Ácidos grasos representativos en el estudio multivariado

If you’re reluctant to eat less, you’ll have to be more active to lose weight. How much more depends on various factors, and it’s important to keep several points in mind when estimating the calories used in different types of activities: • The energy used in weight-bearing exercises

is quite variable. Not only does the caloric out-

put depend on how far you move, but also on how much you weigh (i.e., how much weight is moved). Someone weighing 120 pounds uses about 70 calories walking a mile in 20 min- utes, whereas someone weighing 200 pounds uses about 105 calories walking that same mile. (Good reason to carry an infant in a baby carri- er—or books in a backpack—if you’re looking to use more calories.)

This also means that as you lose weight, you will use fewer calories for the same walk (un- less the infant you are carrying gains what you lose, or you add a counterbalancing number of books to your backpack).

The amount of energy used in walking is affected by other factors as well, including the walking surface (e.g., sand or pavement, uphill, downhill, or flat), distribution of weight on the walker’s body, the walker’s physical fitness. Also, we use fewer calories in an activity as we become better at it. Think of the gracefulness of experienced runners.

• In non-weight-bearing exercise, other physi-

cal factors can make a difference in energy output. In swimming, for example, a more

buoyant person expends less energy staying on top of the water.

• People vary quite a lot in how much energy

they expend in a given activity. Tennis isn’t al-

ways tennis. Suppose you are much better than your opponent. You return shots while standing at the net, and your opponent runs a lot. But the next day you are outclassed. Now it’s your turn to run. You hit harder, stretch further, and pant. The third day you play doubles, and one of the partners is a lawyer who frequently stops to

dispute line-calls and asks the score after each point. Even in a lecture hall, some people sit at the edge of their seats and fidget. Others are slumped in their seats, barely awake.

Most of us are quick to imagine how we can in- crease our output of energy. We imagine ourselves deliberately parking a mile away from work, or taking up biking or swimming. But for many of us, such an active imagination doesn’t translate into an active body. This leaves us receptive to claims of painless shortcuts. It’s important to re- member that there are no magic exercises, just as there are no magic foods.

Increasing the output of energy is a fairly obvious matter (see Fig. 3-1). Once an appropriate activity is chosen—one which uses a good bit of energy and, ideally, involves moving the whole body over a long distance (e.g., a long walk, or a simulated long walk on a treadmill)—the problem is chiefly one of persistence.

Increasing physical activity is helpful in other ways. What we want to lose is fat, but muscle mass is also lost with dieting. Exercise helps maintain muscle mass, and also is a counterbalance to the drop in basal metabolism triggered by a low-calo- rie diet. Furthermore, if the exercise leads to more muscle mass, basal calorie needs increase.

Figure 3-1: Activity time to burn about 200 calories. 0 30 60 90 120 Sitting (122 minutes) Walking (36 minutes) Bicycling (28 minutes) Swimming (18 minutes) Running (14 minutes)

Chapter 3. Putting the Laws of Energy to Work ₃₃

Appetite

Increased physical activity can make a crucial difference in the success of reducing regimens, but unless one controls caloric intake, the effort to spend more energy may not accomplish much. One surprising fact that may help is that a moderate increase in activity (increased calorie use) by those who are sedentary doesn’t necessarily result in a proportionate in increase appetite.3,4 _{(The not-}

so-good news is that increased physical activity beyond a moderate point generally signals graded increases in appetite.)

Appetite is a rather mysterious phenomenon, intimately involving physical and emotional fac- tors. And a key reason why appetite remains such a mystery, defying many experimental efforts at analysis, is that these two factors are subtly and intricately interlocked.

Appetite was once thought to be controlled mainly by the mechanical effects of food diges- tion—as seen by such old phrases as “on a full stomach.” It was believed that the contractions of an empty stomach were among the main cues for eating. It was noted, for instance, that fats, which leave the stomach more slowly, seem to have greater value for long-term satisfaction.

But this theory had to contend with the fact that the stomach usually is emptied within two to four hours of eating (depending on the composition of the meal), without any clear correlation with the arrival of hunger pangs. More sophisticated stud-

ies show the process to be far more complicated. Food in the stomach doesn’t necessarily stop the wish to eat, and an empty stomach doesn’t neces- sarily start it.

The appetite-control mechanism is in the brain’s hypothalamus, the seat of some of the most basic reactions for survival. It has to do with the con- trol of body temperature, with certain sexual phe- nomena, and with the primitive “fight-or-flight” reflexes. The appetite-control appears to have two parts, one to turn on appetite and the other to turn it off—much like the way a thermostat regulates room temperature. Both of these switches seem to respond to changing blood levels of glucose, the simple sugar which is a major fuel used to meet the body’s energy needs.

There are other bodily mechanisms for assess- ing how much fuel is coming in, and it’s thought that the brain mechanism responds to a number of body reactions and controls that have a joint net effect of leading us to eat or to stop eating. The stomach, for example, makes more of the ap- petite-stimulating hormone ghrelin before a meal, and makes less of it after the meal.

