What are calories?

You’ve consumed them, burned them, and cut them—but how much do you really know about calories? Here’s the expert take.
Published August 30, 2023

Ever have those stress dreams where you’re in class, your teacher calls on you with a question, and you have absolutely no idea what the answer is? That’s probably how you’d feel if someone brought you up in front of a crowd and asked you to explain in detail what calories are. Cue the nervous sweating! At first thought, the definition can seem simple enough. “They’re a measure of the energy that we get from food,” says Rhyan Geiger, RDN, a registered dietitian based in Phoenix, Arizona. But dive deeper and things start to get a little fuzzier.

For starters, all calories are not equal, notes Dana E. Hunnes, Ph.D, R.D., a senior dietitian at the Ronald Reagan UCLA Medical Center and an assistant professor with UCLA’s Fielding School of Public Health. In other words, 100 calories of salt and vinegar potato chips and 100 calories of spinach aren’t the same—to your taste buds or your body. Here, your calories crib sheet.

What is the definition of calories and k-cals?

It’s time to get a little technical. “Calories are a way of measuring the amount of heat that a particular substance, namely food, gives off,” explains Hunnes. The process of turning that heat into energy is what we call metabolism. All foods have a specific number of calories; the higher the number, the more energy you can theoretically get out of eating that food (more on how higher calorie foods can lead to weight gain later).

To figure out a food’s calories, scientists stick dried-out versions of the food into an instrument called a bomb calorimeter (cool name!), which measures how much heat it produces. They then calculate the calories based on the amount of heat it emits.

K-cals, on the other hand, stands for kilocalories. “The term is used in the same way as calories—to describe how much energy a food provides,” says Michelle Cardel, Ph.D, R.D., senior director of global clinical research and nutrition at WeightWatchers®. Calories and k-cals tend to be used interchangeably and mean the same thing. What you see on a nutrition facts label—calories or k-cals—mostly depends on where in the world you live. The United States and Canada almost always use calories, while places like the European Union and Australia use k-cals.

Why do we need calories?

Calories have a well-deserved reputation as being one of the most important food-related numbers (it is, after all, featured in large, bolded text at the very top of the nutritional label). That’s because we need calories to live. “They’re what give our bodies the fuel to do everyday activities like moving around, staying warm, growing, and thinking. Even breathing and basic organ function requires calories,” says Geiger.

If you’ve heard the phrase basal metabolic rate, that’s what it’s referring to: The calories you need in order to keep your body functioning even if you didn’t move at all. While your body will use up those calories every day no matter what, you need even more calories if you do anything active.

How do we burn calories?

It occurs in three distinct ways.

  • The first is that basal metabolism (a.k.a. basal metabolic rate) mentioned above, which is the energy your body needs just to…be. This includes breathing, creating and repairing cells, and thinking. While the brain accounts for roughly 2% of a person’s total body weight, it consumes around 20% of the calories we take in.
  • The second way the body uses calories is through the process of digesting food and absorbing nutrients. It’s true: Eating burns calories! This process uses about 10% of your total calories per day.
  • And the third way is via physical activity. That can include exercising, like going on a bike ride, playing tag with your kids, chores like cleaning the bathroom, and even fidgeting. Those daily activities can use anywhere from 100 to 800 calories a day, or even higher if you’re extremely active. That’s because if you take, say, a spin class, you get what’s known as the afterburn effect. Think of it like bonus points. “When you work out super hard, your body temperature increases and this actually burns more calories throughout the day,” Hunnes explains.

How many calories you burn also depends on your height, age, weight, and even muscle mass, notes Hunnes. “The more muscle you have, the higher your basal metabolic rate will be at rest—a pound of muscle basically burns 10 times as many calories as a pound of fat.”

What should my calorie intake be?

The short answer is that women 18 and older need 1600 to 2400 calories a day and men need between 2000 to 3000 a day, per the latest USDA Dietary Guidelines for Americans. But remember: Those are just averages. “There are definitely variables to look at when determining how many calories a person needs in a day,” says Hunnes. “Because if I'm 4-foot-8, I don't need 1600 calories in a day to do my basic activities, whereas if I'm 5-foot-10, I might need 2400 calories in a day.” In addition to height, your calorie needs may also be affected by your age, weight, sex, activity levels, and goals (like if you’re trying to lose weight or not).

If you’re using WeightWatchers, you don’t need to spend time calculating calories. The above variables are all considered when the program creates your Points® budget, which you can spend on whatever foods you like. Points are like calorie counters with advanced degrees—they’re calculated using nutritional info including calories, saturated fat, protein, sugar, fiber, and more.

