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Burning Calories, Burning Wood, Grams to Cals?

  1. Jun 10, 2008 #1
    Alright so I apologize if this questions seems less "physics" based, but I promise there is a reason I'm posting it here- if you can thoroughly explain this, I'd much appreciate it!

    I was recently reading on the forum about the question of burning a log of wood (mass) and that if all of the parts of that wood (ask, smoke, water, CO2 etc) were collected that it would be almost the same weight, someone wrote "Burning a log does not convert mass into energy" what did this mean?

    In another post someone mentioned that energy is stored as mass in food. So for example 100 grams of FAT contains a certain amount of energy (we measure this in calories, and there are 8 calories in 1 gram of fat, 4 calories in 1 gram of protein or carbohydrates.

    The act of Jogging, Swimming, Running etc. causes the body's metabolic rate to increase which causes a "burning" of calories. If someone burns more calories than they consume by eating, they will lose weight.

    Since burning a block of wood really only releases a little energy that cannot be retrieved (heat and light) but in reality the water, CO2, ASH, smoke etc. weighs about the same as the original block of wood. AND since the amount of sweat that comes off a body that is working contains a minimal amount of weight, and is almost entirely water and salt (NOT any of the components in the macro-nutrients ingested) then how can it be that after consuming (say...) 100 grams of FAT, or 800 calories, that running or exercising in any amount (say 45 minutes of jogging) could "burn" off those calories of energy AND consequently reduce the physical mass of the person working.

    Sorry for the long post, and structure- I just want to be sure my question is complete and understandable. In brief, though, I'm wondering how by running for 50 minutes, I'm burning 500 calories which is about 120 grams of FAT, yet I am CERTAIN that I have no "lost" .12 Kg of weight.

  2. jcsd
  3. Jun 10, 2008 #2
    Well consider the log example. When you burn a log the mass of all the outputs approximately equals the mass of all the inputs and yet fires are quite hot (at least in my opinion) so if heat is energy where did the energy come from? The comment "mass is not converted to energy in a fire" refers to the famous equation E=mc^2 and in say a nuclear reactor the energy comes from a portion of a nucleons mass being converted to energy (a fraction of the binding energy per nucleon) but this does not happen in fire. So again where does the energy come from? The energy (given off as heat and light) in a fire comes from the fact that wood (made of cellulose a long, complicated hydrocarbon) is broken down in the presence of oxygen into things like CO2 and the energy comes from the bonds which are broken (i.e. the energy of the bonds in the end product are less then the energy of the original wood and the difference is released in heat). Thus the same number and type of molecules is presence (you still have the same numbers of Carbon, Oxygen and Hydrogen) you just have them in a different form (thus the mass doesn't change). Now after all that on to the body.

    When your body does exercise it breaks down sugar (and ultimately fat) to smaller molecules and uses the energy in the bonds. If you're curious about some of the chemical 'pathways' that do this look up Glycolisis (spelling) or the Kreb cycle. Thus when you are burning calories (a measure of how much heat energy is released by burning, which is just something that facilitates a chemical reaction which break and restructure bonds and releases the energy of them) your body, through glycolisis and such, is converting fat and sugar into things like CO2 and water (and a number of other things) which your body then expels as waste. So you are not actually losing any weight it's just the your body keeps fat around but it doesn't keep waste products so if you exercise you force your body to convert that fat into waste products which it then expels (I'm sure you can imagine what I mean by that). Thus when you talk about the amount of CALORIES in 100 grams that is most definetly not the E=mc^2 amount of energy that 100 grams possesss (That would be an extraordinarily large number) but rather how much energy can be freed by burning it). And just a quick comment on what it means to burn. When you burn something what you are doing is simply giving it enough energy to reach the activation energy it requires to undergo a chemical reaction to a lower energy state (like hydrocarbon + oxygen -> CO2 + H2O) and when it reachs the lower energy form it releases the difference between the two states in the form of heat (and this heat can then cause nearby hydrocarbons to undergo the same chemical reaction and thus you have a chain reaction which is why it is rather difficult to stop a fire from burning once it has started). Hope that helps.
    Last edited: Jun 10, 2008
  4. Jun 10, 2008 #3
    Alright so that all makes sense- but here's where I'm getting hung up. Take this example, 100 grams of FAT, about 8000 calories. I'm not sure I completely understand how the CO2 and water (let alone the Heat!) can WEIGH 1 KG when it seems that the sweat and respiration do no amount to much physical weight. I suppose I'm wondering if someone can explain how HEAT released from exercise can explain a loss of mass. Just seems a little odd that the Steak dinner in my body is turned into gas and water and heat.
  5. Jun 10, 2008 #4
    I think your problem is that you may be american. 100 grams is not a kilogram. it's a tenth of a kilogram. If you really doubt that your body expels that much mass then I guess you could always weigh how much you eat and subtract the weight of your urine and feces and you'll see how much fat your body has converted to waste products
  6. Jun 10, 2008 #5
    If you run for like an hour you'll only burn like 500 calories btw. Not even close to 8000
  7. Jun 10, 2008 #6
    Well, "maverick" while I can't say I appreciate the glib remarks on my Nationality- and yes I am an American and no typing 100 when I meant 1000 is not an unforgivable action. Let's not be rude for no reason.