Unfortunately, the fact that big reserves of stored fat are available for energy—and perhaps are even being put to work—doesn’t necessarily shut off the drive to eat. Appetite control seems to concern itself mainly with whether or not enough fuel is coming in to take care of current energy needs. The fat person feels just as hungry as the very thin person when lunch time passes without food. And for similar reasons, the person who is trying to lose weight may feel unsatisfied if less fuel is consumed than the body is burning.

On the other hand, contrary to popular belief, experimental evidence makes it rather clear that fatter people don’t have an unusually active appe- tite or find taste extremely stimulating. But it does appear that overweight people are more prone to eat in response to external cues for eating (e.g., a candy dish on the coffee table, or the clock show- ing it’s lunch time).

There’s also a difference among people of dif- ferent body types. The thin ectomorph seems to have a sharper cut-off of appetite than does the bulkier endomorph or mesomorph, who appears to be more inclined to eat extra food after feeling satisfied (“I couldn’t eat another bite—but did you say chocolate?”). The mesomorph may well say to an ectomorph, “You only have a bite of pie left on your plate. Why didn’t you finish it?” In fact, the mesomorph may even be tempted to eat that last morsel “so it won’t go to waste.”

Cerebrum

Cerebellum Brain stem Pituitary

34 Part 1. Energy and the Human Machine

A strong emotional influence runs throughout all these themes, which blurs logic all the more.

Appetite and the Emotions

We’ve seen that few people who are fat are truly gluttonous—rather they’re more likely to be the victims of small steady excesses of intake over need, with perhaps occasional episodes of extreme excess. It’s easy to see how even minor emotionally-oriented distortions of the appetite might lead to obesity.

The wish to eat has powerful emotional aspects. Feelings of all kinds, from joy to pain, can override the physical signals of a need or lack of need for fuel. Feasting is an ancient way to celebrate. Conversely, in the emotional disturbance

anorexia nervosa, people may suddenly stop

eating—until life itself is seriously threatened— with no feelings of hunger.

The psychological effects on appetite are well known to have a strong placebo effect for many. Almost any system of dieting, or any kind of pill, will magically wipe away hunger for a number of people, if that’s what they’re told it will do.

In this case, the placebo effect is a decrease in appetite due to expectation alone. A placebo (from the Latin for “I please”) is a substance or procedure administered to create the illusion of medication or treatment, either to pacify or for test purposes.

This powerful link between appetite and emo- tion begins in the first hours of life—as we have our first taste of both love and food in our moth- ers’ arms. This maternal nurturing is a key to sur- vival. Love and food are interwoven; the need for both becomes a part of our very will to survive; and food becomes a potent symbol of this whole basic emotional system.

The giving of food quickly begins to symbolize the giving of love, but also begins to embody the sense of healing, and the relief of stress and pain. In contrast, the withholding of food becomes a sign of love denied, of rejection, hostility, and an important kind of punishment. Almost every cul- ture greets its guests with food or drink.

As the child develops, a reverse image of these emotions appears: to be loved, one must accept the

gift of nurture. How would you feel if you invited guests to dinner, and they sat without eating, say- ing they just weren’t hungry? In many cultures, rejecting an offer of food or drink is an insult.

So, in addition to being a way to soothe and please ourselves, eating also becomes a way to please others. For many parents, the good child is the one who cleans the plate. (“Eat your dinner. I spent hours cooking just for you.”)

This subtle kind of reward-and-punishment game can be used in other ways: “If you don’t eat your vegetables, you can’t have dessert.” “Be good at the dentist and we’ll go out for ice cream afterwards.” A child can thus be taught to view foods as palliatives for the pains and stresses of living.

One of the most widely accepted theories about the emotional development of humans is the in- ternalization of our parents and their ideas. In a sense, growing up is partly the business of becom- ing our parents. We learn symbolically to slap our own hands if we want to reach for something that doesn’t belong to us. We abuse ourselves if we’ve been wrong and feel guilt. We learn to soothe our own pains, and we learn to reward ourselves for behavior that we think is good.

If our parents have used food for these purposes, then the parents who live in our minds will also. Similarly, our early training often leads us to see any deprivation of food as emotional evidence that we are being denied love, or perhaps even being punished for some sin.

Some psychologists have theorized that such feelings on the part of the dieter can lead to repeated episodes of painful crash dieting. It’s as if there’s a need to punish ourselves for crimes of gluttony.

“Appetite-Killing” Diets

Many reducing diets are advertised with the promise that, though they are low in calories, they are fully satisfying. There can be a thread of truth to such claims, but it’s generally exaggerated.

For example, because fat tends to stay longer in the stomach, it can be more satisfying. This does have some reality. But there’s the further reality that fat has far more calories (9 calories/gm) than protein or carbohydrate (4 calories/gm).

Chapter 3. Putting the Laws of Energy to Work ₃₅

Many of the very narrow “crash” diets, such as liquid diets or other monotonous diets, seem to act as limits on calorie intake to some extent simply because they quickly become boring and unappetizing, and people aren’t tempted to overeat (or stay on them for very long). An “overdose” of almost anything seems to be a sure way to arrive at some caloric reduction. A diet of all the bread and grapefruit you can eat—but nothing else—can work. And one can expect that a diet of nothing but chocolate ice cream and steak would soon become effective too.

