3.7: Energy

We use the word energy rather loosely. We say we’re full of energy when we wake up in the morning feeling good, ready to “hit the books” or go to work. In science, energy is a very specific term. It’s defined as the capacity to do work (e.g., move or heat something). Scientifically speaking, we’re full of energy when we’re fat. The energy values of body fat and food and the energy the body uses are expressed as calories.*

Calories in food are measured in a bomb calorimeter, which has a small metal chamber surrounded by water (Figure 3.4). A precise bit of food is put in the chamber, and the chamber is filled with oxygen. The food sample, ignited by an electric spark, burns in a flash (“bomb”). The heat (calories) generated by this burning is measured by the increased temperature of the surrounding water.

Calories that a person uses can be measured in an apparatus similar to a bomb calorimeter. In this case, the chamber is a room, often big enough to even hold furniture. The measurements take much longer because the body doesn’t burn its fuel in a flash, but in a slow and controlled manner.

This apparatus for measuring energy expenditure is expensive and has obvious limitations. It certainly can’t measure the calories we use playing tennis. But the calories used playing tennis can be measured indirectly by measuring the amount of oxygen used. Calories measured by either method (measuring heat produced or oxygen used) agree within a fraction of 1%.

Our bodies use much of the energy released from food to do work. For this, our cells make a chemical form of energy called ATP (adenosine triphosphate). It’s made by capturing some of the energy that’s released when our cells break apart fat molecules and other energy-providing nutrients (the covalent bonds that link the atoms together in molecules hold energy). ATP is used for energy-requiring activities like muscle contraction and synthesis of enzymes, hormones, and tissues.*

When we “burn” body fuel, the released energy isn’t completely captured as ATP. More than half is “lost” as heat, just as we lose some fuel-energy as heat when we burn gasoline by driving a car. This heat can be useful (e.g., maintain normal body temperature) and is the reason we feel warm when we exercise.

In an endurance event, ATP is made and used quickly, and an athlete thinks of the energy lost as heat as wasteful and counterproductive to performance. In contrast, a person struggling to lose weight would like to “waste” more energy/calories as heat, rather than eat less or be more active.

^{*1 calorie = the heat (energy) needed to raise the temperature of 1 milliliter of water 1°C. A “food calorie” is actually 1000 calories—a kilocalorie—but consumers (and this book) call a kilocalorie a calorie.
*The workings of key enzymes used to make and use ATP were discovered by Paul Boyer, John Walker, and Jens Skou, who were awarded the 1997 Nobel Prize in chemistry}