Marks Daily Apple
Serving up health and fitness insights (daily, of course) with a side of irreverence.
2 Jul

Dear Mark: How Much Glucose Does Your Brain Really Need?

We now know that the oft-repeated “your brain only runs on glucose!” is wrong. I’ve mentioned it before, and anyone who’s taken the time to get fat-adapted on a low-carb Primal eating plan intuitively knows that your brain doesn’t need piles of glucose to work, because, well, they’re using their brain to read this sentence. Obviously, you eventually adapt and find you have sufficient (if not much improved) cognition without all those carbs. That said, some glucose is required, and that’s where people get tripped up. “Glucose is required” sounds an awful lot like “your brain only uses glucose” which usually leads to “you need lots of carbs to provide that glucose.” And that’s the question today’s edition of “Dear Mark” finds itself attempting to answer: how much glucose is required?

Let’s get to it.

Hi Mark,

I have a little problem. Even though I’m able to function at work, maintain conversations, and go about my daily life without having segments of my brain suddenly stop working while eating Primal, my friends are worried about my brain. All they know is that the brain needs glucose. What can I tell them? How much glucose does my brain actually require to keep working?



I wouldn’t be too hard on your friends. They mean well and it’s a common misconception. Instead of chiding them, rubbing their faces in the knowledge that you can function quite adequately on a high-fat diet, educate them.

How much glucose the brain requires depends on the context. There’s not one single answer.

If you’re on a very high fat, very low carb diet – like a traditional Inuit diet – your brain will eventually be able to use fat-derived ketones for about 50-75% of its energy requirements. Most ketones are produced in the liver, but astrocytes in the brain also generate ketones themselves for use by neurons. You think we’d have that kind of set up in our brains if ketones weren’t useful to have around? If all we could do was burn glucose up there, what would be the point of even having localized ketone factories? Anyway, since the brain can use about 120 grams of glucose a day (PDF), that means you’d still need at least 30 grams of glucose while running on max ketones.

If you’re merely on a lower carb diet – staying under 150 grams per day or so – or eating medium chain triglycerides (coconut oil, MCT oil) to directly generate ketones, you’ll have access to ketones without being in full-blown ketosis, and your brain will be accessing some of them for energy. Take the story of Dr. Mary Newport, who lessened her husband’s Alzheimer’s symptoms simply by adding a couple tablespoons of coconut oil to his regular diet. The MCTs in the coconut oil were converted to ketones, which his brain began using. You’ll probably need more than 30 grams of glucose, but you won’t need the full 120 grams on a lower carb Primal way of eating (especially if you eat some coconut).

If you’re involved in strenuous exercise, your brain will be running primarily on lactate. Yep, lactate – that unwanted metabolic byproduct of muscle metabolism. During exercise, when the muscles are using up most of the available glucose to lift things and move a bunch of intelligent primate flesh through three dimensional space, and where inadequate oxygen (hence breathing hard) leads to incomplete glucose and pyruvate breakdown and increased lactate levels, the brain will draw upon lactate as a direct energy source. Not only that, but lactate appeared to make the brain run more efficiently, more snappily, and when both are available, the brain prefers lactate over glucose. Other research has found that the brain also prefers lactate in the hours and days immediately following a traumatic brain injury. I’m not sure how much glucose the brain requires when it’s accessing lactate, but it’s definitely fewer than 120 grams.

Of course, even when you need some glucose, that glucose needn’t necessarily come from dietary carbohydrate. It can famously come from gluconeogenesis, the process by which the liver converts amino acids into glucose. It can also come from glycerol, a byproduct of fat metabolism. In deep fasting situations, glycerol can contribute up to 21.6% of glucose production, with the rest presumably coming from gluconeogenesis. The glycerol can come from both dietary fat and adipose tissue (the authors of that glycerol fasting study even suggest that fasting burns body fat in order to provide glycerol for glucose production), while the amino acids can come from dietary protein (if you’re eating) or muscle (if you’re starving).

Overall, recent research into the metabolic demands of brain slices (“living” pieces of brains isolated and used for research) shows that incorporating other energy substrates – ketones, lactate, or even pyruvate – into the glucose solution improves oxidative metabolism and neuronal efficiency. Before you say “but this was in vitro, my brain’s not sliced up and submerged in a weird syrupy solution,” know that the whole point of the study was to better replicate the conditions of the kind of real, actual, living, thinking brains we find in human heads. The authors note that the glucose-only solution normally used to fuel brain slices in other studies is limited, because “in the intact brain, complex machinery exists that coordinates energy substrates delivery and adjusts energy substrate pool composition to the needs of neuronal energy metabolism.” In other words, glucose solution is an easy, dependable way to fuel brain slices, but it’s an incomplete representation of how brains work in heads. The authors conclude that “in slices as well as in vivo, the ability of glucose to maintain energy metabolism is limited and neuronal energy supply should be supported by other oxidative substrates.” 

