2.1: The Bodily Energy Crisis

When we want to drive somewhere, we put fuel into a car and go. When we want to vacuum the rug, we take out the vacuum cleaner, plug it in, and turn it on. When we are finished with these machines, we simply turn them off, knowing that we can start them up again whenever we need them.

But once the human machine has been turned off for more than a few minutes, it doesn’t turn on again. To stop it is to destroy it. It’s turned on at the moment of conception, and from that moment it must run continuously until the moment of death. This means that we require an uninterrupted supply of fuel. If we run out of fuel, we die.

The demand for fuel fluctuates, but it never falls below a fundamental “idling” rate (basal metabolic rate) that keeps our body functioning at its most basic level. Energy is needed continuously for such basic functions as pumping blood throughout our body and breathing.

Less obviously, a lot of fuel is needed to maintain the brain. Contrary to popular belief, the brain functions at much the same level whether we’re studying intensely or sleeping. When we’re resting, the brain uses more than 20% of the body’s total energy use. Even during sleep, the brain is busy—messages are going continually from the brain to every limb and organ, and other messages return. If this weren’t happening, we wouldn’t turn over when our arm cramps; we wouldn’t wake upon hearing a baby’s cry or smelling smoke; and we wouldn’t pull up the covers when a cold gust of wind blows in through the window.

Still more subtly, our body is a cooperative colony of trillions of cells, each of which needs energy to perform specialized functions. The cells need fuel to produce the more than 100,000 biochemicals, as well as to generate the heat we need to stay warm.

These are our basal, involuntary, and unstoppable energy demands. We also require energy for our voluntary activities, which are just as necessary for our survival. We couldn’t even begin to find food or care for ourselves without them. We also expend energy in digesting and assimilating food—this energy need is about 10% of caloric intake.

---

Basal metabolic rate (BMR): the rate at which energy (calories) is used for involuntary bodily functions.

---

So our total energy needs are basically a combination of the demands of basal metabolism, of voluntary action, and of digesting and assimilating food. Together they constitute an unremitting energy drain. No matter how much we restrict our activity, our energy need is still quite large. Unless we’re extremely active, chances are that basal metabolism accounts for about two-thirds of the energy we use.

An energy supply must be constantly on hand, readily available. If we had to get the fuel immediately for each use, we’d be in constant danger. A missed meal or snack could mean death.

In our affluent society, we rarely are aware of true hunger, and we tend to look upon the ease with which we become fat as a curse. In fact, the ease with which we fill our fuel-storage system is an essential mechanism for human survival.

Fat Cells—Friend or Foe?

For our prehistoric ancestors, food was a sometime thing. It depended on the luck of the hunt and the success of the forage. It depended on the seasons. In summer and fall, the land was lush with ripening berries and nuts; flocks of birds were everywhere; and the game grew fat on abundant forage. But in winter, the trees and shrubs were bare; the flocks had migrated; and the ground was frozen too hard to dig for roots. As the eating became sporadic, what fueled the body machine through the hard days? To find the answer, we must examine the fat that lies beneath the skin (subcutaneous fat).

To the naked eye, this fat appears to be a yellowish, inert mass. But under the microscope, we see a crowd of adipose cells (“fat cells”). Each of these cells is like a collapsible thin-walled tank that grows larger as it fills with fat, and smaller as it’s emptied.

---

Some people call lumpy-looking fat under the skin “cellulite.” In fact, it’s just ordinary fat. The strands of connective tissue that hold the fat in place can cause dimpling in the skin. The lumpiness can go away when the fat layer is thinned by ordinary weight loss. We can’t pound or massage it away, as ads and popular articles would have you believe.

---

The body takes any excess fuel, beyond that needed to meet current energy demands, and converts it to fat, which is then stored in the fat cells (see Fig. 2-1). Fat is used for this reserve because, as we will see, it’s the most compact body fuel.

So when the hungry days came for the caveman, his body could simply draw upon the fat cells for fuel. The release of fuel might have been (and still can be) triggered by many things—too many hours without eating, or a high fever, or by a sudden need for energy to pursue game or escape from an enemy. It’s easy to see the urgent practicality of such a fuel-reserve system. One could hardly stop for a snack while being chased by a saber-toothed tiger.

The blanket of plump fat cells under the skin serves in other ways, too. It’s an excellent insulator against heat and cold, particularly important when one remembers that the body functions well only within narrow limits of internal temperature (usually 98°-99°F). The fat is also a good buffer against injury, a shock absorber against blows, a shield to keep a cut from reaching internal organs. When we fatten, a fair amount of fat forms around these organs, providing additional protection.

Figure 2-1: Fat Cells. The cross-section of a fat (adipose) cell is aptly described as a signet ring.

The alternating cycles of feast and famine, of leisure and violent activity probably set up a nice energy balance for prehistoric man. Fat cells continually filled and emptied, and in the end there was probably little obesity.

But now, the food supplies of our modern society have affected our delicate fat-storage balance—with supermarkets, vending machines, fast-food outlets, and convenience stores nearby; cans of soft-drinks and beer stockpiled in the refrigerator; packages of frozen dinners and ice cream tucked in the freezer; and packages of instant ramen, microwave popcorn, and ready-to-eat cereal safe in the cupboard. Like our primitive ancestors, we feast when food is abundant and store even the smallest excess of fuel as fat—but unlike the time of the caveman, for most of us now the food is always there.

Ironically, to empty some of this unwanted storage from our fat cells, the primitive plan is still the only one that really works. We must either reproduce the historic days of caloric deprivation, or the vigorous, physical exercise—or a combination of the two.

Even when we are slim, our energy reserves are large. Our bodies range from the 5% fat of the very thin man to the 30% fat of the moderately obese woman.

To get some idea of how much stored energy this fat represents, consider a young woman of normal weight, at 5’5” and 125 pounds. About 25 of those pounds are in fat. This much fat is enough to meet her energy needs for about 45 days.

Figure 2-2: Energy Balance—Fat vs. Thin. Weight maintenance, without gain or loss, balances energy intake (food) and energy expenditure (basically, basal metabolism plus physical activity).

This suggests one reason why it’s hard to make rapid headway against excess fat. For it takes a significant energy shortage, over a substantial period of time, to draw down large excess reserves.

Suppose the young woman had an additional 60 pounds of fat and weighed 185 pounds. In societies ravaged periodically by famine, the excess weight would serve her well. And our 185- pound woman is really only at the margin of a height-weight definition of obesity (see Chap. 3).

Although obesity in primitive societies may have been unusual, there’s good reason to think that an obese woman might have been considered beautiful. In the Sahara where famine is a threat, some brides-to-be were forced to drink several quarts of camel’s milk daily until they became obese—and more sexually attractive in their culture. During a famine, a large store of body fat would be expected to carry them successfully through pregnancy and breastfeeding.