A calorie is a unit of energy. Historically, scientists have defined "calorie" to mean a unit of energy or heat that could come from a variety of sources, such as coal or gas. In a nutritional sense, all types of food — whether they are fats, proteins, carbohydrates or sugars — are important sources of calories, which people need to live and function. You can usually find calorie counts listed on food items, and wearables like the best fitness trackers allow you monitor how many calories you're burning by doing different activities.
"Our brains, our muscles — every cell in our body — require energy to function in its optimal state," said Jennifer McDaniel, a registered nutritionist dietitian in Clayton, Missouri, and spokesperson for the Academy of Nutrition and Dietetics. "So for one, we want to nourish our body right and our brain right. If we don’t get enough of those nutrients [that calories provide], there are negative consequences, whether its losing lean muscle mass, not being able to concentrate or not having the energy we need on a day-to-day basis."
The physics of calories
According to an article in the Journal of Nutrition, titled “History of the Calorie in Nutrition,” in 1863, a calorie was defined as the amount of heat needed to raise the temperature of 1 kilogram of water from 0 to 1 degree Celsius. In 1925, calories became scientifically defined in terms of joules, which are units typically used by physicists to describe the amount of work needed to force one newton through one meter. This is why you sometimes see calories being called kilojoules, especially in Europe and Australia. One calorie equals 4.18 joules; 1 joule equals 0.000239006 of a calorie.
The amount of heat needed to make a calorie differs at different temperatures, so scientists decided to create different types of calories according to their water temperature. Different temperatures yield different types of calories, such as the small calorie, also called the gram calorie or the 15-degree calorie. This calorie refers to the amount of heat needed to raise the temperature of 1 gram of water from 14.5 to 15.5 degrees Celsius.
A calorie in nutrition is actually 1,000 of these small calories. Some researchers use the term kilocalories to refer to the nutritional unit of 1,000 small calories. These units of 1,000 small calories are also sometimes called large calories, dietary calories, nutritional calories, food calories and Calories with a capital C.
Therefore, what Americans see on food labels are actually kilocalories, or kilojoules. When the U.S. Department of Agriculture says that one medium-size apple contains 95 calories, it actually contains 95 kilocalories. (This article uses the term calorie instead of kilocalories.)
Different types of macronutrients have standard amounts of calories. One gram of protein has 4 calories. One gram of carbohydrates has 4 calories. One gram of fat has 9 calories, according to the McKinley Health Center.
How many calories should people eat every day?
How many calories a person needs in a day depends on the individual’s activity level and resting metabolic rate, which can be measured at a doctor or dietitian’s office, McDaniel said. “There’s conventional wisdom that men shouldn’t eat fewer than 1,500 calories and women 1,200 calories to ensure that they’re getting a balance of major nutrients and micronutrients.”
The National Institutes of Health (NIH) provides general guidelines of calorie requirements for various ages and activity levels. A middle-age moderately active female should consume 2,000 calories per day. A middle-age moderately active male should consume 2,400 to 2,600 calories per day.
What should those calories be?
The official Dietary Reference Intakes (DRI) published by the National Academies of Sciences, Engineering and Medicine recommends that for an adult, 45 to 65 percent of calories should come from carbohydrates, 20 to 25 percent should come from fat and 10 to 35 percent should come from protein. Children need a higher proportion of fat, between 25 and 40 percent of their calories. No more than 25 percent of total calories should come from added sugars.
McDaniel said that beyond those general guidelines the amount of macronutrients needed depends on the individual’s activity level. An athlete, for example, needs more carbohydrates. Food preferences also play a role. “I love bread so I’d never go on a low-carb diet because I want a sustainable approach,” she said.
What are high-calorie foods?
Foods that are considered high-calorie, or calorically dense, have a high amount of calories relative to their serving size, according to the Mayo Clinic. Oils, butter and other fats; fried foods; and sugary sweets are high-calorie foods. While high-calorie foods are often associated with junk food, some are high in nutrients, as well.
Healthy foods that are high in calories include avocados (227 calories each), quinoa (222 calories per cup), nuts (828 calories per cup of peanuts), olive oil (119 calories per tablespoon), whole grains, and, in moderation, dark chocolate (648 calories per bar), according to the USDA Nutrition Database.
Raisins are an example of a high-calorie food that might surprise some people; you could eat 1 cup of grapes and get the same amount of calories as from one-quarter cup of raisins, according to the Mayo Clinic. Dried fruits are usually calorically dense; for this reason, they are popular among hikers burning a lot of calories.
What are low-calorie foods?
Foods that are considered low-calorie have a low amount of calories relative to their serving size. Fruits and especially vegetables are usually relatively low in calories. For example, 2 cups of shredded romaine lettuce or spinach have 16 calories, a large stalk of celery has 10 calories, 1 large ear of corn has 123 calories, 1 cup of broccoli has 15 calories and an orange has 70 calories, according to the USDA Nutrition Database.
What are empty calories?
Empty calories contain few to no nutrients. They often come from added sugars and solid fats, according to the Choose My Plate campaign run by the USDA. Solid fats are fats that solidify at room temperature, like butter, shortening and fats found in some meats. They can occur naturally but are often added to foods.
