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

by fluidistic
Tags: body, calories, food, human
 PF Gold P: 3,225 Hello, 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.
 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 6,754 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. http://en.wikipedia.org/wiki/Food_energy 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: http://en.wikipedia.org/wiki/Calorie The joule is the SI unit of heat, and one kcal is equal to about 4200 J.
PF Gold
P: 3,225
 Quote by SteamKing 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.
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.

 Quote by =SteamKing http://en.wikipedia.org/wiki/Food_energy
 Quote by wiki Conventional food energy is based on heats of combustion in a bomb calorimeter and corrections that take into consideration the efficiency of digestion and absorption and the production of urea and other substances in the urine.
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?

 Quote by =SteamKing 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.
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:
 Quote by wiki Each food item has a specific metabolizable energy intake (MEI). This value can be approximated by multiplying the total amount of energy associated with a food item by 85%, which is the typical amount of energy actually obtained by a human after respiration has been completed.
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.

 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 6,754 Using calories of food (as a human body), several questions Did you read the related links: http://en.wikipedia.org/wiki/Atwater_system
PF Gold
P: 3,225
. I am guessing they mean that ${\text{Metabolisable Energy}} = \left(\text{Gross Energy in Food}\right) - \left(\text{Energy lost in Faeces, Urine, Secretions and Gases}\right) \approx 0.85 \text{ Gross Energy in Food}$ or something similar to this.