Why use calories to measure 'food value'?

[MyWifesSkin]

Why does an animal eat? To give it energy to move about, in general to perform chemical reactions. (Forgetting about any raw materials it may need). But this is *work* energy. So it seems peverse to measure the food value of food in calories which measure the heat energy that would be given out if the food were consumed by fire; a variety of free energy would be better since this measures work.

 

The free energy in glucose is a very different number than the calories.

 

So does anyone happen to know why food value is measured in calories?

[John Cuthber]

You are mistaken. Energy is energy.

Since the body oxidises glucose etc to obtain energy the available energy is that same as if you burn it.

[Ringer]

To elaborate, when something is burned it uses heat to overcome an energy barrier between bonds. Overcoming this barrier allows bonds to be broken and new bonds to be formed. The excess heat when something is burned is what is left over from the energy input and the energy of the bonds formed. In the same way a steam engine would use the coal's excess heat from burning we use the excess energy from breaking the bonds in our food to do the things our body needs to do.

[MyWifesSkin]

You are mistaken. Energy is energy.

Since the body oxidises glucose etc to obtain energy the available energy is that same as if you burn it.

Thanks. Fraid a bit mystified.

 

For illustration purposes: oxidize the glucose inside a blind ended cylinder fitted with a sliding piston. Takes place at constant temperature. The glucose's bonds break and the smaller particles fly about, pushing out the piston. How far and how forcibly the piston is pushed depends on the entropy, the ordering, of the glucose molecule. Calories does not measure this ordering. So calories do not measure how much work the organism gets from the glucose surely. (Intuitively, don't know, the heat given out by the animal when the work has dissipated is like you say, but this does not measure the work the animal has been able to do).

 

To elaborate, when something is burned it uses heat to overcome an energy barrier between bonds. Overcoming this barrier allows bonds to be broken and new bonds to be formed. The excess heat when something is burned is what is left over from the energy input and the energy of the bonds formed. In the same way a steam engine would use the coal's excess heat from burning we use the excess energy from breaking the bonds in our food to do the things our body needs to do.

Thanks. Bit confused but: Imagine there is negligible energy difference between the initial bonds and the final bonds after burning. The animal can still get energy because the smaller product molecules want to spread out to occupy a larger volume, they have a high pressure or chemical potential. It is like a cylinder of compressed ideal gas, it can drive an air tool. This factor is not measured by calories.

[John Cuthber]

OK, for a start, while I could try to imagine something where " Imagine there is negligible energy difference between the initial bonds and the final bonds after burning." it wouldn't burn.

 

Also, I have news for you.

The stored energy in a compressed gas bottle is measured in calories. (Though these days, Joules are nor popular as a unit of energy.)

If you multiply the pressure of the gas (in Newtons per square metre) by the volume of the cylinder (in cubic metres) you get the stored energy in Joules.

Divide by 4.184 and you get the energy in calories.

[MyWifesSkin]

<br />OK, for a start, while I could try to imagine something where " Imagine there is negligible energy difference between the initial bonds and the final bonds after burning." it wouldn't burn.<br /><br />Also, I have news for you.<br />The stored energy in a compressed gas bottle <u>is</u> measured in calories. (Though these days, Joules are nor popular as a unit of energy.)<br />If you multiply the pressure of the gas (in Newtons per square metre) by the volume of the cylinder (in cubic metres) you get the stored energy in Joules.<br />Divide by 4.184 and you get the energy in calories.<br />

<br /><br /><br />Good oh, this is getting absurdist.

 

I said 'negligible', i.e. enough for reaction to take place but not enough to have to add it in and obfuscate the point.

 

Measure the mass of the ideal gas before compression. Compress it and measure it again. The masses are the same. So, E = mc^2, the stored energy is the same - for these purposes there is no stored energy. So how does it spin the air tool? It uses its low entropy to convert the thermal energy of the surroundings into work. If the compressed gas was burned it would yield no heat. We are not talking about some flammable gas obviously.

 

The reason I asked the question was that the 'retardation of aging and disease by dietary restriction' is now officially said to be a matter of *calories*. I want to know if this is just loose talk or whether they mean it literally.

[John Cuthber]

It is indeed absurd. You talk about things that burn but without getting hot, then you misunderstand relativity.

If you compress the gas it does gain mass.

 

Anyway, to get to your point re "The reason I asked the question was that the 'retardation of aging and disease by dietary restriction' is now officially said to be a matter of *calories*. I want to know if this is just loose talk or whether they mean it literally. "

Since food's energy is measured in calories less food means fewer calories.

What's the difference?

[swansont]

<br /><br /><br />Good oh, this is getting absurdist.

 

I said 'negligible', i.e. enough for reaction to take place but not enough to have to add it in and obfuscate the point.

 

Measure the mass of the ideal gas before compression. Compress it and measure it again. The masses are the same. So, E = mc^2, the stored energy is the same - for these purposes there is no stored energy. So how does it spin the air tool? It uses its low entropy to convert the thermal energy of the surroundings into work. If the compressed gas was burned it would yield no heat. We are not talking about some flammable gas obviously.

