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.

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.

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.

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.)

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.

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!