2.2: Food and Body Fat as Fuel

When we understand how much of an energy deficit is needed to make real headway against even moderate fatness, we can see that the most common promises of so many weight-loss schemes are unrealistic. Fat can’t be lost quickly without starvation, illness, or extraordinary physical activity. Dramatic weight loss is usually dramatic water loss. But we don’t want to believe it. We all would like something for nothing—weight loss without effort.

How can we test the reality of the rosy vision that fat can be lost quickly without hunger or extraordinary effort? We can test them against the basic formula for determining fat gain or loss:

Energy Intake (food) - Energy Expenditure = Fat gain or loss

We can vary in our efficiency in extracting the energy from food and in expending calories. This can be from differences in genetics, our mix of intestinal bacteria, etc., but it’s a basic law of physics that the calories are accounted for.

The focus is on calories, rather than on the foods themselves. The body seeks only to meet its energy needs. In this respect, it’s similar to a rechargeable battery. It doesn’t matter to the battery whether the electrical energy used to recharge it comes from the burning of oil, coal, or even wood. In turn, the ability of the battery to provide energy for work is simply a question of how many energy units—in the case of a battery, watts—it can take in and hold. For us it simply ends up as a matter of calories.

Measuring the Energy of Food and Work

The energy unit in the human body is the calorie. This word calorie takes us into a bit of confusion. The word itself comes from the worlds of physics and chemistry, as a measure of heat—the amount of heat needed to raise the temperature of one gram of water by one degree centigrade (1°C). But this amount of heat is quite small, and the energy in food is quite high.

A gram of water is only about one-fifth of a teaspoonful. The food energy in a single almond can add 1°C to two gallons of water. An ounce of hamburger could do the same to 20 gallons.

The physicist’s calorie turns out to be an inconveniently small measure for nutrition science. So nutritionists use a kilocalorie (kcal) or Calorie (with capital C), 1,000 of the small calories of physics, as a measure.

Consumers generally ignore the capital C and use calorie with a small c to mean kilocalorie. In effect, they ignore the tiny unit of measurement used by the physicists. So does this book.

The Energy in Food

How do we find out how much energy a food can supply to the body? We start by measuring how much heat is produced when we burn a sample of that food.

The body uses food energy differently. It does not burn its fuel as a log burns in the fireplace—combining rapidly with oxygen at a high temperature. Nevertheless, by actually burning a food sample in the laboratory and measuring the heat produced, we can find out how much energy a food potentially can give us.

That energy potential is essentially the same no matter how we put it to use. Burning food as we would a log would be an inefficient way to derive its energy. If the body operated this way, the fuel would be used up very fast, without much regard for how much energy was really needed at that moment or whether energy was needed for heat or for work. It would also be quite uncomfortable for our innards.

The body does use its fuel (food) through a kind of chemical “burning,” and it does combine with oxygen in the process. But fuel burning in the body is an intricate matter, a kind of extremely slow burning which goes through many steps and is exquisitely controlled.

The point is that the amount of energy elicited from a given fuel is much the same whether the fuel is used rapidly or slowly. Whether gasoline is sipped by a Prius, gulped by a Land Cruiser, or exploded in a Molotov cocktail, the energy potential of the gasoline is the same. In a similar manner, the caloric potential of a food is much the same whether it’s burned in a flash in the fireplace or burned slowly in the body.

To learn how many calories a specific amount of food can yield, a sample of the food is burned in an instrument called a bomb calorimeter (see Fig. 2-3). The food sample is placed in the chamber, together with oxygen. The container is sealed, and the oxygen is ignited electrically and burns the food in a flash (like a bomb). The heat of the burning food heats the surrounding water, and the rise in the water’s temperature is a measure of calories of energy released.

Figure 2-3: Bomb Calorimeter.

Caloric vs. Non-Caloric: When it’s said that an apple has 80 calories, this isn’t exactly correct. It would be more accurate to say that an apple can provide the body with about 80 calories. If the caloric content of an apple were measured in a bomb calorimeter, it would be found to have more than the 80 calories that the body can use.

All of the calories in a food can’t be used if the food isn’t completely digestible or isn’t completely “combusted” to carbon dioxide and water in the body. An apple contains fiber, for example, which has calories as measured in a bomb calorimeter. But since we can’t digest this fiber, it doesn’t provide us with energy (it’s “non-caloric”)—just as a chewing a pencil doesn’t provide us with any calories. (Colon bacteria can, however, provide us some calories from fiber.)

