Automobiles burn gasoline to get the energy they need to move. Your body burns (metabolizes) food to produce energy in the form of heat. This heat warms your body and (as energy) powers every move you make. Nutritionists measure the amount of heat produced by metabolizing food in units called kilocalories.

A kilocalorie is the amount of energy it takes to raise the temperature of 1 kilogram of water 1 degree on a Centigrade (Celsius) thermometer at sea level. In common use, nutritionists substitute the word calorie for kilocalorie. This information isn’t scientifically accurate: Strictly speaking, a calorie is really 1⁄1000 of a kilocalorie.

But the word calorie is easier to say and easier to remember, so that’s the term you see whenever you read about the energy in food. And few nutrition-related words have caused as much confusion and concern as the lowly calorie. Read on to find out what calories mean to you and your nutrition.

Counting the Calories in Food

When you read that a serving of food — say, one banana — has 105 calories, that means metabolizing the banana produces 105 calories of heat that your body can use for work. You may wonder which kinds of food have the most calories. Here’s how the calories measure up in 1 gram of the following foods:

• Protein: 4 calories
• Carbohydrates: 4 calories
• Alcohol: 7 calories
• Fat: 9 calories

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 high in calories, while low-fat foods, such as bagels (minus the cream cheese, of course), are not.

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 is (sorry about this) full of it. A ounce serving of skinless chicken provides 140 calories, while a ounce burger yields 230 to 245 calories, depending on the cut of the meat.

Empty calories

All food provides calories. All calories provide energy. But not all calories come with a full complement of extra benefits such as amino acids, fatty acids, fiber, vitamins, and minerals. Some foods are said to give you empty calories.

This term has nothing to do with the calorie’s energy potential or with calories having a hole in the middle. It describes a calorie with no extra benefits. 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.

People who abuse alcohol aren’t always thin, but the fact that they often substitute alcohol for food can lead to nutritional deficiencies, most commonly a deficiency of thiamin (vitamin B1), resulting in loss of appetite, an upset stomach, depression, and an inability to concentrate.

Of course, it’s only fair to point out that sugar and alcohol are ingredients 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 have 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.

But many Americans who can afford enough food nevertheless 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 can cause significant health problems, such as having weak bones; being underweight (yes, being too thin can be a problem); getting bleeding gums, skin rashes, and other nasties; and developing mental disorders, including depression and preventable retardation.

Every calorie counts

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 seems to arise like a phoenix in every generation of dieters is the high-protein diet.

The high-protein diet says 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 until your eyes bug out, 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. Boy, wouldn’t it be nice if that were true? The problem is, it isn’t.

Here’s the absolute truth: 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 less than you use up, you’ll lose weight. This nutrition rule is an equal opportunity, one-size-fits-all proposition that applies to everyone.

How Many Calories Do 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, 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.

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 signals to every part of your body.

The energy that your resting body uses to do all this stuff is called (surprise! surprise!) resting energy expenditure, abbreviated REE. The REE, also known as the basal metabolism, accounts for a whopping 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 the number in pounds by 2.2. For example, if you weigh 150 pounds, that’s equal to 68.2 kg (150 ÷ 2.2). Plug that into the appropriate equation in Table below — and bingo! You have your REE.

How Many Calories Do You Need When You’re Resting?

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

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

Sex, glands, and chocolate cake

A gland is an organ that secretes hormones, which are chemical substances that can change the function — and sometimes the structure — of other body parts. For example, your pancreas secretes insulin, a hormone that enables you to digest and metabolize carbohydrates.

At puberty, your sex glands secrete either the female hormones estrogen and progesterone or the male hormone testosterone; these hormones trigger the development of secondary sex characteristics, such as the body and facial hair that make us look like either men or women.

Hormones can also affect your REE, 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 influence 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 call to battle stations.

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 the sex glands, on the other hand, last as long as you live. Read on.

How your hormones affect your energy needs

If you’re a woman, you know that your appetite rises and falls 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. Yes, you really are hungrier (and need more energy) just before you get your period.

