Counting the Calories in Food

When someone says that a serving of food — say, one banana — has 105 calories, that means your metabolizing the banana produces 105 calories of energy your body can use for work.

Different foods have different amounts of calories depending on their nutritional components. For example, high-fat foods have more calories than low-fat foods because a gram of fat has more calories than a gram of carbs or protein or alcohol:

Protein has 4 calories per gram.
Carbohydrates have 4 calories per gram.
Fat has 9 calories per gram.
Alcohol has 7 calories per gram.

In other words, ounce for ounce, proteins and carbohydrates give you fewer than half as many calories as fat. That’s why — again, ounce for ounce — high-fat foods, such as cream cheese, are calorie-rich, while low-fat foods, such as fruits and vegetables, are not.

MEASURING THE NUMBER OF CALORIES

Nutrition scientists measure the number of calories in food by actually burning the food in a bomb calorimeter, which is a box with two chambers, one inside the other. The researchers weigh a sample of the food, put the sample in a dish, and put the dish into the inner chamber of the calorimeter. They fill the inner chamber with oxygen and then seal it so the oxygen can’t escape. The outer chamber is filled with a measured amount of cold water, and the oxygen in the first chamber (inside the chamber with the water) is ignited with an electric spark. When the food burns, an observer records the rise in the temperature of the water in the outer chamber. If the temperature of the water goes up 1 degree per kilogram, the food has 1 calorie; if it goes up 2 degrees, it has 2 calories; 235 degrees, 235 calories — or one 8-ounce chocolate malt made with powder mix and whole milk.

Sometimes foods that seem to be equally low calorie really aren’t. You have to watch all the angles, paying attention to fat in addition to protein and carbohydrates. Here’s a good example: A chicken breast and a hamburger are both high-protein foods. Both should have the same number of calories per ounce. But if you serve the chicken without its skin, it contains very little fat, while the hamburger has a lot. So a 3-ounce serving of skinless chicken gives you 140 calories, while a 3-ounce burger yields 230 to 245 calories, depending on the cut of the meat and its fat content.

Empty calories

All food provides calories. All calories provide energy. But some foods are said to give you empty calories. This term has nothing to do with the energy the calorie provides. It simply describes a food whose protein, fat, and carb calories come “naked” without the additional nutrients such, as dietary fiber, vitamins, and minerals, that improve the nutrition value of the foods on your plate.

The best-known empty-calorie foods are table sugar and ethanol (the kind of alcohol found in beer, wine, and spirits). On their own, sugar and ethanol give you energy — but no nutrients. (See Chapter 8 for more about sugar and Chapter 9 for more about alcohol.)

Of course, sugar and alcohol are often found in foods that do provide other nutrients. For example, sugar is found in bread, and alcohol is found in beer — two very different foods that both provide calcium, phosphorus, iron, potassium, sodium, and B vitamins.

In the United States, some people are malnourished because they can’t afford enough food to get the nutrients they need. The school lunch program started by President Franklin Delano Roosevelt in 1935 and expanded by almost every president, Republican and Democrat, since then has been a largely successful attempt to prevent malnutrition among poor schoolchildren.

Many Americans who can afford enough food are malnourished because they simply don’t know how to choose a diet that gives them nutrients as well as calories. For these people, eating too many foods with empty calories may cause significant health problems, such as weak bones, bleeding gums, skin rashes, mental depression, and preventable birth defects. Too many empty calories may also lead to obesity, an epidemic in current American society and, increasingly, around the world, which I outline in Chapter 4.

Every calorie counts

Although your body burns some calories faster than others — your first source of energy is carbohydrates, which you metabolize before fats — people who say that “calories don’t count” or that “some calories count less than others” are usually trying to convince you to follow a diet that concentrates on one kind of food to the exclusion of most others. One common example that arises like a phoenix in every generation of dieters is the high-protein diet.

The high-protein diet, most commonly known today as the Atkins diet, tells you to cut back or even entirely eliminate carbohydrate foods on the assumption that because your muscle tissue is mostly protein, the protein foods you eat will go straight from your stomach to your muscles, while everything else turns to fat. In other words, this diet says that you can stuff yourself with protein foods because no matter how many calories you get, they’ll all be protein calories, and they’ll all end up in your muscles, not on your hips. Wouldn’t it be nice if that were true? The problem is, it isn’t. All calories, regardless of where they come from, give you energy. If you take in more energy (calories) than you spend each day, you’ll gain weight. If you take in fewer calories than you use up, you’ll lose weight. This nutrition rule is an equal opportunity, one-size-fits-all proposition that generally applies to everyone and everybody.

