(This number differs a bit depending on age and body shape. To calculate your laziest possible caloric expenditure, also called "basal metabolic rate" or BMR, go here.)
Your brain alone, even watching Here Comes Honey Boo Boo, takes up a good 400 of these calories.
The brain's energy and oxygen demands are much subtler than those of the body. When riding a bike, for instance, and you decide to drop that fred, you quadruple the volume of air you take in, from a resting rate of 5 liters a minute to a whopping 20 liters a minute.
The expansion is a wonderful thing and hints at a range of possible ways to exist--to exist at X breaths per minute, or at X * 5 breaths per minute.
A friend of mine, a cyclist and possible cyborg, likes to solve the Rubik's cube while holding his breath. The effort of solving the cube, he says, takes a lot of oxygen--something you don't have much of if you aren't breathing. Doing nothing at all, he might last four minutes on one breath, but the brain in frantic motion cuts that down to two minutes.
The brain is "only 2% of the body weight, but it receives 15% of the cardiac output and consumes 20% of the total body oxygen" (Magistretti and Pellerin, 1996; Quastel and Wheatley, 1932). This is surprising to me, possibly because I don't feel my brain to be all that high powered and demanding. And possibly it isn't.
Back when we were trying to explain what, oh what, explained Lance Armstrong's magical first Tour de France win, we pointed to something called "efficiency." Ed Coyle proclaimed that Lance had increased his efficiency 8% between 1992 and 1998, even as his VO2max was stuck at 84. VO2max measures how much oxygen our body's fire is burning; efficiency measures how much of the energy from this bodily fire makes its way into the bicycle.
Incidentally, that's why VO2max is not a good predictor of winners of bike races--because it's only one variable in performance. Efficiency is another.
Efficiency has long perplexed me. It's what, in statistics, is called a delta or Δ. You get a delta by taking the difference between two things--in this case:
power consumed (calories burned) - power put into pedals (Watts) = Δ
This thing that is left over is called the delta.
It doesn't explain efficiency, of course. It's like a tax rate: every April when you look up your tax rate, you have no idea why you have to pay 70% of your income to the government, you just know that, given your income, this is your tax payment required.
I mean, one guy could be considered super efficient because he has a tiny, undemanding brain, so that the energy that would have been used for thinking up witty limericks and differential equations for application in knitting of doilies, instead simply sends that energy to his legs.
Or it could be that his liver is undemanding of energy.
It could be any number of things that make efficiency.
It remains, however, that even the most efficient of us is only 25% efficient.
It isn't much. Most of us, when we are burning our brightest, can spare only 200 to 400 Watts an hour.
Most things we do don't use our energy that well; bicycles, thankfully, do. They use all but 3-7% of the energy we put into them. Compare this with that ex-girlfriend, or even with combustion engines, which, at their best, are 30% efficient.
Why bicycles exist is clear; they are pure get-from-A-t-Z-machines. Energy applied to the moving parts of the bike make it obvious: you see it in the drive train, the turning of the wheels, the pivot of the fork, in the arcs the wheels draw in dirt. The bicycle's orbits are clean like energy at its elemental, like the movement of planets.
Our own purpose is less clear. Energy is used, directed, and wasted in, yes, sustaining life and thought, but the rest hangs behind a hood you can never be sure you've popped. There's no fire to see, no red-hot embers, no electrical charge. The brain is not just a flame from a log tossed on a fire; it's not just one more cool thing energy does.
I don't mean to get wrapped up in minutia, except to suggest a great freedom we are given--to live anywhere between our basal and peak metabolic rates, to breathe 4 or 20 liters a minute, to suck down a closet or a gymnasium of air in a day, whatever is your preference.
This is something we cyclists can claim, above anything else: that we have taken more breaths than anyone else our age. This doesn't make us better people; it just means we understand and believe in exertion. We are whatever the opposite of this is: sitting on the couch all day, eating soylent and watching Here Comes Honey Boo Boo.