by E. Paul Zehr
No discussion of diet would be complete without vitamins and minerals. We have already seen how useful calcium is for muscle and bone. Vitamins and minerals are also needed to ensure that all the energy pathways work properly. They act as facilitators (actually as enzymes and catalysts of reactions) of your metabolic pathways. You get most of them from your dietary intake. Some key fat-soluble vitamins (A, D, E, and K) are absorbed with fats. Many of the vitamins that Batman’s body needs and must be in his diet are shown in Figure 6.2. This figure also shows some of the main functions of the vitamins. Minerals such as iron and calcium are absorbed and linked to the needs of the body. If you look at food labels when shopping or eating, you will see vitamins listed. For example, niacin is found on breakfast cereal labels. When you eat niacin, remember that it is an important vitamin for energy production, the function of your muscles, and the health of your skin!
Figure 6.3 shows how minerals help with metabolism for both catabolic and anabolic activity. (Recall that anabolic activity creates and that catabolic activity breaks down. I wonder what batabolic activity would do.) Some minerals, like magnesium, potassium, calcium, and manganese, are dual workers—they are involved in both building up and breaking down. By the way, Batman seems well aware that he needs these vitamins and minerals in his diet. In a panel in the graphic novel Batman: Venom (1993), Batman is talking with Commissioner Gordon, who comments on the increased “muscling” of Batman’s build. Gordon actually tells Batman that he looks “Bigger. Beefier.” Batman makes an off-the-cuff joke about switching cereals: “I’m eating the kind that’s fortified with vitamins and minerals.” Unfortunately, as we will discuss later in Chapter 13 in the context of steroid use in athletes, the reason for the change in Batman’s physique is much more sinister. However, don’t worry about any enduring transformation in the Dark Knight’s character—the change is temporary!
Figure 6.2. Some of the functions of vitamins in the body. Data from McArdle et al. (2005).
Figure 6.3. Utility of vitamins and minerals in anabolism (processes that build up) and catabolism (processes that break down) in the body. Data from McArdle et al. (2005).
At this point, I have probably convinced you that a fair amount of work actually goes on inside your body at all times. You may be wondering where the energy for the biological work comes from. It may come either from absorbed nutrients that we get from eating food or from stored nutrients that we packed away earlier from anabolic processes. Usually the metabolic state of the body is broken into two modes: fed or fasted. Now, fasted doesn’t necessarily mean “fasted,” as in not eating for days on end. Rather, it defines the period during which you aren’t eating and nutrients aren’t being absorbed into the body.
In the fed state, nutrients from whatever meal Batman has just eaten are being digested and absorbed. The fats, carbohydrates, and proteins that were in Batman’s food are initially broken down into their basic components (refer back to Figure 6.1). This is an anabolic state in which energy is transferred from the components of the food into something your body can use directly in metabolism—that is, free fatty acids, glucose, and amino acids—and then used up or stored away. This contrasts with the fasted state in which nutrients from the meal are no longer in the blood. This occurs typically within a few hours of eating. The fed state is a catabolic one in which larger molecules and their stored energy are converted into smaller ones to be metabolically used for energy. A superficial overview of what happens to molecules in our food is that they are metabolized quickly (as in blood glucose used by the nervous system), used in the creation of other molecules, or stored primarily as glycogen and fat. As you saw in Figure 6.1, most of the carbohydrate winds up as glucose and is used up throughout the body and in the nervous system. In fact, about 30% is used for liver metabolism and the remaining roughly 70% is used for nervous system, muscle, and other tissues.
The liver is very important because it is the great carbohydrate “storage tank” in the body. Actually, your muscle tissue is an even bigger tank for storing carbohydrates. Much of the excess glucose gets packed away as glycogen and stored in the liver for use later. In addition, glucose is stored in all of your muscles as glycogen. If you look back at Figure 6.1 again, at the bottom the boxes with dotted lines show the “storage” fate of nutrients. The trick is that the glycogen in your muscles can only be broken down and used up by that same muscle, whereas your liver breaks down glycogen when it needs to and releases glucose into the blood to go everywhere. Whatever glucose remains is stored away as fat.
