By Dr. Ian Halim

You’ve probably heard the expression, “like oil and water,” to describe two people who don’t get along. It’s not just a throwaway phrase. The opposition between oil and water is key to understanding how living things work – including, especially, an under-appreciated bodily fluid called bile.

But before we get to that, you can test the idea that oil and water don’t mix, using just what’s in your cabinet at home. Try pouring vegetable oil into a glass of water, for instance. You’ll see pockets of oil form, or a separated layer of oil. Like is attracted to like, with oil sticking to oil, and water sticking to water. And unlike repels unlike, making oil and water separate. If you try to rinse a greasy pan in the sink with just water, it won’t work. The water will flow over the oily surface, without really mingling with it. The water doesn’t seem to get in there and shake up the oily gook.

But why? To explain this, we have to dip into chemistry. Water is made up of tiny particles too small to see with the naked eye, called molecules. And water is polar, meaning that one part (or pole) of each water molecule has an electric charge that’s more negative, while the other part is more positive. The more positive hydrogen atoms of one molecule attract it to the more negative oxygen atoms of nearby molecules. The same principle explains why substances that divide into tiny electrically charged ions, like table salt, dissolve in water.

This starts to explain why water sticks to water, but it doesn’t explain why oil sticks to oil, or why water and oil repel each other. Oil molecules are different from water. They are nonpolar, with a pretty even distribution of electrical charge, so they have only very weak electrical attractions to one another. Intro chemistry courses sometimes use these weak attractions to explain why oil sticks together and separates from water. But there is a better, more accurate explanation. It’s a little tricky, but oil and water’s mutual repulsion is so important in biology that it’s worth trying to be accurate.

When water and oil separate, each water molecule gains the opportunity to interact with a larger number of adjacent water molecules–like extroverts at a party separating from introverts, so that they have more extroverts to talk to. And it’s actually this freedom for more different water-water interactions that favors the separation. In chemistry and physics, arrangements of matter and energy tend to become more disordered over time–a principle called entropy. But if oil molecules were more widely dispersed in water, these scattered and isolated oil molecules would block many of the polar attractions between water molecules, creating a more ordered system, with less freedom for different water-water interactions–like introverts blocking extroverts from talking. The tendency toward entropy or disorder opposes this kind of limitation on water molecules’ freedom to interact. So, water and oil separate, and water molecules have a field day with disorder and entropy, relishing their various polar interactions. If this seems too difficult, that’s why intro chemistry classes usually fudge it.

This isn’t just about oil and water. Other biological substances are broadly categorized as fat-soluble or water-soluble. Or, to use the technical terms, hydrophilic substances are attracted (-philic) to water (hydro-) and lipophilic substances are attracted (-philic) to fats (lipo-). This is one of the basic organizing principles of biological systems, explaining how many things work in the human body. For instance, water soluble wastes are more readily eliminated in the urine, which is mostly water. And fat soluble substances–like steroid skin ointments–pass more easily through the non-polar surfaces of cell membranes (such as the surfaces of skin cells).

Soaps, however, are different–and now we’re starting to approach bile, our main subject here. The molecules that make up soaps have polar portions, with a positive or negative electric charge, as well as non-polar portions, without an electrical charge. This dual characteristic is known as amphiphilic, meaning “liking both.” Because amphiphilic substances, like soaps, have this dual quality, they can mix with both water-soluble substances as well as non-polar, fat-soluble substances. But, even more than that, amphiphilic substances allow these water-soluble and fat-soluble substances to mix with each other. That’s why soap is so helpful for cleaning food off of dishes. Food contains water-soluble, hydrophilic substances (like sugars), as well as oils, fats, and other fat-soluble, lipophilic substances. Adding soap to a dirty dish allows grease and water to mix freely so that these things may be more readily cleaned off.

The food we eat, just like the food residue on the dishes piled up in our sinks, contains a mixture of water-soluble and fat-soluble substances. So in order to extract energy and nutrients from our food, fat-soluble and water-soluble substances must be able to mix freely with one another in our gut too. Therefore, we need something similar to dish soap within us–a kind of internal soap–not for cleaning, exactly, but rather to allow these water-soluble and fat-soluble substances to freely intermingle.

In this sense, bile is the answer. Bile salts are the amphiphilic, soap-like ingredient that allows grease, oil, fats, and water-soluble substances to mix freely with one another, so that we can absorb fats and fat-soluble vitamins. This special stuff is made by the liver, from which it either drains through bile ducts and into the small intestine, or collects in a pouch-like structure called the gallbladder. When bile is needed, the gallbladder contracts, forcing a squirt out into the small intestine.

Bile’s fat-dissolving function is crucial, allowing us to harness not just energy from dietary fat, but also the critical fat-soluble vitamins A, D, E, and K (medical students rely on the word-like mnemonic, “ADEK”). Vitamin K, for instance, is essential for the proper clotting of blood. So when we’re deficient in vitamin K, our blood can’t block off the flow from a small cut, and we’re at risk of it becoming a runaway hemorrhage.

Of course, bile has other functions too. Red blood cells are packed full of a substance called hemoglobin that carries oxygen from our lungs out to our tissues. When our body retires red cells, after about 120 days, we eliminate hemoglobin as waste. As part of this process, we convert it into bilirubin, which gives bile its characteristic green-yellow color.

Bile also dissolves lipophilic cholesterol and excretes it in our stool. And it helps kill any bacteria introduced into our gut by what we eat and drink. And it’s also alkaline or basic–the opposite of acidic–so it helps neutralize the stomach acid entering the small intestine to prevent acid damage and ulcers. It does many important things.

Bile can also be involved in diseases. Normally, it’s a green-yellow liquid, but it can thicken into sludge, or even harden into bile stones, known as gallstones. Sometimes these stones are harmless, but sometimes they can be very painful when the gallbladder squeezes against them. Gallstones can also block the tubes that carry bile into the small intestine, even plugging up these ducts and obstructing the flow of bile. The presence of gallstones in the bile ducts has an unwieldy name, “choledocholithiasis” meaning bile (chole-) duct (docho-) stones (-lithiasis).

When the flow of bile is blocked, bilirubin can accumulate in the body, even yellowing the whites of the eyes, the underside of the tongue, and the skin (a condition called jaundice). Bile stones can also make it easier for the gallbladder itself to get infected, causing cholecystitis (a word meaning gallbladder inflammation, but usually referring to the infection of the gallbladder, specifically).

For now, though, you would do well just to remember that one of the most important things that bile does is act like a soap, allowing all the fat-soluble and water-soluble things in our food and drink to intermingle with one another, so that we may absorb all of those intermixed nutrients, and excrete cholesterol. The next time you eat a rich and fat-laden piece of chocolate, you can thank (or curse) the bile that permits you to harness all that energy. In our gut, oil and water can mix after all.

Somerville Bagel Bards member and physician-humanist, Ian Halim, writes about how medicine relates to everything from ethics to botany – aiming to make science accessible to the widest possible audience. Ian earned his PhD in Greek & Latin literature and his MD at Columbia University in New York City and is now training at a hospital in Boston.