There is the idea that mental work is tiring. Does the brain as a whole need to consume more glucose and oxygen when we are "mentally active"?

  • In the physiology course we learned that this is surprisingly not the case (i.e. the brain always consumes the same amount of energy, no matter the workload). Surely you also took the course and learned the same? ;-) But +1, since I always wondered the same and actually intended to ask the same question. May 16, 2011 at 19:47
  • @Konrad: I'm retaking the course at the moment and the answer is still the same that it was when you took the course ;) Unfortunately there no script for the course and therefore I can't link to it. [for those of you who don't know, we are studying at the same subject at the same university]
    – Christian
    May 16, 2011 at 20:09
  • 2
    Some new research? Someone want to write it up? I am mentally exhausted at the moment.
    – Oddthinking
    Jul 22, 2012 at 3:35
  • @Oddthinking: There nothing new about the research you refer to. A linked paper like "Carbohydrates and mental function: feeding or impeding the brain?" comes from 2007. Over at biology.stackexachange there are answer to this question which reflect the scientific consensus.
    – Christian
    Jul 22, 2012 at 19:26
  • @Christian: Your comments throughout seem to indicate you are on top of this issue. Any chance you want to write a summary answer from those sources? The answers here don't strike me as definitive (e.g. Ye/No/It's more complicated/no-one knows for sure.)
    – Oddthinking
    Jul 23, 2012 at 2:35

3 Answers 3


I think Chris Dennet's answer is a bit misleading, and I would like to see quoted sections for such a complex question rather than some lonely links. In my opinion, there is no direct link between fMRI signal and neuronal activity.

I looked for research on the changes of brain activity (sleep, resting, cognitive tasks...) and overall energy consumption of the brain. I found two very good articles putting your question into a bigger picture, quite academically written, but they focus on the statements concerning energy consumption. The articles also discuss what actually can be derived from brain imaging data (fMRI, PET) and how higher conscious brain functions relate to physiological changes measured by these techniques.

The basic conclusion is that the brain doesn't vary its energy consumption much, whether resting, tasked. In fact, it needs high average activity (high metabolism, energy consumption) to make specific functionality possible at all. So it's not like a computer where you start a program (analogous to higher conscious brain function, e.g. playing chess) and then the processor and memory consumption rises. Instead, the energy consumption is already and constantly on a high average level, otherwise the operating system (brain) couldn't run distinct software (function) at all.**

I've quoted the most important parts, but both articles give a pretty good overview and draw the bigger picture around your question.

brain represents about 2% of the body weight. Remarkably, despite its relatively small size, the brain accounts for about 20% of the oxygen and, hence, calories consumed by the body. This high rate of metabolism is remarkably constant despite widely varying mental and motoric activity

showing that the maximum values of oxygen consumption and spike frequency achieved during stimulation were approximately the same from both baselines (i.e., both levels of anesthesia). The authors assert that an overall level of ongoing activity must be achieved for a particular function to occur

This high metabolic activity is present when we are completely passive and resting as well as when we are observably doing something. Two lines of investigation have recently converged in their analysis on how this energy is being used. Both have focused on the metabolic requirements associated with glutamate signaling in the brain. This focus would seem reasonable, considering that greater than 80% of neurons are excitatory and greater than 90% of synapses release glutamate (6, 7). Attwell and Laughlin (8) have taken a bottom up modeling approach using extant data on the blowfly retina and the mammalian cerebral cortex. Estimates from their approach indicate that most of the energy used in the brain is required for the propagation of action potentials and for restoring postsynaptic ion fluxes after receptors have been stimulated by the neurotransmitter. In contrast, maintenance of the resting potential in neurons and glial cells accounts for less than 15% of the total energy consumption. Shulman and his colleagues (9, 10) in a very different approach using MRS in anesthetized rats have shown remarkably converging evidence that a very large fraction (≈80%) of the energy use in the brain is correlated with glutamate cycling and, hence, active signaling processes

An intriguing hypothesis has emerged that the responsiveness of neurons to changes in their input depends on a continuous, high-level but balanced input of both excitatory and inhibitory activity (for review, see ref. 29). Importantly, it is the balance between this continuous excitatory and inhibitory input that determines the gain or responsiveness of the neurons to correlations in their input. In this formulation, spontaneous ongoing activity becomes a critical enabling factor in the creation of functional connections within circuits responsible for specific behaviors. Furthermore, this correlation-induced functional connectivity can be modified without causing variations in the mean firing rates of the involved cells. As Salinas and Sejnowski have pointed out in their review (29), balanced neurons have rich dynamics and can react to external stimuli on effective timescales that are much smaller than the membrane time constant of a single neuron.

So, how might this relate to our analysis of the energy budget of the brain? It should be noted that most of the neurophysiology discussed above concerns synaptic activity at the input to neurons. Because the highest energy-demanding processes in the brain are centered at these sites (27, 28), it suggests that much of the ongoing or baseline metabolism is devoted to processes occurring there. We might therefore posit that, in the brain, a large majority of its metabolic activity is devoted to ongoing synaptic processes associated with maintaining a proper balance between excitatory and inhibitory activity. Maintenance of this balance allows neurons to respond appropriately to correlational changes in their input and establish the functional connectivity as required for a particular task.

Thus, we may entertain the possibility that the very high baseline or ongoing metabolic activity of the brain not only supports processes necessary for the maintenance of the proper responsiveness of neurons for the transient and ever changing functions of the brain but also instantiates a sustained functionality.


