How Much Do You Really Know About Calories?

The short answer: a calorie is a unit of energy. The cells in your body need energy to move blood, breathe air, and pump muscles—and they get it from three sources: carbohydrates, proteins, and fats (otherwise known as macronutrients). You get nine calories per gram of fat, compared to four calories per gram of carbohydrate or protein. Surplus calories are stored as fat, which can cause weight gain—so it’s important to balance what you’re using with what you’re consuming—for a handy way to calculate what you burn during a run, try our Calories Burned Running Calculator. 

Everybody has what is called a basal metabolic rate (BMR), or the amount of energy your body burns just to maintain essential functions like breathing, thinking, and pumping blood. Your BMR is determined by your height, weight, and age, and can range from around 1,400 calories for women to 2,000 for men, give or take.

One way to think of your BMR is like an electric bill: You know the basic cost per month, but it can vary depending on whether the AC or heat is on. “When you add activity, your electric bill goes up,” says Douglas Kalman, PhD., sports nutritionist with Florida International University athletics. Estimate how many calories you burn with our Daily Calories Calculator.

Though the USDA keeps a chart of estimated calorie needs per day by age, gender, and physical activity level, the long-standing guideline is this: For active adults age 31-50, men need 2,800 to 3,000 calories daily and women need around 2,200. Of course, the magic word is “active,” which varies per person and depends on factors like intensity and metabolic health (i.e., how effectively your body burns calories). For most exercise-conscious people, these numbers are a solid foundation. However, the more active you are, the more calories you need.

How do you determine your specific needs? “Keep a food journal to record your daily calories for a week,” says sports nutritionist Heather Mangieri, MS, RDN, author of Fueling Young Athletes. That way you can adjust your intake depending on how you feel. “If caloric intake satisfies your hunger and maintains your body weight goals, but not your performance goals, try adjusting your meal timing.” Mangieri adds. “Sometimes, it's just a matter of reorganizing when you take in those calories.” 

The short answer is no. “In general, people who exercise for less than 75 minutes do not need extra calories,” says Kalman. There’s an exception if you have not eaten in six or more hours or are extra lean, but otherwise there is no real advantage to stuffing the tank before a short to moderate workout. “The average body has 100,000 calories of stored fat, so it can access fuel when it needs to.”

Yes, and it’s important to get them at the right time, from the right sources. You’ve probably heard about the anabolic window—around 30 minutes post-run—when the body is primed to accept nutrients for optimal muscle tissue repair.  How much do you need? The exact amount will depend on your expenditure, and “is not as important as the proper macronutrient balance,” says Mangieri. Her rule of thumb is a 2:1 carb to protein ratio.

When you are briefly calorie-deficient, the body taps into stored glycogen for energy. If you move into a more long-term deficiency, your body will start to metabolize fat, which is why long, low-intensity workouts are great if you’re trying to get lean. The problem is that if you don’t bring enough fuel, your body will begin to burn protein (essentially eating its own muscle), which can hurt your performance. “This is why you need to monitor your calorie-intake when exercising for longer than 75 minutes,” says Kalman. Whenever possible you want to avoid forcing your body to burn protein. 

Yes; not all calories are created equally. And while a doughnut might give you the same energy burst as a banana on the bike, one is packed with vitamins while the other is packed with processed sugars and fats, which will ultimately hurt you in the long run. Whenever you can, try to get your calories from foods as close to their natural state as possible. (That said, fast foods might actually help you recover.)

Fitness trackers can give you a good estimate—especially if you’re just looking at your relative output day-to-day—but no tracker is 100 percent accurate. In fact, a 2016 study in JAMA Internal Medicine found that most of the popular fitness trackers have a margin of error of around 200 calories per day in either direction. 

Related: Take Your Fitness Tracking to the Next Level With These Training Variables