Changes of metabolic function have also been claimed for some crash diets, again with a thread of truth. Ketosis, a state which derives from a lack of carbohydrate for fuel (as in an extremely low- carbohydrate diet or starvation), can, for example, produce mild nausea, which, of course, kills the appetite. But purposely altering the body’s chemistry in such ways is generally considered to be unwise, especially if the abnormal state is continued for very long.

“Good” Weight-Loss Diets

What makes a good reducing diet? The simplest answer, of course, is a diet with fewer calories than are used by the body. But when we understand how much time it takes for a realistic caloric deficit to have a real impact on stored fat, we can readily see that a good reducing diet must also be adequate nutritionally.

There’s little that a physician—or self-treating dieter—can do to speed the process of fat loss, de- spite the breathless stories about magical potions, foods, and diets. Although these regimens offer an endless variety of distractions, all of them gen- erally have a low intake of calories as an essential component.5

Are 1,000 calories or less a day okay? We’ve already seen that in terms of pure survival, one can go a long time without eating and still have energy reserves to draw upon. And most of us would stop dieting when we became emaciated.

But energy isn’t all there is to nutrition. Instructions for most crash diets include the warn- ing to see your doctor first (although few people do) and to “take a vitamin pill.”

Yet only some of the needed nutrients are found in vitamin-mineral supplements. We also need the “structural materials”—too “big” to fit into a pill—for the building and repair of tissue. Extremely obese people who are treated with complete fasting, burn not only fat but muscle tis- sue as well. In a sense, the body consumes its own meat to get needed protein.

The widespread use of so-called “protein‑

sparing” diet—-the most popular of which was

marketed as Optifast—was given a lot of public- ity. Originally developed by physicians for medi- cally-endangered obese patients, it was designed to be used only with such patients, and under close supervision according to strict medical pro- tocols. Caloric intake was severely restricted, and special supplements provided vitamins, minerals, and protein (to help spare body protein—“protein sparing”).

Where used as intended, these liquid diets were useful in achieving rapid weight loss with relative safety. But the rapid weight-loss aspect was so at- tractive that its use spread. The medical control aspect loosened, and these diets became wide- spread, under various commercially promoted ti- tles using various supplements and a wide variety of supervisory standards.

As protocols and products have varied, so have long-term results. Health risks come with inadequate supervision and control, and long- term weight-loss was disappointing, despite the well-publicized examples of dramatic weight loss.

The power of such programs is also its weak- ness: Participants living for months on liquid sup- plements don’t face up to their regular exercise and eating habits or perceptions about food—ele- ments which are essential for true weight control by the chronically overweight.

Healthy weight loss Assess food intake Calculate energy needs Assess physical activity Eat a well- balanced diet Set realistic long-term goals Lose weight slowly

36 Part 1. Energy and the Human Machine

So pills and potions aren’t the answer. Eventual- ly, the dieter must eat real food. Most dietitians agree that a nutritious diet usually requires at least 1,200 calories a day, and more for people who are larger than average. At this calorie level, much care in food selection is needed to insure sufficient intake of the needed nutrients. So a more generous calorie allotment is better.

Planning changes in diet and activity, and pre- dicting weight loss are made easier by the use of personalized online modules, such as the Body Weight Planner http://bwsimulator.niddk.nih.gov

In cutting calories, two key principles are usu- ally followed. The first is to limit the intake of

“calorie‑dense” foods—foods which have a lot

of calories packed into a small amount of food. Foods rich in fats and oils and sugars are typical. As we shall see in later chapters, a good reducing diet is basically the same as a general, healthy diet—both focus on a diet rich in grains, vegetables, and fruits.

Foods with low caloric concentrations should be favored—foods with high content of water and fiber, such as grains, vegetables, and fruits. Their high bulk, relative to their calories, makes them good dieting foods.

A second principle is that of choosing nutri‑

ent‑dense foods—foods that have lots of nutrients

for the calories (e.g., carrots). The simplest way to understand the principle is to think of calories as money. We want to use our limited “budget” of

calories to buy as much good nutritive value as we can (see Table 3-3).

This concept explains why added sugar is usu- ally restricted in reducing diets. In effect, it “di- lutes” foods nutritionally. As shown in Table 3-3, a canned peach packed in heavy syrup is 150 cal- ories, whereas a canned peach packed in juice is 90 calories. In heavy syrup, the peach “costs” us far more calories (and has less nutrients per cal- orie) than the peach packed in juice. The better choice is the more nutrient‑dense peaches packed in juice.

Reduced-Calorie Foods

There’s a variety of commercial food prod- ucts we can substitute for commonly consumed foods at a lower caloric value. It could be a soft drink sweetened with artificial sweetener like NutraSweet instead of sugar. Or it could be “light” sour cream, reduced-fat cheese, or non-fat milk.

But the body’s controls over caloric intake