Proteins are absorbed and broken down into amino acids. Amino acids are sometimes used as cellular energy but typically only when glucose levels are extremely low. Starvation represents probably the best example of a time when protein would make up the bulk of the energy needs. Excess protein beyond that needed for cellular building and repair is stored as fat. Fat is stored as fat in liver and in adipose tissue.
The key trigger for the fasted state is a drop in glucose levels in the blood, which signals the end of the fed state. This is kind of like the fed state being the eating of a meal plus the full cleaning up after. As soon as everything is put away and cleaned up and every last scrap of food—including leftovers on dirty plates—is taken care of, you are entering the fasted state. This is a state of catabolism. I mentioned that the level of blood glucose was so important because your nervous system needs it, and your nervous system is so important because one of its main functions is to maintain homeostasis. And without homeostasis, life isn’t possible for very long. So, the drop in glucose is something your body closely monitors. In the fasted state the liver is the primary source of glucose. In fact, byproducts of other metabolic activity in muscle are sent to the liver to be made into glucose.
Our systems work mostly in balance. In biology this energy balance is reflected in the amount of oxygen our cells consume. You may be surprised to know that this brings us to the question of how much energy is there in the food that we eat. This is similar to asking how much energy is in the gasoline we use for fuel in our vehicles. Indeed, the process of consuming fuel in a car engine is similar to the principle of consuming foodstuffs to run our cellular engines. Both involve a kind of burning known more properly in chemistry as “oxidation.” Measures of overall metabolic energy content can be made directly using a technique called “bomb calorimetry,” described in detail below. This name refers to the unit used in describing the energy content of food: the calorie, which is defined as the amount of heat energy needed to raise one gram of water by one degree Celsius. This definition isn’t particularly useful for metabolic calculations and nutrition however. In nutritional applications, energy is instead calculated from the composition of the foodstuffs consumed. In these calculations a calorie (or kcal) is used, which is equal to approximately 4.18 kJoule, if you are keeping count.
We typically think of the energy content of food we eat as coming from protein, carbohydrate, or fat (remember Figure 6.1). Conventionally, proteins and carbohydrates have been described as having about 4 kcal per gram (which is about a third of an ounce) and fats as having about 9 kcal per gram. Soon we’ll tackle the question of why there are 9 kcal in fat and only 4 in carbohydrate or protein. But the precise measurement of energy content of food can be taken using the bomb calorimetry mentioned above. This involves placing the material inside a chamber and burning it while measuring the total heat energy released. You could do this with everything you wanted to eat or drink, if you had a bomb calorimeter. However, by convention we use the estimates of energy content in food based upon the approximately 4 or 9 kcal per gram given above. In that case, all we need to know is the mass of the food and the relative proportions of fat, carbohydrate, or protein it contains.
So now you have an idea of how to measure the energy we take in when consuming food. How do we get a handle on the energy we use just being alive? Answering that question will guide us further toward answering the question we are actually most concerned about. How
much energy does Bruce Wayne expend while being Batman?
To find the answers we need to know about metabolism in our cells and in particular about the concept of cellular respiration. Respiration is the process by which biochemical energy in the food we eat is transformed into energy that can be used in our cells. In addition to producing energy, respiration produces carbon dioxide, water, and heat. Above we started discussing energy in food and the caloric content of foods that we consume. Energy in the foods we eat is in the wrong format for our cells to use. To transform the energy sources from food into cellular energy means turning those carbohydrates, fats, and proteins in our dietary intake into a form that our cells can understand—molecules of ATP, the high-energy compound that we were introduced to in Chapter 4 and saw again earlier in this chapter.