    Second, here was my intended follow up question: When we burn the log we see that Smoke + H2O + CO2 make up the majority of the lost mass. In the event where the body burns fat, the H20 and CO2 are released, but there is no "smoke" component. In other words it would seem that a lot of the burned mass is released in the form of HEAT and not other physical forms (like the smoke and water). That said I'm wondering if someone could verify this:

    When a person exercises, the majority of the weight loss is due to the release of heat.

    ...Or is it still more the result of the loss of water and CO2.

    And if the latter is true, then does this mean that burning a log will result in greater loss of mass (referring to the remaining ashes) than burning fat (referring to the remaining calories) if we could assume equal amount of work is used to burn the log and the calories? As in: more of the original mass (log or fat) is lost to the environment?
  8. Jun 10, 2008 #7
    The american comment was in reference to your unfamiliarity with the metric system not an attack on your intelligence
    Last edited: Jun 10, 2008
  9. Jun 10, 2008 #8
    Most of the following points are off-topic, but for the record:

    The calories given for foods are really Calories (ie, kilo-calories). Keep that in mind if you start doing energy calculations.

    Contrary to a statement above, the heat released when burning wood (or any other chemical reaction) really does reduce the mass of the constituents - but only by a tiny bit (on the order of 10^-10 grams per gram of material burned). This is too small to be measured by chemical lab instruments. The mass comes from the difference in the binding energy of the molecules involved. Nuclear power works the same way, but it is the difference in binding energy of the nucleus that is released. The forces there are about a million times stronger, hence the vast difference in scale of energy release. But it is still a tiny mass - even a large nuclear plant is converting about 3 or 4 grams mass into energy per day (and this is out of a core with a mass of about 100,000,000 grams).

    Back to your original question - I think the weight you lose due to exercise is is down to the waste elimination - what was fat is turned into waste that mostly disappears down the toilet.
  10. Jun 10, 2008 #9
    Once again it's important to realize that virtually no mass is being lost if you consider all the end products. However, when a nutrional label says the amount of calories this figure is arrived by burning the food in a calorimeter (a device used to determine heat loss) but in the human body you're not burning, it's not the same chemical reaction, so the end products are different (in fact the entire chemical reaction is VERY different and more complicated and can be seen here http://en.wikipedia.org/wiki/Glycolysis#Overview ). So this and considering the fact that an hours running only burns about 500 calories (so if you say 1 KG equals 8000 then that's like 60 grams) then you've got about 60 grams of waste product (some of which is amino acids like lactase which your body can use and thus the actual mass is even less) which is really not that much.

    So, when you see Calories (which is actually kilocalories) on the side of food that's just a general approximation of how much energy can be obtained from this food by breaking down its molecules to smaller molecules (which once again produces no loss of mass).
  11. Jun 10, 2008 #10
    Ok this is true, but ultimately confusing. The mass loss due to bond breaking is the exact E=mc^2 amount which is released by heat but if you were to take a scale and weigh the wood and oxygen and then weigh the end products you would really get the same number (since as you say the difference is mass in extremely small) so don't be come confused by this (i.e. when you exercise to lose weight the 'weight' you're losing for all intents and purposes is being lost through waste products and not because the breaking of bonds cause a reduction mass (which is totally negligible)).
  12. Jun 10, 2008 #11

    Andy Resnick

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    Other people have adequately addressed the premises, but there seems to be a lot of confusion about the question.

    First, as maverick_starstrider mentions, caloric content printed on the label of food (or the 9 kcal/g of fat number) comes from bomb calorimetry measurements, not true metabolic measurements. So don't take the numbers to be exact. We can also assign caloric rates from excersise the same way- measuring how much heat energy is generated by certain activities. Again, don't take those numbers too literally either.

