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.
Calories in the foods we eat provide energy in the form of heat so that our bodies can function. This means that we need to eat a certain amount of calories just to sustain life. But if we take in too many calories, then we risk gaining weight.
“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.”
When we talk about eating and exercise, we often talk in terms of calories. Menus at chain restaurants quote the number of calories in every dish. Treadmills tell you the number of calories burned. Dieters count calories religiously at each meal; other people joyfully declare that calories don’t count on the weekend.
What Calories Are
Many people think of calories as “things” in your food that, if you eat too many of them, will put weight on you. Eating more calories than you need will likely lead to weight gain, that part is certainly true. But calories aren’t “things” that you can see or touch. You can’t pick them out of your food or push them to the side of your plate. Love ‘em or hate ‘em, they’re something you simply cannot live without.
Food contains energy that has the potential to be converted into useful work once metabolized or broken down by your body. That energy is measured in units called calories. What is a calorie and how do we know how many calories are in each food that we eat?
This is where things can get confusing.
Many people don’t know that by using this definition, you are actually describing what is also called a kilocalorie, or kcal. This is the typical calorie that you see on Nutrient Fact labels.
A calorie is simply a unit of energy—it’s a measurement, just like inches or degrees or kilograms. In technical terms, a calorie (or, when accurately used to describe the calories in food, a kilocalorie) is the amount of energy that is needed to raise the temperature of a kilogram of water by one degree Celsius. So what does that have to do with the calories on your plate?
A calorie is simply a unit of measurement of the energy in the foods that you eat. Your body doesn’t use energy to raise the temperature of water, but you need energy (measured as calories) to fuel all of your daily functions. This includes your basic metabolic processes, as well as all the activity you engage in throughout the day. In order for your body to tap into this energy, it first has to be released from the foods you eat.
Providing energy to the body is often compared to the way you provide energy to your car. When you put fuel in your car’s tank, there is energy (which could also be measured in calories!) “locked up” in the gasoline. But just having gas in the gas tank isn’t enough to make the car move. In order for that to happen the fuel has to be ignited in the engine, which releases the energy from the gasoline—energy that can be used to propel the car.
Similarly, the food (fuel) that you eat has energy in the form of calories that are locked up in the protein, fat and carbohydrate (and sometimes alcohol) that you eat. Much like the energy is released when the gasoline ignites in your car’s engine, the food you eat has to be digested and metabolized to release the energy that can be used to fuel your body.
These calories are absolutely necessary to life. Your resting metabolic rate (the number of calories your body uses every day for the most basic processes just to keep you alive) accounts for about 75 percent of the calories your body uses every day. The remaining amount that you burn during the day are used to fuel your muscles, as you move around throughout your day and engage in exercise. And a very small amount is used to digest and process your food.
General dietary guidelines suggest that the average adult woman needs to consume about 2,000 calories a day, and the average adult man about 2,500, Wright says. While these numbers provide a ballpark metric, calorie requirements vary drastically from person to person, she says. How much energy your body needs varies based on many factors, including activity level, age, height and more.
When you eat more calories than you need, the body will convert that unused energy into muscle (short-term storage) or fat tissues (long-term storage). This is a survival mechanism — if you aren’t able to eat enough, your body will tap into these stores to fuel itself. “The body once again is this amazing machine,” Wright says.
Consuming too much energy can lead to weight gain and health problems. A major culprit in the U.S., where obesity affects more than 93 million adults, are foods with “empty calories.” Soda is a good example, Wright says. It provides no nutritional benefit and a lot of calories. In your diet, you want to get the most bang for your buck, she says. You want foods that deliver calories as well as other nutrients like vitamins, minerals, proteins and fiber.
Ultimately, calories aren’t the enemy, Wright says. Many people hoping to lose weight get obsessed with the numbers; but, really, you should be thinking about calories in terms of your individualized energy needs, she says.
Whether you’re counting calories or ignoring them, those numbers are just a measure of energy. Making the most of that energy is up to you.