A currency exchange analogy is good to use here. As such, ATP is the local currency of our cells. However, your body takes in “tourists,” who “pay” with carbohydrate, protein, and fat, which then have to be converted into ATP dollars. There’s a different but fixed exchange rate for each one—that’s where the 4 versus 9 kcal per gram part comes in—but the end objective is always to get that conversion into ATP. Throughout both the plant and animal kingdoms—from the bat flower Tacca chantrieri, to flying bats, and then onto Batman—ATP is the universal energy currency. The reason that the exchange rates are not the same for the three main types of food has to do with the biochemical makeup of protein, carbohydrate, and fat and their basic elements: amino acids, glucose, and fatty acids, respectively. Fatty acids are the most complex and contain the greatest number of chemical bonds. Because potential energy is stored in the chemical bonds, more energy is found in fat. In contrast, protein and carbohydrate are simpler molecules with respect to their chemical bonds. Because of that, less energy can be stored or extracted from them. Regardless, we can still consider all of them to be consumed in cellular respiration in generally the same way that gasoline or diesel fuel can be consumed in the engine of a vehicle. In both cases oxidation occurs, and the oxidation liberates the energy contained in the biochemical or petrochemical bonds. Unlike an internal combustion engine, such as the one you have in your car that relies on oxygen, cellular respiration can proceed both with and without oxygen. The terms aerobic (with oxygen) and anaerobic (without oxygen) respiration or metabolism are used when describing these two sets of pathways.
Aerobic metabolism occurs in cellular structures called “mitochondria,” which are our cellular engines and power-generating stations and produce the bulk of our energy needs. These mitochondria create large numbers of ATP molecules by the oxidation of glucose (mostly), fatty acids, and amino acids (not as much). Despite the fact that mitochondria are found inside our cells, they have their own DNA and genetic history independent from the genome of the cell nucleus. Some evidence suggests that mitochondria were once independent single-celled bacterial organisms that were incorporated into our cells about two billion years ago.
Anaerobic metabolism occurs also inside the cell, but it occurs outside the mitochondria in the spaces around all the other organelles. Anaerobic metabolism is extremely fast but much less efficient than aerobic metabolism. In muscle, one glucose molecule can provide up to 36 ATP molecules from aerobic metabolism, but only two ATP molecules are generated from the same glucose molecule during anaerobic metabolism. By the way, your heart muscle does even better with aerobic metabolism, yielding 38 ATP molecules in the same process. Overall, anaerobic pathways are supplements to the bulk of energy production obtained during aerobic metabolism. To return to our car engine analogy, consider fuel economy. If you went around in your car always accelerating to top speed after every stop, you would have very poor fuel economy. In contrast, gradually increasing speed to some constant is much more efficient. Well, of course you cannot always gradually accelerate your car. Nor can Batman slowly move around all of the time. All of us need to produce extreme bursts of high-speed maximal effort from time to time. The point is that if maximal efforts are all that is done, we will run out of fuel very quickly.
Carrying on with our theme of threes that runs throughout this book, we can think of there being three related but functionally different energy systems. They are related to the anaerobic and aerobic concepts mentioned above and can be thought of as immediate, short-term, and long-term energy systems. These descriptions refer to the time period over which you might do an activity and which energy system would be most used in that activity. Immediate energy comes from the splitting of ATP-CP (the CP here refers to creatine phosphate, an energy source found in skeletal muscle), short-term from the anaerobic breakdown of glucose (called glycolysis), and long-term from the aerobic metabolism of glucose, fatty acids, or amino acids.
Why have all these different fuel stores? It might seem to be easier if the analogy to the tank of gasoline was actually easier to deal with. You don’t have special immediate, short-term, and long-term energy tanks to supply fuel in your vehicle, so why do you have that kind of setup in the body? The answer lies in how the energy systems work and the immediate capacity each has for energy production.
The contribution for each system is as generally shown in Figure 6.4. The duration of exercise activity is shown at the bottom of the figure going from seconds up to five minutes or more. A quick glance will tell you that for maximal efforts, like running as fast as you can, much of the energy comes from the immediate energy system, which mostly uses anaerobic metabolism. At the far right you can see that if you go for an eight-kilometer (five-mile) run (which will take much more than the five minutes indicated) you will use primarily the long-term energy system, which mostly uses aerobic metabolism.