    Metabolic processes work via the *free energy* released in a reaction- this is not "energy released in breaking a chemical bond" by any stretch. Fortunately, thermodynamics teaches us that energy can interconvert between free, kinetic, potential, electrical, whatever form we wish to express it in.

    So, say you expend 1000 kcal (based on what the treadmill tells you, for example). That loosely corresponds to using ~100 grams of fat (or more likely a mix of carbohydrates and fats) for fuel. That does not mean you lost 100 grams of weight. It means your body accessed 1000 kcal of available energy stores- ATP and phosphocreatine at the very beginning (and is depleted nearly instantaneously), followed by glycogen (approximately 1200-1600 kcal stored in muscle, 300-400 kcal stored in the liver, then fatty acids (130,000+ kcal stored in adipose tissue).

    The energy source, whatever the source, is oxidized and converted into various waste products over time, such as CO2, H2O, urea, and so on.

    For weight loss, the "rule of thumb" is that over a 24 hour period, a resting metabolic rate consumes 2000 kcal (YMMV). An intake less than that means that over time, the body will convert more fuel into waste than was consumed. Ditto increasing the caloric load- burning more fuel than in consumed will result in weight loss over time.
  13. Jun 10, 2008 #12
    "Metabolic processes work via the *free energy* released in a reaction- this is not "energy released in breaking a chemical bond" by any stretch. Fortunately, thermodynamics teaches us that energy can interconvert between free, kinetic, potential, electrical, whatever form we wish to express it in."

    Energy is energy. It's either kinetic or potential (or mass). In the case of a chemical reaction which is releasing heat it is converting the potential energy the atoms have through the bonds in the molecule which are converted to kinetic when the bonds are broken or re-organized. *Free energy* is a thermodynamic metric (assuming you mean gibb's free energy) which is conserved in certain situtations and not actually a type of energy (of which there is only one).
  14. Jun 10, 2008 #13


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    Briefly, the loss of mass you can't account for is in your poo and wee.
  15. Jun 10, 2008 #14


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    Remember that fat in your body also contains a lot of water.
    So when people say that 3500 calories puts on 1lb of fat, it actually creates very little fat but adds nearly 1lb of water.
    That's how a snickers bar can add more than it's own weight to your hips!
  16. Jun 10, 2008 #15
    Maverick said, "Ok this is true, but ultimately confusing."

    You're absolutely right, I lost it on that one. Sorry.

  17. Jun 10, 2008 #16


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    Though technially, Einstein's Relativity always applies, the amount of energy you are dealing with in these processes is too small to notice the associated change in mass.

    These processes are chemical processes. The energy that is released is energy stored in chemical bonds, and is released when bonds are broken or formed (like snapping a stretched rubber band). There is no noticeable change in mass associated with these processes: the mass of the products equals the mass of the reactants.
  18. Jun 10, 2008 #17


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    While we're clarifying things: a "calorie" of food is not a calorie, it's a kilo calorie. Your daily intake, energywise, is 2,000,000 calories.
  19. Jun 11, 2008 #18

    Andy Resnick

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    Again, let me clear up a continuing misconception:

    Cellular biochemical reactions generally use hydrolysis of ATP to drive thermodynamically unfavored reactions- for example, to pump sodium out of a cell via the Na-K-ATPase membrane protein.

    Many books make the claim that the energy is due to "breaking the high-energy phosphate bond" in ATP. This is totally untrue.

    The reason ATP is usable as an energy source is becasue the cell keeps the relative concentration of ATP and ADP out of equilibrium- approximately 13 orders of magnitude out of equilibrium. So converting on molecule of ATP to ADP releases energy due to the change in numbers of ATP molecules relative to ADP molecules. That is not the same as releaseing the binding energy contained in a phosphate bond. Either the Helmholtz or Gibbs free energy can be used to describe the process, but the Gibbs free energy is better becasue it's a constant temperature, constant pressure process (more physiologcally relevant).
  20. Jun 13, 2008 #19
    thats probably the best answer right there.
    also note that in 1 hour of running you are likely to consume much more than just 500 calories. An 80 kg person (~170 pounds) will burn about 600-1000 depending on the speed. Of course, at 1000calorie/hour speed you probably wouldnt be able to run for an hour though :)
  21. Jun 18, 2008 #20
    burning calories

    i'm going to point out one fact before i continue. molecules of fat are not what you see in the mirror. the fat on your body is adipose tissue, or groups of fat cells. you may have a lot of fat molecules traveling around in your body that do not show up as flab. i hope that makes sense.
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