I weigh about 80 kilograms. Most of that, let's say 64 percent, is water though you can't tell by looking. I mean, as organisms go, I like to think thatI look fairly solid. After water, the next largest proportion ofme is protein, about 16% not just in my muscles, but also in things like the tinysodiumpotassium pumps in my neurons, and the hemoglobin in my blood, and the enzymesdriving the chemical reactions in every one of my 37 trillion cells. Then another 16% of me is fat, which I'mtotally OK with;
Four percent of me is minerals, like the calciumand phosphorus in my bones, and the iron in my blood; and 1 percent is carbohydrates, most of whichis either being consumed as I talk to you, or is sitting around as glycogen waiting tobe used. But here's the thing: It's not like I just ate 80kilograms of food and then all this happened. Instead, my body, like yours, is constantlyacquiring stuff, extracting some of it to keep, burning some of it for energy, and gettingrid of the rest. But even the stuff that my body does holdonto doesn't last forever. Some of the chemicals that I absorb in my food eventually becomea part of me. But enzymes wear out, and membranes
break down, and DNA gets oxidized. So, theyget discarded. And then I need more of those chemicals toreconstruct the material that I've lost. As a result, over the course of my lifetime,my cells will synthesize somewhere between 225 and 450 kilograms of protein â€¦ That's like 3, or 4, or 5 separate me's just made of protein. And all of the protein and fat and carbohydrates nucleic acids that make up me, of course, come from food. Every organism has to keep taking in and breakingdown food, to keep resupplying itself with
the raw materials it needs to survive. And all that activity requires energy, whichwe also gain from food. So, how do our bodies actually convert whatwe eat into energy and raw materialsé The answer is a neverending series of reactionsthat are dedicated to doing two vital, and totally contradictory, things: One set of chemical reactions destroys thereactants that you give them, reducing big, complex substances into molecular rubble. And the other set reassembles that rubbleinto new and bigger products that are put
together again to make you. So our bodies are constantly reinventing themselves in a perpetual state of loss, but also always rebuilding. And even though all of this is happening at thecellular level, its consequences could hardly be larger. These two sets of reactions are where everythingthat we've learned so far about the digestive, endocrine, circulatory, and respiratory systems really starts to come together. Together, these processes make up your metabolism. Now the sciencey word metabolism hascome to have a meaning in popular speech, but metabolism isn't just one thing.
People talk about metabolism as meaning, like,how fast your body burns the fuel in your food, or how high your personal energy levelis. And that's fine for use by personal trainersand fitness magazines. But physiologically, metabolism really describes everysingle biochemical reaction that goes on in your body. And maybe more importantly, it reconcilestwo conflicting chemical processes that are always, simultaneously underway inside ofyou. One of those chemical forces is anabolism. Anabolic reactions construct things and consumeenergy.