13: Energy Requirements

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I think the devil will not have me damned, lest the oil that’s in me should set hell on fire.

Sir John Falstaff in Shakespeare’s The Merry Wives of Windsor

Cells constantly use a lot of energy and can’t afford to run out. Even the thinnest of us keeps an ample back-up supply of fuel as fat.

As discussed in Chapter 3, energy in food and energy-use in the body are measured in calories. Food calories are measured by burning the food and measuring the heat. Calories we use (“burn”) are measured either directly by measuring the heat we generate, or indirectly by measuring the oxygen we use.*

We release the energy in food via the reactions of metabolism (Chap. 9). We capture about 40% of the energy as ATP. The rest (60%) is released as heat. A 40% fuel efficiency may not sound high, but even a well-tuned car uses gas at less than 30% efficiency.

In cold weather, the generated heat is useful in heating our bodies or heating the car. But hot weather can be a problem. We instinctively avoid overheating our bodies by being less active (less heat made) and by wearing less clothing (more heat lost). When the car engine is overheating, we do try to stop the car (less heat made), but most of us don’t think of turning on the car’s heater (more heat lost), though this will help.

How many calories do we need? Simply the number of calories needed to maintain a healthy weight. This answer is, of course, deceptively simple. Adults need energy for:

Basal metabolism: Maintain normal body functions (breathe, pump blood, maintain tissues, etc.)—what it takes to sustain life. Except for the extremely active, basal metabolism makes up the biggest part of our energy need.
Physical activity: Moving!
Energy expended upon eating: Incorporating food into the body (digestion, etc.).

^{*It usually doesn’t matter whether we use glucose or fat as fuel. But endurance athletes try to use glucose sparingly, since they fatigue when their muscles run out of glucose (Chap. 9). They can gauge their “fuel mix” by measuring how much carbon dioxide they make, relative to how much oxygen they use. As shown in Figure 9.2, if only fat is used, it’s 16CO₂ /23O₂ =0.7; if only glucose is used, it’s 6CO₂ /6O₂ =1.0; “mixed fuels” fall between 0.7 and 1.0. By this method, it’s been shown that caffeine taken an hour before an endurance event spares the use of glucose (caffeine raises fatty acids in blood) and delays the onset of fatigue from a lack of glucose.}