Determining How Many Calories You Need

Think of your energy requirements as a bank account. You make deposits when you consume calories. You make withdrawals when your body spends energy on work. Nutritionists divide the amount of energy you withdraw each day into two parts:

The energy you need when your body is at rest
The energy you need to do your daily “work”

To keep your energy account in balance, you need to take in enough each day to cover your withdrawals. As a general rule, infants and adolescents burn more energy per pound than adults do, because they’re continually making large amounts of new tissue. Similarly, an average man burns more energy than an average woman because his body is larger and has more muscle (see the upcoming section “Sex, glands, and chocolate cake”), thus leading to the totally unfair but totally true proposition that a man who weighs, say, 150 pounds can consume about 10 percent more calories than a woman who weighs 150 pounds and still not gain weight. For the numbers, check out the next section and Table 5-1.

Table 5-1 How Many Calories Do You Need When You’re Resting?

Gender and Age	Equation to Figure Out Your REE
Males
18–30	(15.3 × weight in kg) + 679
31–60	(11.6 × weight in kg) + 879
61 and older	(13.5 × weight in kg) + 487
Females
18–30	(14.7 × weight in kg) + 496
31–60	(8.7 × weight in kg) + 829
61 and older	(10.5 × weight in kg) + 596

The National Research Council, Recommended Dietary Allowances (Washington, D.C.: National Academy Press, 1989)

Resting energy expenditure (REE)

Even when you’re at rest, your body is busy. Your heart beats. Your lungs expand and contract. Your intestines digest food. Your liver processes nutrients. Your glands secrete hormones. Your muscles flex, usually gently. Cells send electrical impulses back and forth among themselves, and your brain continually sends messages to every tissue and organ.

The energy that your resting body uses to do all of this is called resting energy expenditure, abbreviated REE. The REE, also known as the basal metabolism, accounts for 60 to 70 percent of all the energy you need each day.

To find your resting energy expenditure (REE), you must first figure out your weight in kilograms (kg). One kilogram equals 2.2 pounds. So to get your weight in kilograms, divide your weight in pounds by 2.2. For example, if you weigh 150 pounds, that’s equal to 68.2 kilograms (150 ÷ 2.2). Plug that into the appropriate equation in Table 5-1, and there’s your REE.

What do you do with this information? First, simply appreciate its scientific value in describing the most basic fact about how many calories you need to survive. Second, and more pragmatically, regard it as a base on which to build a nutritional, real-life daily menu.

Sex, glands, and chocolate cake

A gland is an organ that secretes hormones — chemical substances that can change the function and sometimes the structure of other body parts. Hormones secreted by three glands — the pituitary, the thyroid, and the adrenals — influence how much energy you use when your body’s at rest.

Your pituitary gland, a small structure in the center of your brain, stimulates your thyroid gland (which sits at the front of your throat) to secrete hormones that impact the rate at which your tissues burn nutrients to produce energy.

When your thyroid gland doesn’t secrete enough hormones (a condition known as hypothyroidism), you burn food more slowly, and your REE drops. When your thyroid secretes excess amounts of hormones (a condition known as hyperthyroidism), you burn food faster, and your REE is higher.

When you’re frightened or excited, your adrenal glands (two small glands, one on top of each kidney) release adrenaline, the hormone that serves as your body’s wake-up call. When you release adrenaline, your heartbeat increases. You breathe faster. Your muscles clench. And you burn food faster, converting it as fast as possible to the energy you need for the reaction commonly known as fight or flight. But these effects are temporary. The effects of your sex glands, on the other hand, last as long as you live.

If you’re a woman, you know that your appetite may rise and fall in tune with your menstrual cycle. In fact, this fluctuation parallels what’s happening to your REE, which goes up just before or at the time of ovulation. Your appetite is highest when menstrual bleeding starts and then falls sharply. Being a man (and making lots of testosterone) makes satisfying your nutritional needs on a normal American diet easier. Your male bones are naturally denser, so you’re less dependent on dietary or supplemental calcium to prevent osteoporosis (severe loss of bone tissue) later in life. You don’t lose blood through menstruation, so you need only less than half as much iron (8 milligrams for an adult male; 18 milligrams for a premenopausal woman who is neither pregnant nor nursing). Best of all, you can consume about 10 percent more calories than a woman of the same weight without adding pounds.

It is no accident that while teenage boys are developing wide shoulders and biceps, teenage girls are getting hips. Testosterone, the male hormone, promotes the growth of muscle and bone. Estrogen gives you fatty tissue. As a result, the average male body has proportionally more muscle; the average female body, proportionally more fat.