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) late in life.

You don’t lose blood through menstruation, so you need only two-thirds as much iron. Best of all, you can consume about 10 percent more calories than a woman of the same weight without adding pounds.

Teenage boys’ developing wide shoulders and biceps while teenage girls get hips is no accident. 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 requires fewer calories than he to hold her weight steady. And here’s a really rotten possibility. Muscle weighs more than fat. This interesting fact is one that many people who take up exercise to lose weight discover by accident.

One month into the barbells and step-up-step-down routine, their clothes fit better, but the scale points slightly higher because they’ve traded fat for muscle — and you know what that means: Sometimes you can’t win for losing. (Sorry, but I just couldn’t resist.)

Energy for work

Your second largest chunk of energy is the energy you withdraw to spend on physical work. That’s everything from brushing your teeth in the morning to hoeing 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 —— 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 2 defines the energy level of various activities ranging from the least energetic (sleep) to the most (playing football, digging ditches). Table 3 shows how many calories you use in an hour’s worth of different kinds of work.

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)

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)

Enjoying the extras

Are you a sensible foodie? If you’re supposed to have no more than 2,000 calories a day, can you pack all the vitamins, minerals, protein, heart-healthy fats, and carbs you need into 1,800 calories? Do that, and the folks who wrote the Dietary Guidelines for Americans 2005 say, reward yourself.

Use the “leftover” 200 calories — called discretionary calories — for anything that makes your mouth water. Naturally, some expert spoilsports disagree. They say that giving you an inch (those leftover calories) means you’ll take a mile (three pieces of chocolate cake). Prove them wrong and celebrate your smarts. Yum!

How Much Should You Weigh?

Through the years, a number of charts have purported to lay out standard or healthy weights for adult Americans, but some set the figures so low that you can hardly get there without severely restricting your diet — or being born again with a different body, preferably with light bones and no curves.

Studying weight charts

Table 4 is one moderate, eminently usable set of weight recommendations that originally appeared in the 1990 edition of Dietary Guidelines for Americans published by the U.S. Department of Agriculture and the U.S. Department of Health and Human Services.

How Much Should You Weigh?

Height Weight	Weight (Pounds) for 19- to 34-Year-Olds	Weight (Pounds) for 35-Year-Olds and Older
5' 5'1" 5'2" 5'3" 5'4" 5'5" 5'6" 5'7" 5'8" 5'9" 5'10" 5'11" 6' 6'1" 6'2" 6'3" 6'4" 6'5" 6'6"	97–128 101–132 104–137 107–141 111–146 114–150 118–155 121–160 125–164 129–169 132–174 136–179 140–184 144–189 148–195 152–200 156–205 160–211 164–216	108–138 111–143 115–148 119–152 122–157 126–162 130–167 134–172 138–178 142–183 146–188 151–194 155–199 159–205 164–210 168–216 173–222 177–228 182–234

The weights in this chart are listed in ranges for people (men and women) of specific heights. Naturally, height is measured without shoes, and weight is measured without clothes.

Because most people gain some weight as they grow older, Table 4 does a really sensible thing by dividing the ranges into two broad categories, one for men and women ages 19 to 34, the other for men and women ages 35 and older.

People with a small frame and proportionately more fat tissue than muscle tissue (muscle is heavier than fat) are likely to weigh in at the low end. People with a large frame and proportionately more muscle than fat are likely to weigh in at the high end.

As a general (but by no means invariable) rule, that means that women — who have smaller frames and less muscle — weigh less than men of the same height and age.

I feel honor bound to tell you that the 2000 and 2005 editions of the Dietary Guidelines leave out the higher weight allowances for older people, which means that the healthy weights for everyone, young or old, are the ones listed in the column for 19- to 34-year-olds.

