"The kitchen's a laboratory, and everything that happens there has to do with science. It's biology, chemistry, physics. Yes, there's history. Yes, there's artistry. Yes, to all of that. But what happened there, what actually happens to the food is all science." - Alton Brown
Metabolism is one of those things that people talk about a lot. But as much as they talk about it, they don’t always have a complete understanding of what it is. Many like to talk about it in terms of speed (fast vs slow), but when asked what metabolism means, the majority will probably talk about its relationship to how quickly you burn fat or how quickly you burn calories. This isn’t entirely untrue, but it’s only part of the picture. There’s a good reason people have a hard time understanding and describing metabolism. It’s a complex network of biochemical reactions. In fact, metabolism is simply the collection of reactions that occur in a living organism. This includes reactions that break molecules down, a process called catabolism, and reactions that build molecules, anabolism.
In medical school, we dedicate an entire block, several weeks, of our first year to studying these reactions, and even then it’s a cursory overview. There are scientists who spend their careers investigating just a portion of this complex process. We may try to simplify the process: carbohydrates are converted into energy, fats are stored energy, and proteins make muscle. But in doing so we lose sight of the intricate web that is human metabolism. For those interested in a more detailed and interactive representation of the complexity of human metabolism visit reactome.org.
Image source: KEGG Pathway
Macronutrients or “macros” is a term that gets thrown around a lot when people are talking about nutrition. But what exactly do they mean? Macronutrients are the major categories used to classify some of the main sources of energy in the web of metabolism. Think of them as the starting points to the various metabolic pathways. When talking about macronutrients from a dietary standpoint, we are generally referring to carbohydrates, proteins, and fats. They are the basic fuels that our bodies utilize. Technically, alcohol is another source of energy. However, not all energy sources are created equal, and thus the amount of energy that we derive from them is variable. While biochemists refer to energy in terms of a high-energy molecule called adenosine triphosphate (ATP), many people are more familiar with measuring energy in terms of calories. Nutrition labels and the treadmill’s screen are simply telling you how much energy is being eaten or burned, respectively. Hover over the boxes below to reveal how many calories each macronutrient has in one gram.
Carbs
4
Proteins
4
Fats
9
Alcohol
7
As you can see, energy is derived from each macronutrient. That’s because while carbohydrates are considered to be the primary energy source of our cells, what goes into our bodies as a carbohydrate, fat, or protein, doesn’t necessarily stay as such. There is an ebb and flow between catabolic and anabolic pathways. Catabolic pathways are those that break down molecules and release energy. Think of these as occurring during times of activity, when energy requirements increase, such as running on the treadmill or lifting weights. On the other hand, anabolic pathways are those that build molecules and store energy. As you might guess, these occur during times that energy requirements are low or there is an excess of energy available, so the body converts it to fat for storage in fat cells, glycogen for storage in the liver, or protein for growing muscle tissue. These pathways are all coordinated by a host of signals that are released, turning on those enzymes that lead to catabolism or anabolism. Some of these signals include insulin, glucagon, epinephrine (adrenaline), steroids, sex hormones (testosterone and estrogen), and thyroid hormones.
As I previously mentioned, there is a fair amount of crossover between these major pathways. Proteins are broken down into a collection of amino acids, and while these amino acids can certainly be used to build more protein in your body, they can also be broken down further and chemically altered to enter the same metabolic pathway that converts carbohydrates into energy. This allows us to derive the 4 calories of energy per gram of protein as noted above. Amino acids enter this pathway in the form of the molecules pyruvate and acetyl-CoA. These two molecules, especially acetyl-CoA, are the link between these macronutrients within the network of metabolic pathways. It is through these molecules that both glucose from carbohydrates and amino acids from proteins can also be converted into fatty acids to be stored in fat cells. Fatty acids generally can only be stored in fat cells or converted acetyl-CoA for energy production. Fatty acids cannot be converted into protein or carbohydrates the way that proteins and carbohydrates can be converted into fatty acids. This explains why consuming more calories, regardless of the type of macronutrient, can result in weight gain and increased body fat. The same is true for alcohol, which through a series of oxidative reactions is converted into acetate and then further converted into acetyl-CoA, underpinning the effect that alcohol can have on weight gain as well. Below is a video that I came across in medical school, which does a great job of explaining the concept of metabolism and how the three main macronutrients are related to one another.