1_j9tvt4t wrote

Why do vitamins turn your urine into neon glow stick liquid?

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CrateDane t1_j9v2u3r wrote

Riboflavin, AKA vitamin B2, is intensely yellow and water-soluble, so whatever your body doesn't need just colors your urine.

It's even used as food coloring sometimes.

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Likeabalrog t1_j9txna1 wrote

What about people with Gilbert's syndrome? Does your body just pass the non processed bilirubin?

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Sable-Keech t1_j9r7uio wrote

Oh so that’s why the urine in the toilet turns more yellow the longer I pee.

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k_alva t1_j9th35d wrote

The volume is a large part of that. If it's a little pee and a lot of water, it's quite diluted. As you pee more, the pee to water ratio changes

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Alpacaofvengeance t1_j9tiyyt wrote

> This urobilinogen oxidizes with air once you pee and converts to urobilin which has a yellow color

So is the pee in your bladder not yellow? Does this oxidation happen instantly while urinating? Or is there oxygen in your bladder which means the oxidation is happening there?

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viber_in_training t1_j9two2v wrote

Huh, would not have guessed breakdown of blood cells was reaponsible not only for urine color, but poo color as well

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inventordude01 t1_j9u7rhd wrote

Dude, your knowledge is profound! I was just on a diet and my stools were concrete grey. (They are normal now that I'm off it, but I would really like an explanation of this.)

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ackermann t1_j9s8zk6 wrote

> and urobilinogen which ends up in your bladder

Huh, so your gallbladder is connected to your bladder?
I always thought it drained only into the intestine, and your bladder was filled only by your kidneys.

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meathole t1_j9s9p19 wrote

It’s not connected. Urobilinogen is excreted into your intestines where you reabsorb it into your blood and your kidneys filter it out.

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Lupicia t1_j9r10gj wrote

Pee isn't (just) from what you drink. There's not a 1-to-1. Eating and drinking goes through digestion. Food and liquids are absorbed in your small intestine, enter your blood stream, and are processed by the liver. Glucose and nutrients are transported to cells by your red blood cells. They also carry oxygen from your lungs.

Waste of various kinds, byproducts of cells using energy, enters your bloodstream and is carried by red blood cells to your lungs (carbon dioxide) for air-soluble waste and kidneys (urea, salts, etc) for water-soluble waste. Kidneys help keep the balance of water and salt just right. Water soluble waste happens to be yellow. It comes from cells metabolism, breathing, bile, excess soluble vitamins and nitrogen wastes (from protein) and it is dissolved in water.

The rest of what you eat and drink is digested, or is indigestible, and leaves as solid waste. Bile and dead red blood cells and fiber and other non-soluble waste becomes poop.

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[deleted] t1_j9s3k4m wrote

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green_print_business t1_j9slkpq wrote

The typical colour range of urine is from light yellow to deep amber. The urobilin pigment, commonly known as urochrome, is the main cause for this colour.The colour of the pigment depends on whether your pee is water-diluted or more concentrated. The colour of your urine gets lighter as you drink more water and become more hydrated.
The chemicals and pigments in the foods you consume, as well as the drugs you take, also affect the colour of your urine. These modifications are fairly common and generally pass quickly.
An underlying medical problem may be indicated by certain colour changes.

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aSleepingPanda t1_j9sznhp wrote

I'm going to post a comment I found on a chemistry youtube video. Basically the youtuber made a joke about how chemicals often turn yellow after reaction and a commenter explained why in chemistry terms.

"Regarding 13:23 - I'm sure you already know it, but for anyone who wonders: many organic stuff are yellow because they have a lot of conjugated bonds in them (in the case of the imine here the bezene ring is conjugated to the nitrogen via its free electron pairs). This causes the electronic excitation energy to drop, bringing it from the close UV range into the visible range, i.e. the molecule absorbs deep-blue/violet photons, and we're left with a yellowish-looking product."

Which in all honesty I don't know how applicable this is to the urine we produce but was interested if anyone could confirm or deny it for me here.

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Bad_DNA t1_j9qw2tc wrote

Most all of those pigments are either degraded or go out the other chute. Urine is the body's way of managing a variety of salts and soluble byproducts of protein catabolism. Urea in various concentrations is... yellow

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kovadomen t1_j9qyzvq wrote

The yellow color of urine is because of urobilin, the end product of bilirubin catabolism. One part of bilirubin is excreted by GI tract, where it gives the brown colour - stereobilin. The other part is excreted by the kidneys and the urogenital system, it gives the yellow colour - urobilin.

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Bad_DNA t1_j9r12w9 wrote

Pay attention kids -- this poster is right and I knew urea was colorless. Man, where did that brain fart come from.

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EmilyU1F984 t1_j9t8zcs wrote

It’s relevant but not the whole explanation, also all organic molecules get their colour from those conjugated double bonds, whether they are blue yellow or red doesn‘t matter.

It‘s just that yellow requires the lowest amount of bonds, so any break down products are likely have enough bonds to go from only absorbs UV to also absorbs blue.

In urine the natural healthy colour is made from the molecule that oxygen is transported around in your body.

It’s gets broken down to smaller bits, shorter conjugated electron systems hence yellowish colour.

Additionally: the way our body metabolizes random molecules, like pigments in plant food etc, is by oxidizing them into more water solubles derivatives.

This oxidation is usually pretty efficient at double bonds (which make conjugated systems) And if your body breaks the double bond in the middle of a conjugated system, you usually end up with something that doesn’t have a large enough conjugated system to even absorb blue. So it appears colorless.

Take the imine in your example: break that, and you end up with just a benzene ring, which is colorless.

That‘s why foods and other colorful substances rarely dye the urine.

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Inevitable_Bar1607 t1_j9uoc63 wrote

You don't pee out the liquid you drink. The kidneys act as a filter for blood and one of their most important jobs is maintaining the right amount of water vs. saltiness in your body. Too much saltiness is bad, and too much water is bad. If you have too much water, the kidneys will produce more-watery pee.

Pee is yellow because of the chemical urobilin. People will say urea is responsible, but this is not true. Urea is the main component of urine (pee) but has no color. The color is urobilin. If your kidneys decide you have too much water, then your urine becomes more watery and this dilutes the urobilin, reducing the yellowness of the urine. Similarly, if you do not have much water at all, then the urobilin will be highly concentrated giving a dark yellow/orange color.

If your pee is red, you almost certainly are peeing blood, which means something very bad is happening in your kidneys or bladder. Pee can rarely be other colors - if your pee is green, your kidneys are filtering something blueish into the urine stream, etc. This is not necessarily a bad thing.

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