I’m going to go out on a limb here to say that I prefer the 1990 recommendations because they’re:

• Achievable without constant dieting
• Realistic about how your body changes as you get older
• Less likely to make you totally crazy about your weight

. . . which is a pretty good description of how nutritional guidelines need to work, don’t you think?

Another way to rate your weight: Calculating your BMI

As you run your finger down the chart in Table 4, remember that the numbers are guidelines — no more, no less. Squeezing people into neat little boxes is a reassuring exercise, but in real life, human beings constantly confound the rules. We all know chubby people who live long and happy lives and trim and skinny ones who leave us sooner than they should.

However, people who are overweight have a higher risk of developing conditions such as arthritis, diabetes, and heart disease, so you need a way to find out whether your current weight puts you at risk.

One good guide is the Body Mass Index (BMI), a number that measures the relationship between your weight and your height and offers some predictive estimate of your risk of weight-related disease. The Body Mass Index (BMI) provides a second way to determine who’s tipping the scales.

In the United States, a BMI below 18.5 is currently considered underweight, 18.5–24.9 is normal, 25.0–29.9 is overweight, and 30.0 and higher is obese. Other countries have slightly different standards.

For example, in Australia, a BMI below 19 is underweight, 20–25 is normal, 26–30 is overweight, and 31+ is obese. In Canada, a BMI below 18.5 is underweight, 18.5–24.9 is normal, 25–29.9 is overweight, 30–34.9 is Class 1 obese, 35–39.9 is Class 2 obese, and 40+ is Class 3 obese. In Great Britain, a BMI below 20 is underweight, 20–25 is normal, 25–30 is overweight, and 30+ is obese.

To calculate your BMI, perform the following steps:

1. Divide your weight (in pounds) by your height (in inches) squared.
2. Multiply the result of Step 1 by 705.

Currently, the healthiest BMI seems to be 21.0. A BMI higher than 28 (168 pounds for a 5'5" woman; 195 pounds for a 5'10" man) appears to double the risk of diabetes, heart disease, and death.

How reliable are the numbers? Considering confounding variables

Weight charts and tables and numbers and stats are so plentiful that you may think they’re totally reliable in predicting who’s healthy and who’s not. So here’s a surprise: They aren’t. The problem is that real people and their differences keep sneaking into the equation.

For example, the value of the Body Mass Index in predicting your risk of illness or death appears to be tied to your age. If you’re in your 30s, a lower BMI is clearly linked to better health.

If you’re in your 70s or older, no convincing evidence points to how much you weigh playing a significant role in determining how healthy you are or how much longer you’ll live. In between, from age 30 to age 74, the relationship between your BMI and your health is, well, in-between — more important early on, less important later in life.

In other words, the simple evidence of your own eyes is true. Although Americans sometimes seem totally obsessed with the need to lose weight, the fact is that many larger people, even people who are clearly obese, do live long, happy, and healthy lives.

To figure out why, many nutrition scientists now are focusing not only on weight or weight/height (the BMI) but on the importance of confounding variables, which is sciencespeak for “something else is going on here.” Here are three potential confounding variables in the obesity/health equation:

• Maybe people who are overweight are more prone to illness because they exercise less, in which case stepping up the workouts may reduce the perceived risk of being overweight.

• People who are overweight may be more likely to be sick because they eat lots of foods containing high-calorie ingredients, such as saturated fat, that can trigger adverse health effects; in this case, the remedy may simply be a change in diet.

• Maybe people who are overweight have a genetic predisposition to a serious disease. If that’s true, you’d have to ask whether losing 20 pounds really reduces their risk of disease to the level of a person who is naturally 20 pounds lighter. Perhaps not: In a few studies, people who successfully lost weight actually had a higher rate of death.

Adding to the confusion is the fact that an obsessive attempt to lose weight may itself be hazardous to your health. Every year, Americans spend $30 billion to $50 billion (yes, you read that right) on diet clubs, special foods, and over-the-counter remedies aimed at weight loss.