Many typical American foods have a lot of empty calories. Choose My Plate lists ice cream, sodas, cheese, pizza and processed meats like hot dogs and sausages as examples of popular foods high in empty calories. Some of these foods, like cheese and pizza, also contain nutrients (cheese is high in calcium and contains protein; pizza sauces, toppings and crusts can have nutrients) but other foods, like sodas and most candies, contain only empty calories. Choose My Plate calls these empty-calorie foods.
Calories and weight loss
Though it is important to consume sufficient calories, counting and cutting calories can help many people shed pounds. Calories are expended through physical activity. For example, running a mile might burn around 112 calories, according to Runner’s World magazine. The Centers for Disease Control and Prevention (CDC) refers to the balance of calories burned and calories consumed as caloric balance. It functions like a scale; when you are in balance, the calories consumed are balanced by the calories burned. This means you will maintain your body weight.
According to the CDC, if you are maintaining your weight, you are in caloric balance. This means that every day, you are consuming roughly the same amount of calories you are burning. If you are in caloric excess, you are eating more calories than you are burning and you will gain weight. If you are in caloric deficit, you are burning more calories than you are eating, and you will lose weight.
When people want to lose weight they often attempt to have a caloric deficit. But even a person with a caloric deficit needs to consume sufficient calories to function and stay healthy. Eating enough calories helps maintain muscle mass during the weight-loss process, McDaniel said. It also helps make weight loss sustainable. “If someone slashes calories to lose weight, where do they go from there? Does that mean they continue eating less and less?” she asked.
Consuming sufficient calories during the weight-loss process is also important to maintaining a healthy metabolic rate. “Chronic under-eating — eating 800 or 1,000 calories a day over time — lowers your resting metabolic rate, [which impacts] how many calories you burn, and your metabolic machinery,” said McDaniel.
Counting calories and cutting calories for weight loss
Counting and cutting calories can be a practical approach to weight loss. There are, however, other approaches to weight loss for those who don’t want to count calories, such as regimens that focus on changes in dietary behavior rather than calorie counts.
Whether someone chooses to count calories or take a behavioral change approach, McDaniel said it is important to “find ways to not just slash calories but replace them with healthier options that are still emotionally and physically gratifying for that individual.”
Someone interested in losing weight should consider how many calories he or she is eating, how many calories he or she needs and the differences between those numbers, McDaniel said. If someone is eating more calories than necessary, that person will need to change his or her behavior.
It’s important that behavior change to a “sustainable pattern that reduces calories,” McDaniel said. She gave the example of a man who eats 300-500 calories of ice cream every night after dinner. “We might change that to putting a frozen banana in the blender and adding a little cocoa powder to make soft serve ‘ice cream’ … This cuts the calories in half and he’s still getting the same mouth feel, pleasure and sweetness.”
The National Institutes of Health (NIH) defines low-calorie diets as meal plans that provide 1,000 to 1,200 calories per day for women and 1,200 to 1,600 calories per day for men. That number is sometimes adjusted for age, weight and activity level. Low-calorie diet plans typically consist of regular food but sometimes contain meal replacements.
McDaniel does not endorse low-calorie diets. “Low-calorie diets are not something we’ve seen supported in research or in my practice as something that works long term,” she said.
“It is not sustainable to slash calories. It also puts you at risk of losing lean muscle mass,” she said. People often regain the weight that they lose on low-calorie diets when they return to regular caloric intake. “Sometimes, they’ve been following a low-calorie diet for so long that they find themselves desiring high-calorie foods or falling into a binge eating pattern,” McDaniel said. The unsustainability of low-calorie diets coupled with loss of muscle mass means that regained weight is usually fat, not muscle.
When that happens, McDaniel said, it is doubly harmful. “Not only were they unable to sustain something, leading them to feel like they’ve failed, but then they’ve also disrupted the right ratio between good weight [muscle] and bad weight [excess fat] and put more [bad weight] on the body.”
Though McDaniel does not recommend low-calorie diets be used consistently, she does say there might be a time and place for low-calorie meal plans. “The concept of intermittent fasting is gaining some popularity,” she said. Intermittent fasting might consist of a day of low-calorie intake once a week. Intermittent fasting gained attention after a series of animal studies showed that it appeared to increase longevity. A 2014 review published in the journal Cell Metabolism found that in humans, intermittent fasting might help “reduce obesity, hypertension, asthma and rheumatoid arthritis.”
“Something like that might be one of the tools in someone’s toolbox they can lose for weight loss,” said McDaniel.
Very low-calorie diets
Experts distinguish between low-calorie diets and "very low-calorie diets." According to the NIH, very low-calorie diets are special diets in which all meals are replaced with prepared formulas, often liquid shakes. These are not the same as shakes and other meal replacements found commercially.
Very low-calorie diets provide up to 800 calories a day. These kinds of diets require a doctor or other health care provider’s guidance and regular monitoring. You should not start one on your own. People prescribed very low-calorie diets are usually obese and facing serious health problems like diabetes, high blood pressure and high cholesterol.