trib767

Just wondering, when they say a food item has a number of calories in it, how is this worked out ?

la petite moi

One calorie, also known as a kilocalorie, is the amount of heat required to raise the temperature of 1 kilogram of water 1 degree Celsius.

Rodney

As for how they measure the calories: they literally burn the food, then measure the heat released.

This is the main fallacy in calorie-counting. Wood, for goodness sake releases calories. But you, a human, can't absorb nourishment or calories from wood. You're not equipped to break wood down.

By the same token, burning a potato or an apple or banana may well release more calories in heat than your body could ever liberate from the same fruit from the digestive process. So it's not just about calories, but about how easily the calories in a particular food are absorbed by the body. And this varies a lot.

Stompy

I don't have the time to look up the exact numbers at the moment, but all food is, of course, made up of protein, fat, and carbs, each of which has their own caloric value.

I think carb/protein are 4 calories per gram and fat is 9 calories per gram.. not 100% sure on that, though.

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Slavakion

That's about right. People today are all afraid of calories. Calories are just energy. Granted, that energy can turn into love handles, but that's probably because you either don't exercize enough or ate too many calories.

Also, it's the amount of energy required to raise 1 gram of water 1 degree celsius. Another, more modern, definition is 1 calorie = 4.18 joules.

Cynthetiq

actually I tend to think that people are more afraid of FAT than calories, which is why Snackwells and other low fat items sell so well even though the total caloric value isn't less.

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vinaur

1 Cal as it is most often used in food industry is in fact 1 kcal, which is 1000 cal or 4184 Joules. So a food calorie (Cal) is the energy required to raise 1kg of water by 1 Kelvin (or Celsius, doesn't make any difference in this case).

Regarding the calories of individual component, Carbs do in general give you about 4 Cal and so do fats. Proteins give you 9 Cals.

However, I remember reading somewhere recently, that you cannot judge the actual energy you obtain by these values for few reasons. First reason was mentioned by Rodney. The other one is that the more complex the molecule gets the less energy (per gram) your organism will be able to get from it even if it manages to break it down completelly. This is due to the fact that during every step of the breakdown (which increase with the complexity of the molecule), a small fraction of heat is lost and therefore is not used to make ATP.

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trib767

Really? That thought did occur but I thought it can't be how it's done. So they'll take a ready-meal and torch it ? How do they measure the heat released, with a thermometer ?

I'm just really curious for the sake of it. it's one of those things that's been at the back of my mind for years.....

Redlemon

It's called a bomb calorimeter. It is essentially a sealed container placed in a water bath.

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Slavakion

You can also use a styrofoam coffee cup, with a bit (lot) less accuracy.
Well, that's a pain in the ass. If some says gram, I expect them to mean gram, not kilogram.

Poloboy

Slavakion, yeah it is a pain in the ass. Don't be worried if you're getting confused, it's a confusing system of naming they've developed.

1 calorie is the energy to raise 1 gram of water 1°C.

1 kilocalorie is 1000 calories, which means it's equivalent to the energy to raise 1 kg of water 1°C.

Food manufacturers are actually listing kilocalories on their food labels, not calories. But, by spelling it "Calorie" with a capital C, instead of calorie, they assume that's enough of a difference to be clear.

So basically it's the food industry that messed everything up when they started labelling things with capital letters and assuming that changed the definition.

*sigh*

trib767

Ok, so assuming I set fire to my dinner and work out it's calories using one of those cool bomb things, I now look at the other side of the equation: How do calorie meters, etc, work out what you've burned in the gym. Surely there are loads of assumptions there, right ? I guess there is a technically accurate method but those meters in the gym don't come close ?

vinaur

My guess would be that they would measure the actual work you've done and add in your body temperature increase and maybe, somehow, how much heat you've released into the atmosphere.

I'd imagine it being done in a thermally isolated room and measuring the increase of the temperature of the atmosphere.

Oh and those meters in the gym just take the stats of your workout and use the formula derived from the data obtained above to calculate the calories you've burnt.

The reason why the meters would be so much off, is because every person's metabolism is different and the way you do exercises also affects your energy output

Once again this is only the way I would imagine doing it, so it might be, and probably is, incorrect.

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jaco

The only reason i know about all this is because of one of those saturday morning "mr. science" style shows where some guy was having a little girl set fires to high calorie food. Good stuff to know.

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Slavakion

There would have to be a lot of assumptions made. Say you were on a treadmill. The machine would assume that at this speed, and this incline, the average human body burns approximately this many calories. Cheaper machines might dispense with speed and incline and just give you something based on time. It's adequate for rough approximations, but not really accurate.

kutulu

A good treadmill also factors your weight into the equation.