Often the diets, the pills, and the foods don’t work, which can leave dieters feeling worse than they did before they started. The chance that the diet fails is only half the bad news. Here’s the rest: Some foods that effectively lower calorie intake and some drugs that effectively reduce appetite have potentially serious side effects.

For example, some fat substitutes prevent your body from absorbing important nutrients, and some prescription diet drugs, such as the combination once known as Phen-Fen, are linked to serious, even fatal, diseases.

Facing the numbers when they don’t fit your body

Right about here, you probably feel the strong need for a really big chocolate bar (not such a bad idea now that nutritionists have discovered that dark chocolate is rich in disease-fighting antioxidants). But it also makes sense to consider the alternative: realistic rules that enable you to control your weight safely and effectively.

Check out the following:

• Rule No. 1: Not everybody starts out with the same set of genes — or fits into the same pair of jeans. Some people are naturally larger and heavier than others. If that’s you, and all your vital stats satisfy your doctor, don’t waste time trying to fit someone else’s idea of perfection. Relax and enjoy your own body.

• Rule No. 2: If you’re overweight and your doctor agrees with your decision to diet, you don’t have to set world records to improve your health. Even a moderate drop in poundage can be highly beneficial. According to The New England Journal of Medicine (www.nejm.org on the Net), losing just 10 to 15 percent of your body weight can lower high blood sugar, high cholesterol, and high blood pressure, reducing your risks of diabetes, heart disease, and stroke.

• Rule No. 3: The only number you really need to remember is 3,500, the number of calories it takes to gain or lose one pound of body fat. In other words, one pound of body fat equals 3,500 calories. So if you simply:

• Cut your calorie consumption from 2,000 calories a day to 1,700 and continue to do the same amount of physical work, you’ll lose one pound of fat in just 12 days.

• Go the other way, increasing from 1,700 to 2,000 calories a day without increasing the amount of work you do, 12 days later you’ll be one pound heavier.

• Rule No. 4: Moderation is the best path to weight control. Moderate calorie deprivation on a sensible diet produces healthful, moderate weight loss; this diet includes a wide variety of different foods containing sufficient amounts of essential nutrients.

Abusing this rule and cutting calories to the bone can turn you literally into skin and bones, depriving you of the nutrients you need to live a normal healthy life.

• Rule No. 5: Be more active. Doing exercise allows you to take in more calories and still lose weight. In addition, exercise reduces the risk of many health problems, such as heart disease. Sounds like a recipe for success.

How many calories do you really need?

Figuring out ex-act-ly how many calories to consume each day can be a, well, consuming task. Luckily, the Dietary Guidelines for Americans 2005 lays out a list of the average daily calorie allowance for healthy adults with a healthful BMI — 21.5 for women and 22.5 for men — based on the amount of activity a person performs each day. Table 5 shows numbers.

Matching Daily Calories with Daily Lifestyle

Gender/Age*	Calories If Sedentary	Calories If Moderately Active	Calories If Active
Female
19-30 years	2,000	2,000–2,200	2,400
31-50	1,800	2,000	2,200
51+	1,600	1,800	2,000–2,200
Male
19-30	2,400	2,600–2,800	3,000
31-50	2,200	2,400–2,600	2,800–3,000
50+	2,000	2,200–2,400	2,400–2,600

* As a rule, men have proportionately more active tissue (muscle) than women do, so an average man’s calorie requirements are about 10 percent higher than an average woman’s. Dietary Guidelines for Americans, 6th ed. (Washington D.C.: U.S. Department of Agriculture, U.S. Department of Health and Human Services, 2005)

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 1.5–3 mile walks at a speed of 3–4 miles per hour; active means adding physical activity equal to walking 3 miles a day at the 3–4 mph clip.

The last word on calories

Calories are not your enemy. On the contrary, they give you the energy you need to live a healthy life. The trick is managing your calories and not letting them manage you. After you know that fats are more fattening than proteins and carbohydrates and that your body burns food to make energy, you can strategize your energy intake to match your energy expenditure, and vice versa.