 

The reason I asked the question was that the 'retardation of aging and disease by dietary restriction' is now officially said to be a matter of *calories*. I want to know if this is just loose talk or whether they mean it literally.

 

It's still energy. All of the energy from digestion comes from the breaking and forming of bonds, so you can't say it's negligible and then point to it as the source for some other reaction.

 

PV is an energy term. You have to do work, i.e. transfer energy, to compress a piston. But this is moot; most animals don't use any pressure generated by reactions as a source of energy*. As John Cuthber has already pointed out, the reaction is basically combustion

 

 

As for "'retardation of aging and disease by dietary restriction' is now officially said to be a matter of *calories*", that sounds like a (sham) medical claim. But calories (or, more correctly, Calories, since 1 Cal = 1 kcal) is a valid unit and measure of energy intake.

 

* (except, of course, the fartmoth, who can double his speed when evading predators using flatulence as a mode of propulsion)

[MyWifesSkin]

It is indeed absurd. You talk about things that burn but without getting hot, then you misunderstand relativity.

If you compress the gas it does gain mass.

 

Anyway, to get to your point re "The reason I asked the question was that the 'retardation of aging and disease by dietary restriction' is now officially said to be a matter of *calories*. I want to know if this is just loose talk or whether they mean it literally. "

Since food's energy is measured in calories less food means fewer calories.

What's the difference?

I only introduced the negligible energy thing to try to keep things simple. Shove in as much energy as you want from the before and after bonds - it's probably calories. I'm just saying there is then the additional 'energy' that the low entropy glucose, say, can produce by converting the heat of the surroundings to work.

 

Same temperature before and after compression: Most certainly does not gain mass, it is an ideal gas so its internal energy is unchanged by the compression. Could ask the physics lot to pontificate perhaps.

 

'What's the difference?' - That a very good point. You mean, if the type of food is the same, eating less food involves eating fewer calories but also less free energy? Had not thought of that, rather idiotically. However, would have thought having observed that things live longer if they are 'undernourished rather than mal-nourished' they went on to test if it was some particular component(s) of the food that was producing the effect. Carbohydrate, fat, protein, potassium .... And they presumably discovered that the composition of the food did not matter it was simply the calorie content.

 

Some carbohydrate with x calories in it must be expected to have a very different free energy than some protein with x calories.

 

However they were probably comparing well fed rats with half starved rats, so the latter's food would be lower in both calories and free energy. But I'd still like to know whether they are just being slipshod of whether they really should talk about calories.

 

Anyway this has clarified my thinking somewhat.

 

Perhaps should say ??enthalpy instead of calories??

 

It's still energy. All of the energy from digestion comes from the breaking and forming of bonds, so you can't say it's negligible and then point to it as the source for some other reaction.

 

PV is an energy term. You have to do work, i.e. transfer energy, to compress a piston. But this is moot; most animals don't use any pressure generated by reactions as a source of energy*. As John Cuthber has already pointed out, the reaction is basically combustion

 

 

As for "'retardation of aging and disease by dietary restriction' is now officially said to be a matter of *calories*", that sounds like a (sham) medical claim. But calories (or, more correctly, Calories, since 1 Cal = 1 kcal) is a valid unit and measure of energy intake.

 

* (except, of course, the fartmoth, who can double his speed when evading predators using flatulence as a mode of propulsion)

Animals use chemical potentials, same idea as pressure really, for example the concentration difference between the inside and the outside of the mitochondrial membrane of H+ ions (to ponce about, hydroxonium ions). The ions spin windmills in the holes in the membrane as they make their way through, down the concentration gradient - much like an air tool.

 

You saying an organism uses only the potential energy stored in its food - initial Ep minus Ep when oxidized, it chucks away the rest of the work it could derive from the low entropy nature of the food? It is sort of operation like a heat engine, perhaps.

 

Not arguing with you, I don't know; but if true this is wildly important.

 

Retardation of ageing and disease by dietary restriction as official as it gets.

[Fuzzwood]

All of the work done by our bodies is in the end converted into heat.

[swansont]

I'm just saying there is then the additional 'energy' that the low entropy glucose, say, can produce by converting the heat of the surroundings to work.

 

Where does the heat of the surroundings come from?

[MyWifesSkin]

All of the work done by our bodies is in the end converted into heat.

Yes, in the case I'm talking about back into heat.

 

Where does the heat of the surroundings come from?

Could come from anywhere, the sun say. E.g. gas cylinder connected to air tool.

[John Cuthber]

 

 

Could come from anywhere, the sun say. E.g. gas cylinder connected to air tool.

It could in principle, but in practice it will come from the body heat of the animal.

So, what you gain one way, you lose the other.

That's the thing about the conservation of energy.

In order to warm up, the animal had to burn some calories- exactly equal to the number of calories you are talking about with the work done by the expanding gas.

[swansont]

Could come from anywhere, the sun say. E.g. gas cylinder connected to air tool.

 

Since you were talking about glucose I was thinking this was in the context of an animal's body. So, the heat comes from the body. The body being at a high temperature comes from the digestion of food, for us warm-blooded types. All you're doing is making the accounting more complex, but in the end all the energy comes from the food.