Learn more about how Points work with WeightWatchers.

chart outlining 6 things that affects calorie needs
chart outlining 6 things that affects calorie needs

What happens when we don’t consume enough calories?

When you eat fewer calories than you need, your body will rely on energy it’s stored to keep your body functioning: first turning to energy stored in fat, then in muscles, says Cardel. That’s when you start losing weight. But if you eat too far below your energy needs for a long period of time, your body won’t “break down food as efficiently, or you may feel tired or weak,” Cardel explains. “Your body is essentially slowing to conserve that energy.” Eating too few calories can also affect your memory and focus and rob your body of electrolytes that affect your heart rhythm. If things get even more extreme, and you eat far-too-few calories for an extended period of time, this can set you up for higher risk of heart disease and even cardiac arrest.

So how low is too low? Many health organizations recommend a deficit of 500 calories a day for healthy weight loss, which would be a minimum of 1,100 calories a day for the average woman and 1,500 for the average man.

Can calories be “good” or “bad”?

There’s a trap some people fall into of labeling calories in food as either being good or bad, usually depending on how high or low the calorie count is. But high-calorie foods like almonds and avocados can be “amazingly good for your health,” says Hunnes. On the flip-side, some low-cal foods like certain “lite” salad dressings might not be the more nutritious choice because they’re higher in sodium or sugar. That’s why experts recommend you don’t place a moral value on calories.

In general, it’s better to think of foods as having higher nutrient density or lower nutrient density, says Geiger. The nutrient density doesn’t always correlate to calorie count, either. Fruits and veggies, for example, tend to have high-nutrient density and lower calorie counts. “I consider fruits and vegetables to be something that you can eat pretty much endlessly and not really be concerned about because they're so high in fiber and healthy nutrients,” says Hunnes. (At WeightWatchers, we call these ZeroPoint® foods.) “They’re also very satiating–you can only eat so much because they take up so much volume.”

Lower nutrient density foods are sometimes referred to as “empty calories,” which Cardel describes as foods that provide a lot of calories without very many nutrients like protein, fiber, vitamins, or minerals. Think soda, sweetened iced teas, potato chips, and candy. There’s nothing bad about them (no food is off limits!), but they just don’t give you much nutritional bang for your buck.

How do calories affect weight loss or gain?

To lose weight, you have to eat fewer calories than you burn off. That’s the trick to forcing your body to use up its energy stores (a.k.a. fat) in order to keep itself going. When the opposite happens and you consume more calories than you burn, you gain weight.

Could you eat 1,200 calories a day of lower nutrient-density foods and lose weight? Yes, but you won’t feel great or improve your health along the way. That’s why a weight-loss plan has to include much more than just counting calories. Here are a few things to focus on when thinking how calories can play into healthier eating habits.

  1. Think data, not numbers. If you’re trying to lose weight, it’s good to have a general idea of roughly how many calories you’re consuming right now. “If you don't know where you're starting from, it's hard to know where you're heading,” says Hunnes. After you’ve figured out what types of foods you eat more of and what your goal is (to lose 15 pounds, for instance), then you can begin to make tweaks to improve the quality of your diet and reduce your calories.
  2. Keep a log of what you eat. To help you stay on track and reach your goals, try journaling, taking photos of the foods you eat, or using an app like WeightWatchers, which does the calorie-counting and more for you via our Points system, says Hunnes.
  3. Look at the big picture. You don’t want to drive yourself crazy thinking about everything you eat. To lower the stress, focus on your ZeroPoint food list if you’re on WeightWatchers, suggests Cardel. “These are ‘go-to’ foods you can reach for without weighing, measuring, or tracking. They can help you to form the foundation of meals and snacks without worrying about Points,” she says. Another way to think about your meals is looking at the plate in front of you, advises Geiger. Is there protein, fruit, vegetable, and whole grains? Lots of colorful foods?
  4. Give yourself props. Your health and success in going after your goal should not depend solely on how many calories you eat in a given day. “It’s important to celebrate other victories like increased energy, better lab results, looser fitting clothes, and a craving for fresh foods,” Geiger says. After all, in the long run, these measures tell you far more about your health than your calorie intake.

The bottom line

Calories are a number related to how much heat a food gives off and how much energy it will take to turn that food into energy. Your body needs calories to do everything it needs to throughout the day, from breathing to walking. And while calories are important to weight loss–eating fewer calories than you burn is the key to consistently losing weight–they aren’t the only thing to focus on when choosing foods. Eating foods high in nutrients like protein and fiber, even if they are also high in calories, can help you feel better and improve your overall health.