Another example is saccharin, the non-caloric sweetener. It has energy value as measured in a bomb calorimeter and is readily absorbed by the body, but our body can’t break down saccharin and release any of its energy. Thus, saccharin doesn’t provide us with any calories and is excreted in the urine. When it’s said that saccharin doesn’t have any calories, what’s meant is that its energy value is of no use to us. The caloric values we’re familiar with are actually physiological fuel values of food (calories that are available to the body). These values have been determined in numerous human studies.

Energy Used By the Body

In much the same way we measure food energy in a bomb calorimeter, we can measure how much energy the body uses. This method is known as direct calorimetry (heat measured directly), and the apparatus used is analogous to the bomb calorimeter. It’s costly and cumbersome, because it entails enclosing a person in a chamber (a small room). The heat (calories) generated by the person’s metabolic and physical activity is reflected in the increased temperature of the water that surrounds the chamber. Of course, the measurements take much longer since the body doesn’t burn (oxidize) its fuel in a flash.

It goes without saying that this method isn’t very practical if we want to know how much energy the body expends cutting stalks in a cornfield or playing tennis. So for such measurements, another technique is used—indirect calorimetry.

With this method, a person wears a gas-masklike device, a respirometer (respiration measurer), to measure the amount of oxygen consumed while, say, playing tennis. Because the amount of oxygen needed to combine with fuel and generate a certain number of calories is known, the calories expended can be determined indirectly by measuring the amount of oxygen consumed.

Measurements obtained by the two methods— measuring heat production (direct calorimetry) and measuring oxygen consumption (indirect calorimetry)—agree within a fraction of 1%.

	Calories/minute above that used when resting*
1 cal	Sitting while reading, knitting, talking, etc.
2 cal	Writing, driving, lab work, playing cards
3 cal	Walking 2 miles per hour (mph), washing dishes, cooking, carpentry
4 cal	Walking 3 mph, washing the car, house-cleaning, playing ping pong
5 cal	Swimming leisurely, plastering a wall, dancing a waltz
6 cal	Downhill skiing, playing tennis, climbing stairs, bicycling 6 mph
7 cal	Sawing logs with a handsaw, mowing the lawn with a hand-mower
8 cal	Pitching bales of hay, playing football, digging a pit
9 cal	Cross-country skiing, playing basketball, dancing the polka
10 cal	Running 6 mph, strenuous swimming, snowshoeing in soft snow
11 cal	Jumping rope (125 times/minute), playing racquetball
12 cal	Running 7 mph

*At rest, a 140-lb man (10% body fat) uses about 1.1 cal/min., and a 140-lb woman (20% body fat) uses about 1.0 cal/min. Asleep, we use about 10% fewer calories than when resting.

Table 2-1: Calories/Minute for a 140-lb Person.

By such means of measuring food energy and bodily energy use, science has developed two sets of basic data. One consists of tables of the caloric values of foods, the other of the number of calories used during various activities—from sitting to washing dishes to jumping rope (see Table 2-1). These two bodies of information provide keys to testing the reality of ideas about energy production and energy use by humans—and hence, about weight control.

Energy—A Confusing Use of the Word

We use the word energy rather loosely. For instance, we say that we feel full of energy when we wake up in the morning feeling good, anxious to “hit the books” or go to work. In science, however, energy is a very specific term. Energy is defined as the capacity to do work. From a scientific standpoint, we’re really full of energy when we have that extra layer of fat stored under our skin.

Adding to the confusion, the word energy is used even more liberally by those selling dietary supplements or diet plans. As a best-selling book, Fit For Life says:

Because you will be eating to free up energy you will have more energy than you ever had before. Consistently optimizing your energy is a critical part of FIT FOR LIFE.¹

This made sense to those who made this book a best-seller, but it’s nonsense to the scientist.

Much of the confusion over the word energy results from its everyday use to mean both energy in the scientific (physical) sense and vigor. Vigor is, of course, a very real thing, a reflection both of one’s physical and one’s emotional status, and might be described as a feeling of readiness to do work.

The psychological aspects of this feeling can be crucial. The ancient warrior who has just eaten the raw meat of a lion and the runner gulping a handful of dietary supplements may both feel physically ready to triumph, for much the same psychological reasons. They may or may not be physically fit. Someone in the best physical and nutritional condition may, on the other hand, have no vigor at all after a romance turns sour.

