Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: Anne on 12/09/2008 16:17:10

Title: What are calories?
Post by: Anne on 12/09/2008 16:17:10

Anne asked the Naked Scientists:

Why do we measure how fattening a food is based on calories when it actually means the amount of energy required to raise 1 g of water to 1º Celsius?

What do you think?

Title: What are calories?
Post by: Evie on 15/09/2008 18:03:23

A food Calorie is actually one thousand energy calories, or a kilocalorie (you will see this on some foreign packaging - kcal). It refers specifically to the amount of energy in each serving of food gained through digestion. Energy that is not used through physical exertion is stored as fat, that's why you want to consume fewer calories of food than you burn through exercise in a day to lose weight (or equalize the two to stay at the weight you are).


Here's the Wiki article on food energy for more specifics: http://en.wikipedia.org/wiki/Food_energy

Title: What are calories?
Post by: techmind on 15/09/2008 19:39:20

Quote from: Anne on 12/09/2008 16:17:10

the amount of energy required to raise 1 g of water to 1º Celsius

Sorry to be the pedant, but the calorie is
the amount of energy required to raise 1 g of water by 1º Celsius

Title: What are calories?
Post by: DrDick on 19/09/2008 14:56:55

Quote from: techmind on 15/09/2008 19:39:20

Quote from: Anne on 12/09/2008 16:17:10

the amount of energy required to raise 1 g of water to 1º Celsius

Sorry to be the pedant, but the calorie is
the amount of energy required to raise 1 g of water by 1º Celsius

To be even more pedantic, it's the amount of heat required to raise the temperature of 1 g of water from 14.5 °C to 15.5°C.  (The specific heat of water changes with temperature).

Dick

Title: What are calories?
Post by: anthony on 23/09/2008 20:03:46

To actually answer the question. We use calories, defined in that rather odd way, because it seemed like a good idea at the time. Whether it's the energy in food or the energy used to heat something up, it's the same thing, that is energy. The choice of unit and how it is defined is arbitrary.

Just the same as a yard being defined as the distance between the King's nose and the tip of his out-stretched arm. We can then use the unit to measure the length of a field, or the length of a molecule. Scientists also like to use different units at different times so that the number looks good. So the length of a carbon to hydrogen bond may be described in "ångströms" as 1.06, rather than in metres as 1.06 * 10^-10, which is just too fiddly. That's what's going on with the difference in calaries refered to in food and in your science class.

Now if you actually asked what energy was, that would be difficult. Length is something we can experience directly and is easy to understand. Unfortunately we can't experience energy in the same way. Scientists generally hide behind definitions like, "energy is the ability to do work." Which is just another way of ignoring difficult questions and getting on with the day job.

Title: What are calories?
Post by: techmind on 23/09/2008 22:06:37

Well, the energy (Calories) quoted for food is the amount of heat you get from it if you burn it.
And since the body extracts energy from food by oxidation reactions (chemically rather like burning, but without all the drama of fire and flames), this does make some kind of hand-waving sense.

Presumably the Calorie-count is the maximum energy you could get from it. Depending on the efficiency of digestion I guess it could be less - particularly for some foods?