What’s a calorie? Facts you need to know!

What actually is a calorie, and what does it mean if a food is “high calorie” anyway? You’re probably familiar with food labels showing the calorie counts of most things you eat. You’ve also seen your calories burned on Pacer, or on a treadmill or other exercise equipment. What does it mean to eat a 100 calorie snack, or to burn off calories while walking? How does that affect your weight and overall health? Here’s a quick rundown to get you started!

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What is a calorie?

Vegetables with calorie labels concept
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The calorie was first defined in 1824 as a unit of energy. There are actually 2 different meanings of the term “calorie” – small and large. “The dictionary definition of a small calorie is the amount of heat required to raise the temperature of 1 gram of water one degree C at one atmosphere of pressure. A large calorie is basically 1000 small calories, or the heat required to raise 1 kg (1000 grams) of water one degree C.

When you see calorie measures in food or related to exercise, you’re almost certainly looking at large calories. Sometimes small calories are abbreviated with a small “c” in “cal” and large with a big “C” in “Cal,” but in practice, large calories are often simply just called “calories” (or kilocalories).

So what does that mean?

Female walkers eating a fruit snack
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It means that a calorie is not a thing, but a measure of heat or energy. When we say that a donut has a certain number of calories, that refers to the energy potential of the stuff in the donut to power your body in various ways. 

This also means that you don’t actually burn calories themselves. “Calories burned” refers to the amount of energy that your body uses to power itself or do exercise. In practice, your body can burn stored fat, available sugars or broken down muscle.

How many calories are in the things we eat?

Variety of healthy food items
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Different components of our foods have different calorie contents. The main components of food contain the following calorie counts:

  • Fat: 9 Cal/gram
  • Carbohydrates: 4 Cal/gram
  • Protein: 4 Cal/gram

Other nutrients also have their own calorie contents:

  • Alcohol: 7 Cal/gram
  • Sugar alcohols: 2.4 Cal/gram
  • Fiber: 2 Cal/gram

As you can see, fat has more than double the calorie content of carbs and protein, which is why fatty foods tend to have such high calorie numbers. This is true of even “healthier” foods that are high in good fats – like avocados and nuts. This doesn’t mean remove all fat from your diet! It does mean that serving sizes of healthier higher-fat foods are probably smaller than you’d expect.

Alcohol actually has a high calorie count as well. If you’re drinking beer or a mixed drink, you’re getting 7 calories per gram of alcohol plus the carbs from the mixers or beer.

Fiber cannot be completely digested by humans, which is why it has a low calorie count. Fiber is great because it can help you feel full, help to control blood sugar and keep your digestive system regular. Many processed foods, like fruit juices, lack the fiber content of whole foods like bananas.

Your body also needs other important nutrients like vitamins, minerals, and water to function. Eating a variety of foods is important to ensure you’re getting everything your body needs!

What about Kilojoules?

Senior couple eating sparingly at Thanksgiving
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The calorie is a fairly old unit of measurement. While the US and many countries still use calories as a measure of nutrition energy, the International System of Units uses the kilojoule as a measure of energy. The kilojoule can be described in a few different technical and scientific ways, but its use is very similar to the calorie.

Each kilocalorie (large calorie) is equivalent to 4.18 kilojoules. That means that in terms of kilojoules:

  • Fat: 37 kj/gram
  • Alcohol: 29 kj/gram
  • Carbs: 17 kj/gram
  • Protein: 17 kj/gram
  • Sugar Alcohol: 10 kj/gram
  • Fiber: 8.4 kj/gram

What does it mean to “burn a calorie?”

Burn calories concept - calories on paper burning
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Your body needs energy so that you can stay alive and that your bodily functions all work properly. Your heart has to beat and lungs have to circulate air for instance. That requires energy, which your body can get from stored fat or sugar in your bloodstream.

As you move around, your body also requires energy to get you where you need to go. The more you walk, the more energy (and calories) you burn. As you exercise, your muscle fibers break down and need to be repaired. Repairing your muscles and keeping them in good shape requires protein as well.

How does it factor into weight loss?

Woman sweating after treadmill cardio workout
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Your body needs a certain level of calorie intake to maintain your current weight. US food label percentages are based on a 2,000 calorie per day diet, but your actual dietary needs vary based on gender, age, and activity level. Here’s a great way to calculate how many calories you need daily.

1 pound of fat contains about 3,500 calories. That means in order to lose a pound of fat, you’ll need to create a caloric deficit of approximately 3,500 calories. There are several models of weight loss that suggest you may actually need to burn more calories than this, but 3,500 is a good starting point. It’s important to note that walking can help you reduce stubborn fat, but walking does this through burning calories throughout your body. You can’t reduce fat in a particular area of your body by exercising it, but this also means that you can burn fat throughout your body, even on your belly, through walking and exercise.

You can create a caloric deficit by burning more calories through activity or reducing the calories you take in. Walking is a great way to burn calories, but it’s much easier to eat (or drink) 200 calories than it is to walk for an hour! This means that in addition to getting more active, making small changes to what you eat (like swapping out a 200-cal daily drink for water) can go a long way.

If you reduce your calorie intake by 250 per day, you’ll lose about a pound in 14 days! That doesn’t mean that your weight is linear and unchanging. Water weight, hormones, and other factors can influence your weight on any given day. The maximum amount of sustainable weight loss is generally recognized as about 1-2 pounds per week. The more sustainable your weight loss is and the easier it is to follow your diet plan, the better the chances that you’ll have long term success!

Final thoughts

That’s only a quick overview of calories and how they factor into exercise and nutrition. The better you understand how calories work, the better you can plan out your exercise and eating plan.

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