“Food energy – supply and actual uptake.”
This is framed as a chat with a Wikier but is, in truth, my still clarifying to myself some baseline elements of human sustenance.
I grew up, for complex reasons I’ll cover here soon, very aware of the inadequacies of the digestive system but have never really come back to defining the absolutes. I’ve always also felt there to be too little clear understanding of this subject. As I’ve mentioned before, the late Oxford Prof Sir Peter Medawar talked of the desperate need to draw ideas together, draw disciplines together. This is a cross curricular exploration and I think it shows!
Well, in this investigation the first stop is so easily taken. To research the energy contents of food, I went to Wiki:
http://en.wikipedia.org/wiki/Food_energy , which said:
“the convention is to use the heat of the oxidation reaction”, that is the chemistry in the bomb calorimeter.
“This method of estimating the food energy has several defects, the most serious of which is that protein is not oxidized in the body as in the bomb calorimeter, with the possible exception of severe starvation. In normal conditions, the protein is metabolized in processes which require energy such as protein synthesis or replacement, synthesis hormones, nucleic acids, etc. Thus, the food energy derived from proteins could be zero, if the energy saved by the body in using the proteic food components instead of synthesizing them is taken into account.”
“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”
Then, though, from this high point, reality creeps in:
“In general, the efficiency of muscles is rather low: only 18 to 26% of the energy available from respiration is converted into mechanical energy. This low efficiency is the result of about 40% efficiency of generating ATP from food energy, losses in converting energy from ATP into mechanical work inside the muscle, and mechanical losses inside the body. The latter two losses are dependent on the type of exercise and the type of muscle fibres being used (fast-twitch or slow-twitch). For an overall efficiency of 20%, one watt of mechanical power is equivalent to 4.3 kcal (18 kJ) per hour.”
I think you mean “at” an overall efficiency of 20%. . Look:
“Confusion of watts, watt-hours, and watts per hour.
“The terms power and energy are frequently confused. Power is the rate at which energy is generated or consumed.
“For example, when a light bulb with a power rating of 100W is turned on for one hour, the energy used is 100 watt-hours , 0.1 kilowatt-hour, or 360 kJ. This same amount of energy would light a 40-watt bulb for 2.5 hours, or a 50-watt bulb for 2 hours.”
Remember – “One Watt is the use of one Joule per second”? This is that!
“A person having a mass of 100 kilograms who climbs a 3 meter high ladder in 5 seconds is doing work at a rate of about 600 watts. Mass times acceleration due to gravity times height divided by the time it takes to lift the object to the given height gives the rate of doing work or power.
“A labourer, over the course of an 8-hour day, can sustain an average output of about 75 watts; higher power levels can be achieved for short intervals and by athletes.”
Are you starting to see things more clearly, my little light bulb? Over the 8 hour working day this is 2160kJ of energy usefully converted. At 20% efficiency that requires consumption of 5x2160kJ = 10800kJ (2580Cal/kcal) plus enough to cover base metabolic rate.
And we have also to look at efficiency of absorption from the intestine and of metabolic pathways and availability of nutrients held within the food..
Not quite like a big powerful car:
“A medium-sized passenger automobile engine is rated at 50 to 150 kilowatts – while cruising it will typically yield half that amount.”
OK, so you consume your Mars bar with its high energy content but, although it contains enough energy to lift you to the top of a mountain, in practice you’re less efficient than a 1930s coal fired power station. Mind you, your pollution is nowhere near as bad and you do have an inbuilt combined heat and power system.
But your metabolism has a maximum power output, sustained by the conversion to energy of elements within the food that you have eaten. So:
“For example, a manufacturer of rowing equipment shows calories released from ‘burning’ food as four times the actual mechanical work, plus 300 kcal (1,300 kJ) per hour,”
OK, I go with that, more or less – except where do they get 1300kJ from? But then
“ which amounts to about 20% efficiency at 250 watts of mechanical output.”
Which is anything but clear. Sadly it then say:
“It can take up to 20 hours of little physical output (e.g. walking) [What cheek!!] to “burn off” 4,000 kcal (17,000 kJ) more than a body would otherwise consume.”
Let me tell you that in twenty hours of walking I will have covered over one hundred kilometres and used up one considerable quantity of energy. As well as an amount equivalating to my base metabolic rate. But what a bizarre statement anyway, and what has it to do with anything else in its proximity. Then I think “Ah, bless the little volunteers for they know not what they say”. And then I think about the abstract nature of information, fact and encyclopeadiae and that no fact is true fact until it is related to others at which point it may just vanish.
Before it does, though, I summarise:
The Mars bar, quoted post calorimeter, gives up 1500kJ/100g. Yum. However you might only harvest 85% of this. Leaving 1275kJ/100g. And then it goes through your metabolic pathways, your glycolysis, your Kreb’s Cycle and , in your blood stream, travels as ATP type parcels to the point of use where the muscles are contracted, heat generated, neural signals released and so on. In all this, another 80% of the energy goes up in non productive, support sevices.
Leaving just 255kJ of muscle action. At steady hard working adult male rate of 75W that’ll power you through 57 minutes.Thus and assuming it weighs 100g “A Mars a day keeps you going for less than an hour!”