|4832 - How calories work||22/06/2005 - 22:35:51|
|by Julia Layton|
For years now, calories have been all the rage -- people are counting them and cutting them, and you'd be hard-pressed to find something at the supermarket that does not list its calories per serving somewhere on the package. But have you ever wondered what exactly a calorie is?
In this article, we'll find out what calories are and why we need them, and examine the relationship between calories and weight.
What is a Calorie?
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.
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. For the duration of this article, when we say "calorie," we mean "kilocalorie."
What Calories Do
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, by which enzymes break the carbohydrates into glucose and other sugars, the fats into glycerol and fatty acids and the proteins into amino acids. These molecules are then transported through the bloodstream to the cells, where they are either absorbed for immediate use or sent on to the final stage of metabolism in which they are reacted with oxygen to release their stored energy.
Click here for a simplified diagram of these metabolic processes.
(Note: The first number in the equation for females is, in fact, 655. Strange but true.)
Your Caloric Needs
The second factor in the equation, physical activity, consumes the next highest number of calories. Physical activity includes everything from making your bed to jogging. Walking, lifting, bending, and just generally moving around burns calories, but the number of calories you burn in any given activity depends on your body weight. Click here for a great table listing the calories expended in various physical activities and for various weights.
The thermic effect of food is the final addition to the number of calories your body burns. This is the amount of energy your body uses to digest the food you eat -- it takes energy to break food down to its basic elements in order to be used by the body. To calculate the number of calories you expend in this process, multiply the total number of calories you eat in a day by 0.10, or 10 percent. If you need some help determining how many calories you eat in a day, check out these sites:here.
Calories, Fat and Exercise
One thing about exercise is that it raises your metabolic rate not only while you're huffing and puffing on the treadmill. 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.
Lots of people wonder if it matters where their calories come from. At its most basic, if we eat exactly the number of calories that we burn and if we're only talking about weight, the answer is no -- a calorie is a calorie. A protein calorie is no different from a fat calorie -- they are simply units of energy. As long as you burn what you eat, you will maintain your weight; and as long as you burn more than you eat, you'll lose weight.
But if we're talking nutrition, it definitely matters where those calories originate. Carbohydrates and proteins are healthier sources of calories than fats. Although our bodies do need a certain amount of fat to function properly -- an adequate supply of fat allows your body to absorb the vitamins you ingest -- an excess of fat can have serious health consequences. The U.S. Food and Drug Administration recommends that a maximum of 30 percent of our daily calories come from fat. So, if you eat 2,000 calories a day, that's a maximum of 600 calories from fat, or 67 grams of fat, per day. However, many doctors and nutritionists now set the maximum number of fat calories at 25 percent of our daily caloric intake. That's 56 grams of fat per day for a 2,000 calorie diet.
Here are some calorie and fat contents that may surprise you:
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