Muscle is active tissue. It expands and contracts. It works. And when a muscle works, it uses more energy than fat (which insulates the body and provides a source of stored energy but does not move an inch on its own). What this muscle versus fat battle means is that the average man’s REE is about 10 percent higher than the average woman’s. In practical terms, that means a 140-pound man can hold his weight steady while eating about 10 percent more than a 140-pound woman who is the same age and performs the same amount of physical work.

No amount of dieting changes this unfair situation. A woman who exercises strenuously may reduce her body fat so dramatically that she no longer menstruates, an occupational hazard for some professional athletes. But she’ll still have proportionately more body fat than an adult man of the same weight. If she eats what he does, and they perform the same amount of physical work, she still must take in fewer calories than he to hold her weight steady.

Energy for work

Your second largest chunk of energy after the REE is the energy you spend on physical work, everything from brushing your teeth in the morning to planting a row of petunias in the garden or working out in the gym.

Your total energy requirement (the number of calories you need each day) is your REE plus enough calories to cover the amount of work you do.

Does thinking about this use up energy? Yes, but not as much as you’d like to imagine. To solve a crossword puzzle, or write a chapter of this book, the average brain uses about 1 calorie every four minutes. That’s only one-third the amount needed to keep a 60-watt bulb burning for the same length of time.

Table 5-2 defines the energy level of various activities ranging from the least energetic (sleep) to the most (playing football, digging ditches). Table 5-3 shows how to make these numbers personal.

Table 5-2 How Active Are You When You’re Active?

Activity Level	Activity
Resting	Sleeping, reclining
Very light	Seated and standing activities, painting, driving, laboratory work, typing, sewing, ironing, cooking, playing cards, and playing a musical instrument
Light	Walking on a level surface at 2.5 to 3 mph, garage work, electrical trades, carpentry, restaurant trades, housecleaning, child-care, golfing, sailing, and table tennis
Moderate	Walking 3.5 to 4 mph, weeding and hoeing, carrying a load, cycling, skiing, tennis, and dancing
Heavy	Walking with a load uphill, tree felling, heavy manual digging, basketball, climbing, football, and soccer
Exceptionally heavy	Professional athletic training

The National Research Council, Recommended Dietary Allowances (Washington, D.C.: National Academy Press, 1989)

Table 5-3 How Many Calories Do You Need to Do the Work You Do?

Activity Level	Calories Needed for This Work for One Hour
Very light	80–100
Light	110–160
Moderate	170–240
Heavy	250–350
Exceptionally heavy	350+

“Food and Your Weight,” House and Garden Bulletin, No. 74 (Washington, D.C.: U.S. Department of Agriculture)

Calculating Your Daily Calorie Needs

Figuring out exactly how many calories to consume each day can be a consuming task. Luckily, the Institute of Medicine, the group whose Food and Nutrition Board determines the RDAs for vitamins, minerals, and other nutrients, has created a list of the average daily calorie allowance for healthy people from infants to senior citizens who maintain a healthful weight (see Chapter 4) based on the amount of activity a person performs each day.

Table 5-4 shows the calorie recommendations. Note that in this context, sedentary means a lifestyle with only the light physical activity associated with daily living; moderately active means a lifestyle that adds physical activity equal to a daily 1.5-to-3-mile walk at a speed of 3 to 4 miles per hour; active means adding physical activity equal to walking 3 miles a day at the 3 to 4 miles per hour clip.

Table 5-4 Estimated Daily Calorie Requirements for Healthy Adults* Based on Activity Level

	Age (Years)	Sedentary	Moderate	Active
Women	19–30	2,000	2,000–2,200	2,300
	31–50	1,800	2,000	2,200
	51 and older	1,600	1,800	2,000–2,200
Men	19–30	2,400	2,600–2,800	3,000
	31–50	2,200	2,400–2,600	2,800–3,000
	51 and older	2,000	2,200–2,400	2,400–2,800

* These calorie recommendations assume a person of healthful weight, that is women with a body mass index (BMI) of 21.5 and men with a BMI of 22.5 (for more on BMI, see Chapter 4).

Institute of Medicine, “Estimated Energy Requirements (EER), IOM Dietary Reference Intakes macronutrients report,” 2002

Calories are not your enemy. On the contrary, they give you the energy you need to live a healthy life. The trick is to manage your calories and not let them manage you. When you know which foods provide what energy, you can strategize your energy intake to match your energy expenditure, and vice versa. When you do, your body will say, “Thank you,” every day.