# How much energy does the brain use in different circumstances?

## Main Question or Discussion Point

On average, how many calories (approx.) does the brain need during the course of a day? Does more activity (thinking more than normal) cause an increased need for calories? How many calories are used while sleeping?

Thanks,
Jacob

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That's an interesting question. I don't have a clue what the answer is, but it lends thought to such things as "I'm mentally exhausted." Like after you have been studying boring stuff for hours, your mind feels like mush.

I know the brain isn't a muscle in the sense of actin/myosin, but perhaps you can work it? People say you have to use your brain in order for it work well, any truth in that?

I do know that since I've been serious in school, my analytical and thinking abilities have improved.

aychamo said:
That's an interesting question. I don't have a clue what the answer is, but it lends thought to such things as "I'm mentally exhausted." Like after you have been studying boring stuff for hours, your mind feels like mush.

I know the brain isn't a muscle in the sense of actin/myosin, but perhaps you can work it? People say you have to use your brain in order for it work well, any truth in that?

I do know that since I've been serious in school, my analytical and thinking abilities have improved.
It doesn't build up like regular muscles, but I remember hearing that when you use parts of your brain your previously hadn't, more connections build between that part and others, and more nutrients etc. go towards that part functioning well.

Moonbear
Staff Emeritus
Gold Member
You have a good question about how much energy does the brain require, but I haven't been able to find a proper answer on that. There are comments, even in reputable articles, about the brain using a proportionally large amount of the body's energy, but I have yet to locate any studies demonstrating that. My best guess is they are pretty old, pre-dating the date range of the pubmed index. So, unless you're willing to spend a while hunting through the good old-fashioned bound version of Index Medicus, I'm not sure where to find the answer. I admit, I'm also curious how they actually determined this. I don't know of any way you could directly measure energy consumption of the brain without the including rest of the body, or at least the rest of the head, especially if these studies were done more than 40 years ago.

Calorie (glucose) usage of the human brain

wasteofo2 said:
On average, how many calories (approx.) does the brain need during the course of a day? Does more activity (thinking more than normal) cause an increased need for calories? How many calories are used while sleeping?
• Energetic basis of brain activity: implications for neuroimaging.

The complex activities of the brain need not distract us from the certainty that it uses energy and performs work very efficiently. The human brain, which claims approximately 2% of our body mass, is responsible for approximately 20% of our body oxygen consumption. In vivo magnetic resonance spectroscopy (MRS) follows the metabolic pathways of energy production (as glucose oxidation) and work (as monitored by the cycling of glutamate and GABA neurotransmitters). In the resting awake state, approximately 80% of energy used by the brain supports events associated with neuronal firing and cycling of GABA and glutamate neurotransmitters.

Wearable computing researcher Thad Starner, in a paper he wrote (while a graduate student at MIT) on powering wearable computers with energy harvested from the human body, included a table of total body heat dissipation (the corollary of total body energy consumption):

Code:
Table 2 Human energy expenditures for selected
activities (derived from Reference 3)

Activity              Kilocal/hr        Watts

Sleeping                   70            81
Lying quietly              80            93
Sitting                   100           116
Standing at ease          110           128
Conversation              110           128
Eating a meal             110           128
Strolling                 140           163
Driving a car             140           163
Playing violin or piano   140           163
Housekeeping              150           175
Carpentry                 230           268
Hiking, 4 mph             350           407
Swimming                  500           582
Mountain climbing         600           698
Long-distance running     900          1048
Sprinting                1400          1630
This gets us a little closer, but it doesn't answer directly questions regarding how much energy the brain uses during specific activities. As far as total energy used throughout the day, there seems to be broad agreement across multiple allied disciplines that the brain typically uses approximately 20 percent of the total body energy throughout the entirety of any given typical day. Therefore, if your total body burned 2,500 Calories (a calorie with a capital c is a kilocalorie and is the energy unit adopted as standard for food labeling; e.g., food labels state Calories and not calories) in a given day, then if you are typical and that day was typical your brain burned ~500 Calories (and mean power dissipation by your brain would be ~24 watts).

