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Wednesday, March 3, 2010

Fun Food Math, Part 3: Calories

Calories! The number everyone hates to count.

But what is a calorie? Google helpfully suggests that it's equal to 4184 joules. That doesn't help us much!

A calorie is a unit of measure of heat change. It's an obsolete, archaic unit. A confusing term, too. If we give the various calorie units their proper names, we've got:
  • Kilogram calorie - heat required to raise the temperature of a kilogram of water one degree Celsius.
  • Gram calorie - heat required to raise the temperature of a gram of water one degree Celsius.
  • Pound calorie - heat required to raise the temperature of a pound of water one degree Celsius.
  • British Thermal Unit (BTU) - heat required to raise the temperature of one pound of water one degree Fahrenheit. It's roughly equal to the energy released when one wooden match is burned down to the very end. 

All of these units have been superseded by the joule, which is a metric unit of energy. A joule in body heat terms is roughly the amount of heat radiated by a human being every 100th of a second.

But I've strayed from fun food math. We want to know about CALORIES that make us fat!

The calorie that makes us fat is a unit of measure describing how our cells convert food to energy through a process called respiration. The cells combine the food with oxygen, which releases energy.
Think about this - a pound of lettuce will be harder to burn and produce less energy than a pound of cheese, so lettuce has fewer potential calories.

Are calories determined by burning some food in a dish (like the match) and measuring the output?

NO. For many reasons:

1. Technically calories aren't IN food, they are the result of a reaction that happens when food is processed through respiration in our cells. Because this process isn't 100% efficient, scientists have decided to "discount" the potential energy in a quantity of food by 15%. The actual intake of 100 calories of food results in 85 calories of usable energy. We only count the 85.

2. There's another discount applied to calories, because many foods contain indigestable fiber. Our bodies don't digest the fiber and so that potential energy is passed through the body and lost. If we burned the food in a dish the fiber would burn too, and influence the results. Thus food scientists discount the caloric content of fibrous foods.

3. The process of doing these conversions is complex and requires more chemical ability than most people have. There are reference books and databases from which everyone draws their caloric information.

So the calories we eat are used up by the muscles, right?  Yes and No.

1. The potential calories we generate get converted into a substance called ATP that is passed from cell to cell (imagine an energy traveler's check that any cell will accept, but each party has to pay fees when they buy or sell).

The conversion cost with ATP is pretty high, so only about 18-26% of the calories freed by respiration actually get used in the muscles to produce energy (running, bicycling, scooping guacamole with a tortilla chip, etc.)

2. Anything not used quickly by the muscles gets converted into fat (which is like putting your traveler's check into a bank account rather than taking cash). Fat is our reserve from which the body draws between meals. And there's a service charge for converting into and out of fat as well, like changing to a different country's currency.

We'll have to save nutritional labels and calorie counting for another day.

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