To put this in a frame of reference for Batman’s activities, let’s talk about “The Lazurus Pit” (Batman #243, 1972), drawn by the great artist Neal Adams. In this story Batman is—once again—fighting for his life against a misguided henchman of R’as al Ghul. They close to grappling and fighting distance and Batman succeeds in disarming the semi-evil bad guy. I say semi-evil in this case because the henchman has been coerced into attacking Batman. We read at the bottom of the panel that it has taken exactly four seconds for this exchange. The length of time Batman needs to fend off evil and semi-evil bad guys was also nicely described in the 2003 graphic novel Trinity, which was the first Batman, Superman, and Wonder Woman combined story. In that story Batman is shown dispatching a small gang of about six punks. The caption reads that it took “thirty-two seconds to dispatch a roomful of armed and dangerous men.” This is an accurate assessment of the time that Batman would spend on directly fighting individual attackers. This means he makes frequent use of his immediate and short-term energy systems and anaerobic metabolism.
Figure 6.4. Physiological energy systems and their contributions overtime (in seconds). ATP-CP = Adenosine triphosphate—creatine phosphate (an energy source created by the body). Data from McArdle et al. (2005).
To further help understand the interactions of the different energy systems for Batman’s activities I have broken down three fighting scenarios of different durations in Figure 6.5. For Batman against Killer Croc, I have estimated a time of 10 seconds, and for Batman versus Bane I reckoned on 60 seconds. Fighting Catwoman was right in between at 30 seconds. The thing to notice is changes in the various proportions of the different energy systems being drawn upon as the exchanges become longer.
Figure 6.5. Breakdowns of the likely energy sources for fights between Batman and Killer Croc, Catwoman, and Bane for 10, 30, and 60 seconds, respectively.
In these examples Batman is using both aerobic and anaerobic metabolism and will be using up much of his energy stores, or fuel. While amino acids can be used as fuel, generally the typical fuel supply for Batman’s body would come from fatty acids and glucose. The fatty acids are stored as body fat either around the body organs—where it is called visceral fat—or underneath the skin—where it is called subcutaneous fat. The trick with body fat is that it is stored away so well that
our bodies don’t like to use it up. It is harder to access and takes longer to metabolize. Really, fat is an awesome energy reserve in that it contains no water and is a very concentrated high energy storage system.
Glucose floats around in our bloodstreams in its original state but is converted to glycogen when stored in the body. Glucose is stored as glycogen because it is more compact and requires less water for use. In a way it is kind of like a concentrate for glucose. Glycogen stores are found in the liver and in skeletal muscle, as we talked about earlier. Muscle is so important for our functioning that each muscle gets its own storage tank of glycogen. The liver holds the glycogen for use throughout the body, for example, in other organ systems like the brain. The tricky thing about glycogen stores, as we learned earlier in the chapter, is that the liver likes to share and liberates glucose for use throughout the body. However, our muscles do not share their glucose. The molecular formation of muscle glycogen is slightly different from liver glycogen and because of that must stay in the muscle.
You might wonder how much energy Batman could have stored up in his body. If he doesn’t take in any additional energy, he would have about four hours’ worth of glycogen, or enough for four hours of hard, continuous exercise. In contrast, Batman has about one month’s worth of fat stored away to be used in aerobic metabolic pathways. (You and I would have about two months’ worth of stored fat, depending on how much we eat and exercise.)
Because the dominant way the body produces energy is through aerobic metabolism, we can estimate cellular metabolism by measuring the volume of oxygen consumed and volume of carbon dioxide produced. This will help us determine the energetic demands of Batman and Bruce Wayne. To do this we could use a technique called “indirect calorimetry,” indirect because direct would mean having the ability to measure what is actually coming in and out of each cell on a moment-to-moment basis. Instead, we can get an overall view of the sum of metabolism by monitoring the exchange of gases (mostly carbon dioxide and oxygen) from the lungs.