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Brain Energy

The human brain requires a highly stable temperature and a supply of high protein and energy. Evolutionarily speaking, it is a very expensive organ to maintain. Fully one quarter of our caloric intake is used by the brain, which comprises only 2% of our total body mass.

In the United States, the average caloric consumption is 3600 Kcal per person daily. (Youíll note that this is approximately 1000 Kcal over the recommended daily amount.) The kilocalorie (Kcal) is a unit of heat, equal to 1000 calories. Itís important to know that a nutritionistís calorie is different from a physicistís calorie. A nutritionistís calorie is equal to 1 Kcal. What you see printed on food labels or in diet books are nutritionistís calories. I donít know who originated this confusing disconnect in scientific terminology -- but whoever he was, I hope heís getting his "just desserts" now in the netherworld.

Heat is convertible to energy, which is most often expressed as Joules. So 1 kilocalorie (or 1 nutritionistís calorie) is equal to 4,186 Joules. A kilocalorie is the amount of heat required to raise the temperature of 1 kilogram of water by 1įC at 1 atmosphere pressure. Converting an average U.S. citizenís consumption of 3600 Kcal of food per day into energy terms yields 15,070 kilojoules of energy.

It sounds like a lot, but itís not. If this amount of energy were used to heat water, it could in principle bring 10 gallons to a boil (starting from 10oC). Or it could heat up a very small (25 gallon) water heater to the required household temperature (50oC). Expressed in alternate energy terms, our bodies are consuming a little over 4 kilowatt-hours per day, with the brain taking 1 kilowatt-hour of that total for its own purposes.

If we were able to get all our required energy by simply plugging ourselves into an electrical socket, it would only cost us about $0.40/day to survive. Thatís roughly equivalent to keeping a 175W incandescent light bulb burning all the time. So if youíre "having a thought", the little light bulb hovering over your head is only a 40W one -- pretty dim, at best.

PET and MRI studies have shown that brain energy consumption rises and fails with emotional swings, learning new tasks, and rest vs. active status. Surprisingly though, these changes are small in relation to the constant "background" brain energy usage Ė as little as 10% of the total. Itís interesting that the balance of brain energy even in an awake, resting and unstimulated state is devoted to the very same neurotransmission signaling duties as are the small percentages of energy added by challenging tasks or "hard thinking". To explain this, some scientists hypothesize that our brains are constantly engaged in maintaining a "probabilistic model" of anticipated events and are, therefore, not heavily dependent on sensory information at all!

Why am I suddenly thinking of that classic old Sci-Fi movie, "Donovanís Brain"? That was the one with the functioning, conversing brain in a tank.

After having calculated the relatively low wattage produced by the human body, it also makes me question the credibility of the story-line of "The Matrix", wherein humans are kept alive in tanks in order to harvest their energy. You wouldnít really be able to maintain much of a civilization using such poor "organic batteries". For example, in 1998, the total U.S. daily energy consumption  was 260 trillion BTUs, or 75 billion kilowatt-hours. Youíd have to harvest the total energy from 19 billion humans to fulfill that energy demand. At last count, there were only about 6.5 billion people in the world. So this premise is not very plausible at all Ė although there are a few people out there that I would dearly love to throw in a tank and attach electrodes to. (Michael Jackson comes to mind rather quicklyÖ)

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See also Lyris Autran & Kathryn Blairís article in Neurology Reviews, Vol. 9 No. 2, Feb. 2001.

Quite recently, researchers at Panasonic's Nanotechnology Research Laboratory in Japan have developed a way to draw power from blood glucose -- mimicking the way the body produces energy from food. The result could be a device capable of producing electricity from blood, effectively turning bodies into "human batteries".  See here.

And for all my fellow Mystery Science Theater 3000 fans, here are the lyrics to that great song, "When I held Your Brain in My Arms"...



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