Complete proteins and your liver

Complete proteins

It seems like any time I’ve eaten rice and beans, someone inevitably points out that what I’m eating is a “complete protein,” which sounds impressive. And then in reply, someone will say something about how an avocado is a complete protein all by itself. But what is a complete protein anyway? Well, it turns out that being a “complete” protein isn’t that special at all, unless you’re a vegetable. Basically any meat you eat, for example, is also a complete protein. But why do we need protein anyway? My first thought was that we needed protein for energy. But it turns out the primary way your body uses protein has nothing to do with energy at all.

The central dogma of biology says that DNA makes RNA and RNA makes proteins. More specifically, every three base pairs (A, C, T, G) of RNA specifies a particular amino acid, and a protein is simply a folded up sequence of amino acids. Because each amino acid is defined by a sequence of three base pairs, and there are four base pairs to choose from, this means there could be 4^3 = 60 different amino acids. Instead, some sets of three base pairs end up coding for the same exact amino acid, so that in the end, there are only 21 amino acids. In any case, to convert each of these base pair sequences to an amino acid, you need a tRNA molecule carrying that amino acid to float over into the ribosome. If you think of the protein-making process like baking a cake, tRNA molecules are like little helpers that bring you the right ingredient (e.g., baking soda) once you get to the right step in the recipe.

Proteins are what makes up your skin, bones, organs–nearly everything in your body that isn’t water. But proteins degrade and need to be replaced, which means your body is making proteins all the time. To make these proteins, you need a constant supply of those 21 amino acids. To continue the cake analogy, it’s like you’re constantly baking different types of cakes, which means you also constantly need to buy more baking powder. Of those 21 amino acid ingredients, your body can actually make 12 of them itself. But 9 of these amino acids must come from outside–i.e., you need to eat them.

So it turns out that the reason you need to eat protein is because you need to eat amino acids. 9 of them, to be precise. Just like baking cakes requires more flour than baking soda, your body needs those 9 amino acids in different proportions. Any protein source (e.g., rice and beans) that provides you with the right proportions of those 9 essential amino acids is called “complete.” ¹

The liver

Your body is making lots of proteins using the recipes listed in each cell’s DNA. But it turns out that not everyone is sharing the load. In fact, of the 20,000 or so different proteins your body makes, your liver makes about 60% of them.

Your liver! I couldn’t even remember what the liver does. Which is probably because your liver does about 500 different things (and that’s not a made up number). To summarize though, I think a fair word for what the liver does is “metabolism.” This is a very vague word that includes a range of different processes like digestion, protein synthesis, and the elimination of waste. This includes more fun functions such as breaking down drugs like alcohol.

In fact, it’s your liver that gets all the amino acids from the foods you eat (after being broken down from proteins by your intestines) and decides what to do with them.

Given that I know nothing about organs or how they work, I was surprised to learn that the liver is also your heaviest internal organ. But even more interesting to me was that organs are taught in medical school almost as if they are “functions” in a programming or computational sense. For example, the liver has two inputs (one artery, and a vein called the “portal” vein), one output (a different vein), and in between it applies some function. By “apply some function,” obviously this isn’t something like “2+2,” but rather, it can either remove something from the blood (e.g., waste?), or add something to it (e.g., a new protein). You can even “compose” functions of multiple organs by chaining them up in series. For example, the heart sends blood to the stomach and intestines, which apply the function of “absorb food,” resulting in various nutrients (e.g., amino acids) being added to the bloodstream. The outputs of the stomach and intestines are then sent to the inputs of the liver (the portal vein I mentioned earlier), which in turn applies its own functions (e.g., using the amino acids to make new proteins).

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Notes