So, a healthy, efficient brain is one that draws on several different fuels. A healthy, efficient brain is one that uses ketones (and perhaps lactate and other fuels) to spare some glucose. A complete reliance on glucose indicates an underachieving brain, a brain that could do so much better, a brain that could really use a coconut milk curry and some intense exercise every now and again. As far as we can tell, then, the absolute physiological minimum is 30 grams of glucose. I wish I could provide hard numbers for some of the other contexts beyond near carnivory (like basic 150 grams carbs Primal eating with coconut or maybe figuring out how to rely on lactate fueling), but the numbers don’t really matter in practice. What matters is that our brains don’t need the full 120 grams of glucose, especially if we’re following a Primal Blueprint eating plan.

I hope that helps.

Questions? Comments? Concerns? Leave them here. Thanks for reading!

You want comments? We got comments:

Imagine you’re George Clooney. Take a moment to admire your grooming and wit. Okay, now imagine someone walks up to you and asks, “What’s your name?” You say, “I’m George Clooney.” Or maybe you say, “I’m the Clooninator!” You don’t say “I’m George of George Clooney Sells Movies Blog” and you certainly don’t say, “I’m Clooney Weight Loss Plan”. So while spam is technically meat, it ain’t anywhere near Primal. Please nickname yourself something your friends would call you.

  1. Just what I was looking for – for when my husband’s medical friends say that that brain can run on ketones but not well. (I feel this is said ‘smugly’ to myself, a mere mortal without a medical degree, or is that a false perception generated by my ketogenic brain ?)

    Ann wrote on September 4th, 2012
  2. how would you know when you need a glucose
    and what is a doe it work.

    fabiolabutoyi wrote on September 23rd, 2012
  3. i don’t know what is wrong with my brain please can somone help me out. when ever i study my books i easily foget what i have read. please do i need glucose to cure this forgetful act?

    Leo-Brown wrote on December 5th, 2012
  4. Love your website!
    By educating me you have inproved my diet and because im eating better the positive effects tumble over into many aspects of my life.
    Im only 23 but going by my curent health status I expect to live a really long healthy and happy life which you will have contributed to.
    One final thing I want to add is that I have some mental issues which I obviously don’t usually broadcast but had to say that this way of eating has really helped me control them grately!
    I hope you sleep easy at night knowing you have helped so many people.
    Thankyou Mark!

    Ben wrote on January 20th, 2013
  5. So obviously, 1 g glucose = 1 g carbs. But does 1 g carbs = 1 g glucose? How many carbs should I be eating to get enoigh glucose?

    Tigara wrote on April 9th, 2013
  6. Wow this is very interesting.My daughter had just been diagnosed with glut1 ds.Glucose transmitter deficiancy syndrome. Very rare and it means her body doesnt absorb glucose correctly and in turn this has given her epilepsy. We are starting the ketogenic diet on tuesday. Thanks for this interesting read.

    Laura wrote on April 25th, 2013
  7. Brain carbs will also depend on the amount of brain activity you have going on. If you have a thinking job, where you have to solve problems instead of just manual labor… your brain will need more energy and more carbs. Thinking takes work.

    Mark wrote on February 10th, 2014
  8. Does anyone know is there a straightforward way to calculate how much glucose the body is using each day, given as input the average glucose level (A1C)? Or is there a non trivial function about how actively a given glucose level actually gets used by the body?

    If A1C doesn’t let us calculate this, are there any blood tests that would let us know how much glucose our body is burning each day?

    Pone wrote on February 16th, 2014
  9. The author does not fully understand the dangers of ketoacidosis and has a very poor grasp on evolutionary biology. “You think we’d have that kind of set up in our brains if ketones weren’t useful to have around?” Just because a certain structure or process occurs it does not mean that it has no repercussions. As someone else mentioned it is wrong to oversimplify complex things such as human biology. I recommend people to talk to qualified professionals like their doctors before changing diets and causing the body stress. The author advocates to go from one extreme (too much sugar) to the other (as little sugar as possible). “If all we could do was burn glucose up there, what would be the point of even having localized ketone factories? ” The point is that its a fail safe mechanism. Much like trying to hold your breathe voluntarily until you die, it simply won’t happen. There are brain centers that will force you to breathe. Does that mean people should give up breathing and hold their breathe every now and then? “Of course, even when you need some glucose, that glucose needn’t necessarily come from dietary carbohydrate. It can famously come from gluconeogenesis, the process by which the liver converts amino acids into glucose. It can also come from glycerol, a byproduct of fat metabolism. In deep fasting situations, glycerol can contribute up to 21.6% of glucose production, with the rest presumably coming from gluconeogenesis.” Wrong again, excess glucose is stored as glycogen to be broken down and released by glycogenolysis when the body is in need. Specifically in “fasting” situations that can give rise to hypoglycemia, triggering glycogenolysis. This article has facts that were nitpicked to favor an argument that is wrong in its own nature. Please consult your doctor and take the information presented here with a grain of salt.

    Albert Einstein wrote on July 17th, 2014
  10. Hi,
    I have got “18F-FDG brain pet CT STUDY” done
    Report says

    hypometabolism in bilateral mesial temporal cortex

    Can anyone help in understanding this

    Joysha wrote on September 7th, 2015
  11. Do you have cited sources?

    Ryan wrote on December 12th, 2015
  12. When you say 120g glucose a day do you mean carbs? does 1g of carbs =1g glucose?


    indiah wrote on April 20th, 2016
  13. Merely wanna admit that this is very helpful , Thanks for taking your time to write this.

    Verdell Moseley wrote on June 4th, 2016

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