Why does the brain use up so much energy?

December 20, 2006 11:20 AM

Why does the brain use up so much energy?

How comes I would feel ravenous hungry after trying to solve
a complex math problem for 3-4 hrs but not feel that hungry after a 70 mile strenuous cycle in hilly countryside?

It just seems counter-intuitive that an imobile organ of the body can use up so much energy.

Can anyone explain the how or why of this phenomenen?

posted by jacobean to science & nature (13 comments total)

Well, the brain does use a lot of energy on it's own, but probably not more then running a 70 mile stretch. Rather, it's likely that after strenuous exercise, your body wants to relax, rather then digesting food.
posted by delmoi at 11:31 AM on December 20, 2006

It's immobile but not inactive. All those neurons firing use chemical signaling and thus chemical energy to do their thing. A computer isn't mobile, but it uses a heck of a lot of power too.

You're also likely to be so wrapped up in a problem you don't notice you're hungry until after you've solved it - that'd be my answer to your actual question.
posted by voidcontext at 12:02 PM on December 20, 2006

When you're awake and not engaged in heavy activity, about one third of the energy you use is used in your brain. Nerve tissue is inherently very energetic.
posted by Steven C. Den Beste at 12:24 PM on December 20, 2006

Actually, the chemical processes required for your neurons to work are remarkably similar to the chemical processes required for your muscles to work. They both have to pump ions into and out of the cells, produce and regulate all of those wonderful channels that allow them to "fire" (often the same exact channels, actually!), on top of doing all of the housekeeping stuff that all cells of every type have to do.

Neurons have it harder though in that they have to traffick molecules along a very long space. Neurons have long outreaches, and have to get proteins (or their precursors) from the nucleus at the center down all of those outreaches to the end of the cell; this takes a lot of energy. Most other cells, however, are more rounded or squared with a much less spread out structure. Neurons also have a great diversity of chemicals that they have to be able to synthesize and break down. They also "fire" at very different rates, some much faster than muscle cells.

I think I've heard that the brain uses something like 30% of your daily caloric intake, or something like that (I'll look for a proper citation for that...)
posted by twoporedomain at 12:24 PM on December 20, 2006

Well, the brain has no energy stores of its own and is completely dependent on metabolism in other areas of the body to keep glucose levels in the blood high enough for it to function. Just sitting there, thinking, your brain is highly active and those cells require glucose to continue their business. I don't think you can lose weight by doing a whole lot of thinking, but you certainly need to be maintaining blood glucose levels while your brain is guzzling the sugar and that will make you hungry after a while.

And, for me at least, if I just sit for 3 or 4 hours doing nothing at all, I'll start to want a snack. Whole meals only stay in your stomach for about that long.
posted by cyclopticgaze at 12:25 PM on December 20, 2006

Hmm, interesting. A recent review (Annu. Rev. Neurosci., probably accessible if you're on a university campus) states:

In the average adult human, the brain represents approximately 2% of the total body weight but approximately 20% of the energy consumed (Clark & Sokoloff 1999), 10 times that predicted by its weight alone. Relative to this high rate of ongoing or "basal" metabolism (usually measured while resting quietly awake with eyes closed), the amount dedicated to task-evoked regional imaging signals is remarkably small.

And in the next paragraph:

One can estimate that . . . the local energy consumption increase owing to a typical task-related response could be as little as 1%.

Note by "task" they are probably referring to the typically very boring and repetitive fMRI and PET tasks (not a criticism--simplicity is necessary for interpretable results and repetition is necessary to average out noise), so one could argue that 3-4 hours of real math would be significantly more demanding, but still, even if we're talking about a 2 or 3% increase in energy consumption, would that be enough to make you extremely hungry? I haven't the foggiest.

And what is the brain doing with all the energy it uses?

Measurements of brain energy metabolism using magnetic resonance spectroscopy (Sibson et al. 1997, 1998; Shulman et al. 2001, 2004) in a variety of experimental settings have indicated that up to 80% of the entire energy consumption of the brain at rest is devoted to glutamate cycling and, hence, neural signaling processes.

Glutamate is the main excitatory neurotransmitter in the brain (i.e., it stimulates nearby neurons when released into the synapse). By "glutamate cycling," they are referring to the process by which glutamate is pulled out of the synaptic cleft, and made ready for another round of neurotransmission. So this means that much of the energy used by the brain is related to the firing of neurons that are doing something unrelated to the task at hand.
posted by epugachev at 12:35 PM on December 20, 2006

The review (Current Biology, subscription also required) that led me to the review I referenced above discusses the next obvious question: What is all that task-unrelated neural activity good for?

We don't know, but the review lists two ideas:

One theory has been proposed to explain the resting state activity. It suggests that a ‘default-mode’ network is engaged in functionally important processing such as continuously monitoring the external world. . . . [A]reas involved in visuospatial orienting and representation of external space . . . are engaged at rest, maintaining a background level of attention to detect potentially dangerous external events. This pattern of brain activation then shifts when alternative circuits are engaged to support overt behaviours. So, in this perspective, the brain is envisaged to be constantly monitoring the internal or external environment, processing this information, and temporarily engaging systems to perform overt behaviours before returning to the default mode.

Nonetheless, intrinsic resting brain activity may not be devoted exclusively to default-mode processing. It may also make a vital contribution to the off-line processing and consolidation of memories.

I understand "memory consolidation" as the "solidifying" of memories in long-term storage, but I believe it's kind of a fuzzily-defined term.
posted by epugachev at 1:12 PM on December 20, 2006

In the mindhacks book (page 22, hack #10), there is an experiment you can do to test your brains energy use.

They suggest relaxing and having a friend take your carotid artery (neck) pulse for 60 seconds, then for the next minute trying to think of all the animals you can while the friend counts your exercise rate.

They say the increase varies quite a bit, but mine was about 10bpm more, about the same as a brisk walk.
posted by scodger at 1:18 PM on December 20, 2006

If you're staying up late working on stuff when you would otherwise be sleeping, you get hungry a lot faster. :)
posted by perpetualstroll at 1:32 PM on December 20, 2006

I wasn't able to search up anything more authoritative than this weight-loss site to back this up, but as I understand it, muscles can burn both fat and glucose, and that for extended aerobic excercise, fat is mainly burned.

The brain, however, can only burn glucose. In addition to the glucose in your blood there are stores of carbohydrate (glycogen) in your liver and muscles which can be converted to glucose. According to the BBC, the average person stores about 1700 Kcal in this system.
posted by jamjam at 3:07 PM on December 20, 2006

jamjam is dead right on the biochemistry, and almost certainly right that this is why you get hungry from brain work, you need to raise your blood sugar.

Teach me to forget my undergrad biochemistry.
posted by scodger at 3:22 PM on December 20, 2006

So it's "difficulty-related"? A brain uses more energy working on a physics problem than when scanning through Metafilter?
posted by davy at 4:18 PM on December 20, 2006

The body uses about 100 watts at rest and the brain's share is 20% of that.
posted by euphorb at 12:27 AM on December 21, 2006