Indeed, relative to the high rate of ongoing or “basal” brain metabolism,6 the amount dedicated to task-evoked regional imaging signals is remarkably small (estimated to be less than 5%). The brain continuously expends a considerable amount of energy, even in the absence of a particular task (i.e., when a subject is awake and at rest). A significant fraction of the energy consumed by the brain (quite possibly the majority) has been shown to be a result of functionally significant spontaneous neuronal activity.7 From this cost-based analysis of brain functional activity, it seems reasonable to conclude that intrinsic activity may be as significant, if not more so, than evoked activity in terms of overall brain function.


  • As far as the last paragraph goes, most people can unconsciously recall a lot of information that they can't consciously recall. We can feel fear and anxiety towards stimuli without remembering our experience with the stimuli. In a similar way an experienced chess player unconsciously accesses experiences of a lot of his previous games.
    – Christian
    Jul 9, 2011 at 16:53
  • @Christian it just shows, that we still dont know much about information processing methods of the brain. Also i wouldnt compare "fear" with "logical information based reasoning" in chess or poker (poker player dont reduce their thinking time for a action at high limits). First you can achieve by operational conditioning like a very fast body reflex (there is not much information processing involveld), second is based on slow learning, training of information/theories. Kim Peek can save a lot of information very fast and recall it, despite his brain doesnt use more energy than average joes. Jul 9, 2011 at 17:32
  • So there is a small amount of extra energy that is used when thinking extra hard (less than 5%)? Am I reading that right, The entire rest of the answer seemed to be saying the opposite.
    – Jonathon
    Sep 9, 2015 at 22:44
  • The brain never stops working. You may be at rest and doing "nothing", but your mind will still wander about. Daydreaming is like a screensaver for the brain that keeps processing high.
    – T. Sar
    Apr 19, 2017 at 19:57

fMRI scanner images show increased areas of blood flow in parts of the brain during mental work. Increased neural activity causes an increased demand for oxygen, and the vascular system actually overcompensates for this, increasing the amount of oxygenated hemoglobin relative to deoxygenated hemoglobin1.

Because deoxygenated hemoglobin attenuates the MR signal, the vascular response leads to a signal increase that is related to the neural activity2.

However, it is not known if this correlates precisely with neural activity, and it is an ongoing area of research3.

  1. Oxygenation dependence of the transverse relaxation time of water protons in whole blood at high field
  2. Cellular mechanisms of brain energy metabolism and their relevance to functional brain imaging
  3. Baseline GABA concentration and fMRI response
  • 5
    An fMRI scanner shows that specific brain area's consume more energy when they are active. Most fMRI pictures however show different activity in different parts of the brain. Does the brain shift energy uses from one part to the other or does the brain as a whole increases the amount of energy that it uses?
    – Christian
    May 15, 2011 at 21:42
  • Thanks for the edit :) I'm not sure if the brain merely shifts around blood, or manages to obtain more from the rest of the body. I think it's a bit of both. May 16, 2011 at 23:15
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    Following from @Christian's comment: Is there anything here to suggest that the increase is significant enough to cause tiredness? It it an increase of, say, 10% or 0.0001% of the body's oxygen (or ATP?) consumption.
    – Oddthinking
    May 20, 2011 at 16:49

Edit: I've learned that this article should not be trusted, due to multiple weaknesses.

Perhaps Gailliot et al.'s1 work on blood glucose and self-control is a more direct answer than the fMRI results (I'm also a bit skeptical like footnote 3). I think the link with blood glucose is not specific to self-control.

The present work suggests that self-control relies on glucose as a limited energy source. Laboratory tests of self-control (i.e., the Stroop task, thought suppression, emotion regulation, attention control) and of social behaviors (i.e., helping behavior, coping with thoughts of death, stifling prejudice during an interracial interaction) showed that (a) acts of self-control reduced blood glucose levels, (b) low levels of blood glucose after an initial self-control task predicted poor performance on a subsequent self-control task, and (c) initial acts of self-control impaired performance on subsequent self-control tasks, but consuming a glucose drink eliminated these impairments. Self-control requires a certain amount of glucose to operate unimpaired. A single act of self-control causes glucose to drop below optimal levels, thereby impairing subsequent attempts at self-control.

1 Gailliot, Matthew T.; Baumeister, Roy F.; DeWall, C. Nathan; Maner, Jon K.; Plant, E. Ashby; Tice, Dianne M.; Brewer, Lauren E.; Schmeichel, Brandon J. Journal of Personality and Social Psychology, Vol 92(2), Feb 2007, 325-336. doi: 10.1037/0022-3514.92.2.325

  • 1
    Stress can plausibly increase muscle tension. Increased muscle tension consumes glucose outside the brain.
    – Christian
    May 29, 2011 at 21:42
  • Hehe, didn't think of that. But still, it wouldn't explain that low levels of blood glucose predicted poor performance on subsequent self-control tasks, unless the increased muscle tension was somehow necessary to concentrate.
    – Ruben
    May 29, 2011 at 22:25
  • Low blood glucose could trigger parasympathicus activity. Parasympathicus activity reduces vigilance and makes concentration harder.
    – Christian
    May 30, 2011 at 8:08
  • Very nice! I'll go through scholar.google.com/… later and see if that has ever been controlled for.
    – Ruben
    May 30, 2011 at 8:21
  • @Christian I didn't find a study which controlled for this. Now I'm intrigued that you might be right and they would have been measuring an artifact. Do you have references for your claims? I'd also like to chat with you about it, since you apparently have a lot of expertise.
    – Ruben
    Jun 7, 2011 at 17:52

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