 

(edit: xpost with John Cuthber)

[MyWifesSkin]

It could in principle, but in practice it will come from the body heat of the animal.

So, what you gain one way, you lose the other.

That's the thing about the conservation of energy.

In order to warm up, the animal had to burn some calories- exactly equal to the number of calories you are talking about with the work done by the expanding gas.

The long journey has to go on forever....

 

Two different molecules, both, however, give out the same amount of heat when oxidised. If one of the molecules has low entropy, perhaps because the atoms are close together, and the other high entropy then the low entropy one can convert a greater amount of the heat to work. So overall the low entropy molecule does more work. The work quickly dissipates so both molecules, as above, end up yielding the same amount of heat.

 

Since you were talking about glucose I was thinking this was in the context of an animal's body. So, the heat comes from the body. The body being at a high temperature comes from the digestion of food, for us warm-blooded types. All you're doing is making the accounting more complex, but in the end all the energy comes from the food.

 

(edit: xpost with John Cuthber)

There is more to it than making the accounting more complex. What I want to know is whether the retardation of aging and disease by dietary restriction really correlates with the heat in the food, 'calories', like they say, or with the work that may be got from the food, ?Helmholtz free energy?

[swansont]

There is more to it than making the accounting more complex. What I want to know is whether the retardation of aging and disease by dietary restriction really correlates with the heat in the food, 'calories', like they say, or with the work that may be got from the food, ?Helmholtz free energy?

 

That's a different question entirely. Is retardation of aging and disease by dietary restriction a real effect?

 

(And it's not heat in the food; heat is not a substance, it's energy transferred owing to temperature gradients.)

[MyWifesSkin]

That's a different question entirely. Is retardation of aging and disease by dietary restriction a real effect?

 

(And it's not heat in the food; heat is not a substance, it's energy transferred owing to temperature gradients.)

I thought the reason they measure food value in calories might inform the dietary restriction thing.

 

Yes it is. See book of same name. It is a very active area of research. Here's one ref among thousands: http://tpx.sagepub.com/content/24/6/742.refs

[swansont]

I thought the reason they measure food value in calories might inform the dietary restriction thing.

 

Food intake is measured in Calories because it's a unit of energy. Nothing more to it, really.

 

Yes it is. See book of same name. It is a very active area of research. Here's one ref among thousands: http://tpx.sagepub.com/content/24/6/742.refs

 

Ah. What us lay-people would call a diet. (one without a fancy name)

[Jens]

Hi,

 

some points to consider with regards to 'food value':

 

a) Yes as mentioned already above calories (cal) is just a unit of measure. Joule (J) is another one. You can simply convert them by a factor. You can use this unit of measure for Energy (U), Enthalpy (H), Free Energy (F), and Free Enthalpy (G). So using calories does not necessarily say what you actually mean. U is standing for heat at a fix volume. H for heat at fix pressure. F and G consider also in addition an entropy term.

 

b) The value that matters from a biochemical point of view point of view is dG ("delta-G" the free Enthalpy or Gibbs Energy), since this value 0, if the reaction is in exact equilibrium. dG includes energy and entropy. This is biological relevant and not just a theoretical consideration. There are autotrophic microorganisms which cannot survive any more or have to change their energy metabolism completely dependent on the concentration of some anorganic substances in their environment they live from. However, the biochemical standard dG°' assumes 1 M concentration of the substances (besides water) and pH 7. 1 M is not very realistic in virtually all cases. But it is typically still better than taking dU.

 

c) Actually from biological point of view, the bigger issue is not the entropy, but the capability of the body to do something useful (e.g. synthesizing fat, or fueling your brain, or doing muscle work) with the 'food value'.

Example 1: Cholesterol: Even though it is very reduced and has very roughly the same dU or dG value as fat, the body is not able to make any energy out of it, since there is no pathway to produce NADH or FADH₂. You can only make bile acid out of it, which partially gets lost in the gut, since it is not 100% resorbed back again.

Example 2: Starch is a polymer of glucose and has more energy than the corresponding amount of glucose molecules. However, in the gut starch is simply hydrolized to glucose, which produces some heat but no energy form the body can use (no ATP). So there is no difference between the two with regards of useful energy.

Example 3: Cellulose has nearly the same energy than starch (it is just another way of assembling glucose into a polymer). However, you will die of hunger, if you eat cellulose (The body just cannot break it down to glucose units). So a statement that cellulose has about the same 'food value' than starch is a bit misleading.

 

So the real value for the body should better be measured in number of ATP (or equivalent) you can obtain out of it.

[Jens]

a) Yes as mentioned already above calories (cal) is just a unit of measure. Joule (J) is another one. You can simply convert them by a factor. You can use this unit of measure for Energy (U), Enthalpy (H), Free Energy (F), and Free Enthalpy (G). So using calories does not necessarily say what you actually mean. U is standing for heat at a fix volume. H for heat at fix pressure. F and G consider also in addition an entropy term.

 

Terminology correction: It is "Gibbs Free Energy" and not "Free Enthalpy" (I was directly translating from German). And it is "Helmholtz Free Energy" and not "Free Energy" (the same issue).