You might think you can see them or taste them. There they are, congregating in great numbers, in that cheesecake or lasagna or ice cream sundae. But they’re not physical things. So calories remain mysterious to most of us, even as they’re crucial components of fitness: drivers of energy, power, and endurance as well as muscle additions and fat subtractions. Let’s solve some mysteries.

CALORIE (noun) the amount of heat required to raise the temperature of one kilogram of water one degree Celsius. Origin: 1819 by chemical engineer Nicolas Clément in a lecture in Paris.

WHERE ARE FOOD CALORIES?

They’re there—but in the way electricity, measured in watts, is powering the screen on which you’re reading this sentence. Food energy is measured in calories. So, a protein bar might contain 240 calories (units of energy), but within that bar they’ll be distributed differently: more than twice as much in fat than in protein and carbs. It breaks down like this:

  • 1 g. of fat has 9 calories
  • 1 g. of alcohol has 7 calories
  • 1 g. of carbohydrates has 4 calories
  • 1 g. of protein has 4 calories
  • 1 g. of fermentable fiber has 2 calories.

ARE ALL CALORIES EQUAL?

Two clichés are in conflict here. In the one corner: “A calorie is a calorie.” In the other: “Not all calories are equal.” Leaving aside the health effects of white flour, processed sugar, saturated fat, and preservatives in the preserves (hello, donut), there’s no difference between calories in a Krispy Kreme jelly donut and the calories in romaine lettuce. They’re all units of food energy, to be burned or stored.

That said, some foods, usually those that require more chewing—like whole grains, meats, and fibrous fruits and vegetables—use more calories than others during digestion. Other foods, like coffee, chilis, and ginger, include ingredients that can boost the rate of caloric-burning. So, the calories are always equal, but the foods may not be.

Which brings us to an important concept: calorie density. That one donut contains the same number of calories (340) as 42.5 cups of shredded lettuce! Or to, put a finer point on it, 7 oz. of roasted chicken breast and a cup of lettuce equals one delicious jelly donut. We love donuts, but it does illustrate that if you want to lose fat but not go hungry, low-caloric-density, fibrous foods are essential. Conversely, to gain weight, skip pre-meal salads and eat (and drink) more high-caloric-density foods. 

HOW MANY CALORIES DO YOU NEED TO MAINTAIN YOUR WEIGHT?

This number depends on your age, size, and activity levels. Here’s a calculator. (If you know your body fat percentage, switch to Katch-McArdle under settings.) Caveat: The more diesel you are the less accurate that formula will be, because it estimates your basil metabolic rate (BMR) from your height and weight without concern for muscle or fat—and, when at rest, muscle burns calories three times faster than fat does. You may get a more accurate number by carefully tracking your calorie consumption and body weight—and monitoring, if only by the mirror, your body fat percentage.

Helmut Strebl: The Living Anatomy Chart

HOW MANY CALORIES DO YOU NEED TO GAIN OR LOSE A POUND?

This depends on whether we’re talking about adding muscle or subtracting flab—and either way it’s more complicated than usually said. A pound of body fat contains roughly 3500 calories, so most sources say, if you daily eat 500 fewer calories than your maintenance level, you’ll shed a pound per week. But everyone’s BMR is different.

What’s more, a pound of muscle contains only approximately 700 calories. So, if you lose 3500 calories from fat and, simultaneously, lost just 10% that amount from muscle, you’ll shed a pound of bad flesh but also a half-pound of great flesh. (This also explains why weight and strength can vanish rapidly during serious illnesses. If your body uses only muscle for energy, a mere 3500 calorie deficit will vaporize a whopping five pounds of muscle.)

Unfortunately, this doesn’t work in reverse. An extra 100 calories per day won’t add a sinewy pound weekly. It takes approximately 2500-2800 calories to construct a pound of muscle, even though muscle only keeps around a quarter of those calories. Your body wants to store any excess calories in a fat reserve. That’s why you lift weights (to stimulate muscle growth) and suffer through cardio (to burn calories headed for fat reserves or already deposited there).

And that’s why these pound-per-week equations are ultimately grad school, despite the temptation to make them grade school. More grad school syllabus:

• dietary fat will not be converted to glucose (muscle fuel) or amino acids (muscle construction material);

carbohydrates will be converted to glucose but excesses will be stored as fat;

protein’s amino acids are muscle-makers, and, though excess amino acids can be converted to fatty acids and stored as fat, this is unlikely.

So, if you don’t care what kind of pound you lose, a deficit of 3500 on average (dependent on your BMR) will shed at least one pound (and a surplus of 3500 will add a flabby pound). But if, like most of us, you care deeply what kind of pound is lost or gained, your diet’s macro nutrient balance will be more important to your success than a calorie count.