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A calorie is a unit of energy. Specifically, a calorie is the amount of energy, or heat, it takes to raise the temperature of 1 gram of water 1 degree Celsius (1.8 degrees Fahrenheit). One calorie is equal to 4.184 joules, a common unit of energy used in the physical sciences. While we tend to associate calories with food, they actually can apply to anything containing energy. For example, a gallon (about 4 liters) of gasoline contains about 31,000,000 calories.


Most of us think of calories in relation to food, as in "This can of soda has 200 calories." It turns out that the calories on a food package are actually kilocalories (1,000 calories = 1 kilocalorie). The word is sometimes capitalized to show the difference, but usually not. A food calorie contains 4,184 joules. A can of soda containing 200 food calories contains 200,000 regular calories, or 200 kilocalories. A gallon of gasoline contains 31,000 kilocalories.


The same applies to exercise -- when a fitness chart says you burn about 100 calories for every mile you jog, it means 100 kilocalories.


Cellular Metabolism
Simplified diagram of the three stages of cellular metabolism that lead from food to waste products in animal cells. This series of reactions produces ATP, which is then used to drive biosynthetic reactions and other energy-requiring processes in the cell. Stage 1 occurs outside cells. Stage 2 occurs mainly in the cytosol, except for the final step of conversion of pyruvate to acetyl groups on acetyl CoA, which occurs in mitochondria. Stage 3 occurs in mitochondria.



We need calories in order to live. Human beings need calories (energy) to survive -- to breathe, move, pump blood, and to be active. This energy comes from the food we eat. The number of calories in a food measures how much potential energy that food possesses. A gram of carbohydrates has 4 calories, a gram of protein has 4 calories, and a gram of fat has 9 calories. Foods are a compilation of these three building blocks.


Caloric Breakdown

  • 1 g Carbohydrates: 4 calories
  • 1 g Protein: 4 calories
  • 1 g Fat: 9 calories








    If we look at the nutritional label on the back of a packet of maple-and-brown-sugar oatmeal, we find that it has 160 calories. This means that if we were to pour this oatmeal into a dish, set the oatmeal on fire and get it to burn completely (which is actually pretty tricky), the reaction would produce 160 kilocalories (remember: food calories are kilocalories) -- enough energy to raise the temperature of 160 kilograms of water 1 degree Celsius. If we look closer at the nutritional label, we see that our oatmeal has 2 grams of fat, 4 grams of protein and 32 grams of carbohydrates, producing a total of 162 calories (apparently, food manufacturers like to round down). Of these 162 calories, 18 come from fat (9 cal x 2 g), 16 come from protein (4 cal x 4 g) and 128 come from carbohydrates (4 cal x 32 g).


    Our bodies "burn" the calories in the oatmeal through metabolic processes. Your basal metabolic rate (BMR) is the amount of energy your body needs to function at rest. This accounts for about 60 to 70 percent of calories burned in a day and includes the energy required to keep the heart beating, the lungs breathing, the kidneys functioning and the body temperature stabilized. In general, men have a higher BMR than women. One of the most accurate methods of estimating your basal metabolic rate is the Harris-Benedict formula:


    Adult male: 66 + (6.3 x body weight in lbs.) + (12.9 x height in inches) - (6.8 x age in years)

    Adult female: 655 + (4.3 x weight in lbs.) + (4.7 x height in inches) - (4.7 x age in years)

    (Note: The first number in the equation for females is, in fact, 655. Strange but true.)


    So what happens if you take in more or fewer calories than your body burns? You either gain or lose fat, respectively. An accumulation of 3,500 extra calories will be stored by your body as 1 pound of fat -- fat is the body's way of saving energy for a rainy day. If, on the other hand, you burn 3,500 more calories than you eat, whether by exercising more or eating less, your body converts 1 pound of its stored fat into energy to make up for the deficit.


    3500 calories = 1 pound of fat


    One thing about exercise is that it raises your metabolic rate not only while you're huffing and puffing on the treadmill, but also for hours and days afterwards. Your metabolism takes a while to return to its normal pace. It continues to function at a higher level; your body burns an increased number of calories for about two hours after you've stopped exercising. This means that even just sitting doing nothing you will burn more calories.


    As a general rule of thumb if you have a normal lifestyle you can take in about 10-11 calories per pound of body weight to just maintain your weight...not increase or decrease. So if you weight 130 lbs. and you eat about 1300-1430 calories a day, your weight theoretically should not fluctuate too much. If you weigh 180 lbs. you can eat 1800-1980 calories a day to stay about the same weight. Of course, other factors come into play. Men vs. women, activity levels, hormonal disease such as hypothyroidism or diabetes, and your own genetic tendency to either burn or store fat easily.