Does more activity (thinking more than normal) cause an increased need for calories?
According to Arthur Jensen, yes (and no researcher seems to disagree with that, judging by the contents of the abstracts returned by a combined search for the keywords glucose, brain and energy on PubMed). Magnetic resonance studies involving glucose doped with radioisotopic tracers show more glucose use when the brain is active and show more glucose use in areas of the brain involved inspecific activities. This has allowed researchers to see which areas of the brain are activated during given specific types of cognitive activity. For example, a test subject may be asked to perform a certain cognitive task or even take an IQ test while his brain is being scanned. The areas of his brain that are activated during performance of the task are then visible to the researchers.

• Another study [27] investigated glucose metabolic rate (GMR) as a function of the "mental effort" expended on a task. The investigators did not correlate GMR with the same test for each individual, but compared groups of average and high-IQ subjects (mean IQ of 104 vs. 123) on easy tasks and on difficult tasks that were equated for the same degree of either "easiness" or "difficulty" within each group. Regardless of the task's objective demands, tasks for which 90% of the responses were correct (within the average group, or within the high-IQ group) were defined as "easy" for each group, and tasks for which only 75% of the responses were correct (within each group) were defined as "difficult." In other words, the level of a task's subjective difficulty was calibrated relative to each group's ability. For example, the average-IQ group could recall 6 digits backwards on 75% of the trials, whereas the high-IQ group could recall 7 digits on 75% of the trials. The measurements of GMR during these tasks revealed a significant interaction between IQ level and "mental effort" (i.e., level of difficulty relative to the individual's general ability level). Average- and high-IQ subjects hardly differed in GMR on the "easy" items but differed markedly on the "difficult" items. The high-IQ subjects brought more "fuel" to bear on the more difficult task. This increase in GMR by the high-IQ subjects suggests that more neural units are involved in their level of performance on a difficult task that is beyond the ability of the average-IQ subjects.

27. Larson et al., 1995.

Larson G. E., Haier R. J., LaCasse L. & Hazen K. (1995). Evaluation of a "mental effort" hypothesis for correlations between cortical metabolism and intelligence. Intelligence, 21, 267-278.
(Arthur Jensen. The g Factor. pp 158-159, 168, 616-616.)

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wasteofo2 said:
Does more activity (thinking more than normal) cause an increased need for calories?
Specific Brain activities are linked to a higher metabolic activity, expressed bot in the glucose uptake and the oxygen consumption. That is the basis of both in vivo functional imaging techniques and in vitro studies of neuronal activity.

Wow, thanks for all that info hitsquad!

Yes, indeed

yes, indeed. brain uses lots of energy and i have been feeling it. started thinking better and more innovatively, boy, i can say, i have to eat a lot to keep with my thinking.

yes, indeed. brain uses lots of energy and i have been feeling it. started thinking better and more innovatively, boy, i can say, i have to eat a lot to keep with my thinking.
That all sounds like side effects of marijuana. Innovative thinking... and extreme hunger.

One must also take consideration of how intense the activity is and how much interaction is actually occuring. I bet one uses a substantial amount of energy in an activity that requires more interaction of all the senses, like playing the piano or any musical instrument of that sort which requires touch, sight, hearing, memorization,etc. Compare that to just regular studying, which requires just memorization and reasoning skills, the difference in energy intake is huge.
It also depends on the mental capacity of a person. If the person is musically gifted the field of music the energy intake might be less

The brain functions purely on glucose (that's why it is helpful to consume "simple" carbs before a test or mental work). Calorie consumption by the brain varies, depending on what type of work you do, so does the proportion (%) of calories consumed by the brain in relation to calories burned by other functions of our bodies. An average person with an average job has 20% of his daily calories burned by the brain. People, whose primary job/function requires almost exclusively mental/intellectual work with significant amount of mental complexity, burn up to 35% of their daily calorie usage. Please, note: this does not prevent fatty tissue build up, even if you consume less than your daily norm of calories. Our physiology is not built that way. A Russian physiologist Sechenov used to say: "Rest is not absense of activity, it is change of activity". Thus, to have a healthy body, intellectual workers must "change their activity" to physical work daily to avoid fatty tissue accumulation...