So, what are the sources of calories in the foods that we eat? The big three macronutrients—protein, fat and carbohydrate—provide the majority of the calories we eat. Here’s how it breaks down. A gram of protein has 4 calories’ worth of energy; a gram of carbohydrate also has 4 calories locked away. Fat is a more concentrated source of energy: each gram of fat contains 9 calories of energy.
There’s one other calorie source that’s considered separately, and that’s alcohol. That’s because a gram of pure alcohol has 7 calories, nearly as calorie-dense as pure fat.
The values of 4 calories per gram of protein or carbohydrate, 9 calories per gram of fat, and 7 calories per gram of alcohol are used to determine the calorie counts of foods. Let me walk you through a simple example.
If you were to look at the nutrition facts panel on a serving of potato chips, it might say that the chips have 7 grams of fat, 17 grams of carbohydrate, and 2 grams of protein, and 140 calories.
Since fat has 9 calories per gram, the 7 grams of fat contribute 63 calories to the total. Carbohydrate has 4 calories per gram, so the 17 grams of carbohydrate add another 68 calories. And the 2 grams of protein contribute just 8 calories. When you add up all the calories (63, 68 and 8) it totals 139 calories (nutrition facts panels are allowed to round their numbers).
Most foods contain calories from more than one source, with the exception of foods like oils (all fat calories) or sugars (all carbohydrate calories). You might consider pasta to be a carb. And while it’s true that most of its calories come from carbohydrate, pasta also has some protein and even small amounts of fat. The calories in nuts come mostly from fat, but nuts also have some protein and carbohydrate that contribute. Salmon gets its calories primarily from protein, but there are also calories from the natural fat it contains.
Gross Energy Vs. Net Energy
Another way of looking at this is gross energy versus net energy. The gross energy value is assigned to a food after being burned in a bomb calorimeter. The net energy value takes into account something called the coefficient of digestibility, or the percent of the ingested food that actually makes it through the gut and into your bloodstream.
For example, dietary fiber reduces the percent of the food that is usable by the body for energy production. This makes sense—we often hear of fiber as a nutrient that promotes regularity or regular bowel movements.
It turns out that many factors impact how many calories you actually can use in your body. Some of these might be how the food is grown—like the soil conditions, what the animal’s diet contained, how ripe the food was when harvested.
Even how you cook and prepare the meal makes a difference. In general, the macronutrient with the highest coefficient of digestibility is carbohydrates at 97%, followed by fats at 95%, and finally proteins at 92%. An interesting note is that there can also be variation even within a food category.
Thus, as you can see, the process of determining the calories in foods is probably more extensive than you imagined. It may also be a shock to see how inexact the process is.
In fact, the calories listed on food labels are really just approximations because you can be sure that the data from the bomb calorimeter has a degree of variability. Research has estimated an error of up to 25% may be seen in the typical nutrition databases that you might use to calculate your calorie intake. We just don’t know how each individual will respond to the food and how much energy it will take to digest, absorb, transport, and ultimately excrete these foods.
“Now you can probably see how calculating calories might give you a snapshot of the ‘Energy In’ part of the energy balance equation, but there are still some kinks in the system,” Professor Ormsbee said. “It does give you an idea of how things change over time, and I think it is useful to check in with your total calorie intake to keep those things honest.”
Consider the quality of the foods you eat, though, and not simply the quantity. In many cases, you can eat a lot more quality food and still maintain or lose weight.
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.
How the Body Uses Calories
Your body needs calories just to operate — to keep your heart beating and your lungs breathing. As a kid, your body also needs calories and nutrients from a variety of foods to grow and develop. And you burn off some calories without even thinking about it — by walking your dog or making your bed.
But it is a great idea to play and be active for an 1 hour or more every day. That means time spent playing sports, playing outside, or riding your bike. It all adds up. Being active every day keeps your body strong and can help you maintain a healthy weight.
Watching TV and playing video games won’t burn many calories at all, which is why you should limit those activities to no more than 2 hours per day. A person burns only about 1 calorie per minute while watching TV, about the same as sleeping!