Dear Science,

Why is pee yellow?

Urinator

Dear Science,

Fuck gout. Why do I get it?

Pained Toes

Dear Science,

Why can people live to about a hundred years or so? Don’t humans outlive their expected life span based on size and heart rate?

Father Time

The story linking these three questions together begins with the building blocks of DNA and RNA—the nucleic acids of life. The A (adenine) and G (guanine) in DNA’s ACTG contain nitrogen atoms in their molecular structure. As the cells in our body tear down and rebuild these molecules, a steady stream of old and tattered adenines and guanines must be disposed of. Humans turn these purines (the name for these nucleic acids in DNA and RNA, and more broadly a family of molecules from living things that includes caffeine and theophylline from chocolate) into uric acid. We then pee out this uric acid, helping create the color of our urine.

Gout occurs when too much uric acid builds up in our blood and starts to deposit all over the body as painful crystals. How does this happen? Uric acid is actually not that great of a waste product; even only modestly increased concentrations in the blood are sufficient to crystallize out. Anything that increases the concentration of uric acid (lots of cell death, too much coffee, too much red wine) in the blood can trigger a gout attack as the crystals precipitate out of the blood.

So why do humans use such a terrible waste product? Because of a mutation in a gene called urate oxidase. In almost all other animals aside from humans and our closest primate relations, urate oxidase changes the waste product uric acid into something much better: allantoin. Allantoin carries these waste nitrogens out of most animal bodies with far more grace and ease than uric acid. It’s next to impossible to crystallize allantoin out of blood in any concentration that you’d expect in a living thing. But because of the mutation in urate oxidase, humans cannot make allantoin. We’re left with peeing out uric acid—and all the gouty consequences. Instead of being filled with relatively safe allantoin, our blood is filled with relatively dangerous uric acid.

Which brings us to the longevity of humans—and our primate relations. As a consequence of burning up calories to live—like all living things—our metabolism creates a few unbalanced molecules (free radicals) that damage structures inside our cells. This damage, accumulating over time, partially causes aging. Uric acid is a damn good antioxidant—gobbling up the free radicals generated by metabolism. By having tons of uric acid floating in our blood as compared to most any other animal, humans are filled with antioxidants. It’s hypothesized that’s one reason primates can cheat death better than our animal brethren. Here we can see how evolution can take mutations that seem harmful at first and use them as the starting point of a new path that can lead to a new advantage.

Voidingly Yours,

Science

Send your science questions to dearscience@thestranger.com.

Jonathan Golob is an actual doctor.

10 replies on “Dear Science”

  7. Because the yellow colour in urine doesn’t come from uric acid. It comes from the breakdown products of heme, the molecule that carries oxygen in the blood. As heme, it’s red and blue, depending on whether or not it’s carrying oxygen. As it breaks down, it turns green, and eventually yellow (which is what you’re seeing as a bruise fades). These breakdown products are secreted in bile into the intestines, broken down further in the gut by your gut flora, and a portion of them are reabsorbed into your bloodstream. At this point, they are now water soluble enough to be excreted in urine. The yellow-brown colour of feces is caused by these same compounds.

    So #6 is right, while urine does contain uric acid, the yellow colour has nothing to do with uric acid.

  9. Humans don’t really create a terrible waste product (unless you have gout). Most nitrogen containing compounds (and the ones that we get rid of in spades, like amino acids from proteins) are broken down and the ammonia is removed as urea which is no problem for the body. Uric acid is the by-product of purine metabolism and most purines get recycled by the body. Pyrimidines (the other stars of the DNA/RNA show) are also metabolized to yield ammonia, which is excreted from the body as urea.

    Now, while everybody metabolizes purines, pyrimidines and proteins, only a small percentage of the population has gout.
    If this really was such a faulty evolutionary change become advantage, we would all have gout because there would be so much uric acid in our bodies. Which for most people, it isn’t a problem.

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