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Body odor is usually normal, but when it's extreme it can be a sign of something gone wrong.
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Isovaleric acidemia

"Mister Exposition" Kevin MacLeod (
Licensed under Creative Commons: By Attribution 3.0 License
Hank: So, you know in Game of Thrones, there’s this character—smells really bad, they call him “Reek”? Turns out, that could actually be caused by medical conditions that we know of, medical conditions that cause you to emit odors that go way beyond the typical stinky armpit. In some cases, you might reek of boiled cabbage, or sweaty feet, or even rotting fish.

These conditions are rare, but their symptoms can be pungent, and sometimes also downright dangerous. Unusual body odors are often a sign of a bigger problem—specifically, a defect in the way your body is breaking down, or metabolizing, your food.

For example, there’s the condition known as trimethylaminuria— also known as “fish odor syndrome.” Patients with this condition are said to smell like decomposing fish, because their bodies don’t break down a compound called trimethylamine, which emits the je ne sais quoi of fishiness.

Now, everyone’s body produces trimethylamine—specifically, in the gut, where bacteria excrete it while helping us digest foods like eggs, liver, and fish. Normally, having all that trimethylamine in your body is not a problem, because it’s converted into an odorless molecule, thanks to a special enzyme in the liver, known as a flavin-containing monooxygenase.

But people with fish odor syndrome can’t metabolize the smelly compound, because they have mutations in the gene that produces that enzyme. Without enough of that working enzyme, the trimethylamine builds up, and has nowhere to go but out with your bodily fluids—in your sweat, urine, even on your breath.

But people with the condition do have some options. They can change their diets so there are fewer of the precursor chemicals that get broken down into trimethylamine. It’s one of the only times your doctor will actually tell you not to eat your broccoli, or your Brussels sprouts! Infusions of antibiotics can also help wipe out some of the bacteria that are making the trimethylamine.

These rarely solve the problem entirely, but the good news is that apart from the smell, there isn’t any major health problem associated with fish odor syndrome. Which is not the case for a disorder that gives people the distinctive whiff of sweaty feet.

This condition, known as isovaleric acidemia, can cause brain damage, and even death, particularly in young children. Here, patients have a genetic mutation that leads to a deficiency in an enzyme called isovaleric co-enzyme A dehydrogenase. This enzyme is important because it helps break down the amino acid leucine. Without this enzyme, leucine can only be broken down part-way, and the compound that’s left over from this process, an acid called isovaleric acid, starts to build up.

Isovaleric acid smells kind of like cheese, and it’s the same chemical that makes your sweaty feet smell. The bacteria hiding out between your toes produce this acid when they’re chomping away on leucine.

But while isovaleric acid isn’t exactly pleasant to smell outside your body, it can be downright damaging to the inside. It’s not exactly clear why, but a build-up of isovaleric acid tends to have the most dramatic effects on the central nervous system. In large amounts, it’s toxic to neurons, which can result in developmental delays in many patients. And because this enzyme deficiency makes it difficult to digest breast milk or formula, dangerous symptoms can start appearing very soon after birth.

In severe cases, infants just a few days old will refuse to eat and begin to have seizures. There is, so far, no cure for isovaleric acidemia, but some treatments— like avoiding foods rich in leucine, and taking supplements of other, non-threatening amino acids— can help keep patients safe.

Finally, peculiar symptoms and even stranger smells can result from another, similar disorder known as hypermethioninemia. In this case, the problem is having too much of a different amino acid: methionine. Methionine is the rare amino acid that contains sulfur, an element known for its pungent odor.

And when methionine isn’t metabolized properly in your body, it can result in large amounts of dimethylsulfide, which produces a smell similar to boiled cabbage. Sometimes the condition comes about just because you’ve eaten too much methionine, which is in protein-rich foods, like meat and cheese. But if the cause is genetic, it can be due to mutations in one of several genes that are responsible for making the enzymes that help break down methionine.

Without those enzymes, patients sometimes have that cabbagey smell in their sweat, breath, or urine. And strangely, not everyone with the disease has symptoms—in fact, most people don’t. But in some, it can be serious.

In severe cases, the inability to process methionine can lead to neurological problems and muscle weakness, among other problems in the nervous system. Again, treatment usually involves avoiding foods that contain methionine, as well as taking supplements to make sure that the body is getting what it needs. So, run-of-the-mill BO is nothing compared to the very real medical conditions that can create unpleasant smells.

There are a lot of things that can go wrong when your body metabolizes food, and weird odors are just one way to help spot and diagnose them.

This episode of SciShow is brought to you by 23andMe, a personal genetic analysis company created to help people understand their DNA. The name "‘23andMe" comes from the fact that human DNA is organized into 23 pairs of chromosomes. Through genetic analysis, 23andMe users can see which regions around the world their ancestors came from, and learn how their DNA influences facial features, hair, sense of taste and smell, sleep quality and more. You can also connect with people who share similar DNA; and also learn how your DNA may influence your health and wellness.

I've been wanting to do this for so long, and I haven't, and I don't know why. Oh, this is gonna be hard with the green screen. Woo, woo, where is it? They're gonna tell me about me... from my spit.

What? What's the liquid in the funnel for? Oh, woah, did I break it? ...Okay, it's fine. Everything's fine. Alright.Now I just spit in it.

By sending in my saliva, I'll have the opportunity to learn about my health, ancestry, and personal traits through my DNA. I'll also learn about my genetics related to muscle composition, lactose intolerance, and caffeine consumption. Once I mail in my kit, I’ll have the results in a few weeks. To do the same—and to support SciShow —pleasecheck out