Using calories of food (as a human body), several questions

  1. fluidistic

    fluidistic 3,336
    Gold Member

    I would like to understand what exactly is meant by "calories" when we read for example that a chocolate bar has 300 kcal. I understand what is a (kilo)calorie.
    What I do not know is whether the written values are the raw values in calories or if they are the values our body can "extract" from the food.
    In other words, if I eat the chocolate bar of 300 kcal, can my body use 300 kcal energy entirely taken out from the chocolate bar? Or can my body only extract a lesser quantity than that due to some inefficiency?

    Another question I have is, say a quarter of a glass filled with oil has also 300 kcal. If I drink it, can I get as many "useful" calories than with the chocolate bar? By useful I mean, can my body use as many calories as with the 300 kcal from the chocolate bar. Or we do "extract/absorb" different food differently?

    Thank you.
  2. jcsd
  3. SteamKing

    SteamKing 10,999
    Staff Emeritus
    Science Advisor
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    As with any mechanism, the human body is not 100% efficient at extracting the energy from food and converting it to usable work. It's about 25% efficient at doing this, so for every 100 calories you consume, about 25 calories are used to keep you alive and moving around. The rest is lost as heat, since humans maintain an elevated body temperature.

    Different foods metabolize differently within the body. If you consume a cup of olive oil, for example, the oil is digested differently than if you were to consume say a cup of a simple sugar like dextrose or glucose, which can be directly absorbed into the bloodstream during digestion.

    The calorie predates SI units. A small 'c' calorie was defined as the amount of heat required to raise one gram of water 1 degree C in temperature. A big 'C' Calorie, or kcal, is the amount of heat required to raise 1 kg of water by 1 degree C. The larger unit was adopted to measure the energy equivalent of foodstuffs as a matter of convenience:

    The joule is the SI unit of heat, and one kcal is equal to about 4200 J.
  4. fluidistic

    fluidistic 3,336
    Gold Member

    Thank you for your reply. I am still left with uncertainties.
    If I understand you well, you define useful work as keeping the organism alive and allow it to move. And you make a distinction between keeping alive and heating the body. I would have thought that these were the same.
    If I understood you well, if I consume 100 calories, 25 of these calories will help me to move and keep my alive (like the energy required for the heart to beat+breathing I guess?) while 75 calories will heat my body. It looks like overall I will use 100% of these 100 calories and none is lost in the excrement. Does this sound correct? Likely not because it sounds like the human body is 100% efficient at extracting energy from food but not 100% efficient at converting it to useful work which goes against your sentence.

    From this article I read that
    Does this mean that the value written on the chocolate bar "100 kcal" is the useful energy the human body can extract out of it? So that in reality the chocolate bar created over 500 kcal of heat in the bomb calorimeter. Is this correct?

    So if I understand well the only difference between drinking a 100 kcal oil olive and eating a 100 kcal sugar powder is just the time it will take for my organism to convert the energy into useful work, but in the end it will give me the same energy.
    That would be true if they performed the Atwater system when writing "100 kcal" for both nutriments. But not true if I measured a heat production of 100 kcal for both the olive oil and chocolate bar in a calorimeter, because my organism will not have the same efficiency to convert the raw calories into useful calories for the two nutriments.

    Maybe worth to mention is that in the wikipedia article one reads "strange" numbers:
    and that in fact it seems that the calorimetric values written over nutriments should be multiplied by 0.85 if I want to know the useful work I can get out of them. This contradict their claim that the Atwater system is applied.

    I am totally confused.
  5. SteamKing

    SteamKing 10,999
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    Science Advisor
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  6. fluidistic

    fluidistic 3,336
    Gold Member

    I had not read it entirely, now I do.
    It seems to be in use (or some derivative of that system), so when one reads "100 kcal" on a chocolate bar, it is supposed to be the useful energy that one can get from the chocolate bar and not the gross energy (that I mistakenly called "raw calories" previously in this thread because I didn't know the conventional term).
    So in a sense when I read the values in calories of nutriments, I have to forget that the human body has some inefficiency converting that energy because the displayed values already take that into account.

    I am still trying to understand what wikipedia means by
    . I am guessing they mean that [itex]{\text{Metabolisable Energy}} = \left(\text{Gross Energy in Food}\right) - \left(\text{Energy lost in Faeces, Urine, Secretions and Gases}\right) \approx 0.85 \text{ Gross Energy in Food}[/itex] or something similar to this.
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