The physical side of vigor is equally important, and lack of vigor can certainly have a nutritional basis. A person who consumes plenty of calories and has ample energy reserves of fuel may, if that person hasn’t been consuming enough of some nutrients, suffer from a condition like anemia, which hampers the use of energy by delivering less-than-normal amounts of oxygen to the tissues. He or she will lack vigor.

Because the body’s use of fuel is so basic to nutrition, and because all concepts of weight control are so absolutely dependent on how we use and store fuel, it’s worthwhile to look deeper into how food provides energy, and how the body puts that energy to work.

ATP—The Secret of Our Energy

The same natural laws are followed whether it’s food that’s furnishing power to a person or gasoline furnishing power to a car. In both cases, the fuel is combined with oxygen to yield energy, water, and carbon dioxide (plus carbon monoxide in a car due to incomplete oxidation).

Gasoline mixes with oxygen from the air. An electric spark ignites the mixture, and the gasoline burns in a mini-explosion. Each mini-explosion propels the car forward a bit. The mini-explosions are set at proper intervals (timing of the engine) so that the car drives smoothly.

Like a car, the body also needs energy for work. But instead of mini-explosions, the body releases energy from its fuel in an exquisitely controlled process. This process is a part of metabolism, which will be discussed in more detail in Chapter 11. When energy is released in metabolism, some of the energy is released as heat, but a good part of the energy is captured in a high-energy substance (ATP—Adenosine TriPhosphate) for use in bodily processes that require energy.

When the body needs energy to, say, run a marathon, the muscles respond by generating energy (ATP) to fuel the muscle contractions of running. The high-energy ATP is made as needed from the breakdown of the body’s fuel. The runner doesn’t gather ATP ahead of time as the starting gun is anticipated. Just a bit is already there; the ATP fueling the continuous muscle contractions is made then and there as it’s needed.

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Muscle naturally contains creatine phosphate, which helps replenish ATP. Creatine supplements can help in “explosive”-type activities like weight-lifting and sprinting.²

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A lot of ATP is needed for a long-distance run. Fortunately, even the leanest athlete has a more than ample store of fat to fuel a marathon. As the runners start their motion, nerves signal the muscles to contract, and there’s action. ATP provides the energy for that action, and the ATP used is rapidly regenerated for the continuing action.

The energy in ATP is released by the breaking of a special “high-energy” bond within the molecular structure of ATP. The process is similar to releasing an arrow from a bow (see Fig. 2-4). The high-energy bond within ATP is like a drawn bow. And once you break the bond and let fly, you need to reload.

With a regular bow, of course, the string can be drawn over and over again. All you have to do is get another arrow and use some energy to pull. So it is with ATP. The energy released from fuel is used to regenerate ATP. In effect, it fits another arrow to the bow and draws back the string. The body doesn’t need a lot of bows; it can put arrows to those bows very quickly.

Suppose we want to scratch an ear. A signal goes from the brain to the nerves and is flashed to the muscle cells in the hand. At once, the energy in ATP is released. As thousands of cells in the hand are similarly energized with the production of ATP, the finger bends back and forth to scratch the ear.

Fuel Efficiency

When the body “burns” its fuel to generate ATP, more than half of the energy is “lost” as heat—like the heat produced by an automobile engine (which is why it needs a cooling system). The heat generated in the body this way can be useful (e.g., to maintain normal body temperature), but it also can make us uncomfortably warm when we exercise.

Figure 2-4: High-Energy ATP. A. ATP is shown as the arrow of a drawn bow, “bonded” to the bow with a store of energy behind it. B. When needed, energy is released as the arrow flies off

In an endurance event, ATP is used and regenerated at a rapid rate, and the large amount of heat produced as a by-product is in a way wasteful. The athlete would rather that more ATP be produced for power than “wasted” as heat. Also, rapid heat production can hurt performance if the heat can’t be dissipated fast enough.

The slender lean body of a typical marathon runner is ideal for getting rid of heat. A slender shape provides relatively more surface area from which heat can escape, and the minimal amount of fat stored under the skin (along with minimal clothing) facilitates the loss of body heat.

In contrast, the heat generated by vigorous and sustained exercise can easily become more than an obese person’s body can tolerate. There’s less surface area relative to body weight, and the thick layer of fat provides a greater layer of insulation that hinders heat loss.