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Let me introduce myself. My name is Mark Sisson. I’m 63 years young. I live and work in Malibu, California. In a past life I was a professional marathoner and triathlete. Now my life goal is to help 100 million people get healthy. I started this blog in 2006 to empower people to take full responsibility for their own health and enjoyment of life by investigating, discussing, and critically rethinking everything we’ve assumed to be true about health and wellness...

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July 02 2012

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

By Mark Sisson
116 Comments

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?

Thanks,

Frank

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!

TAGS:  dear mark, keto

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116 thoughts on “Dear Mark: How Much Glucose Does Your Brain Really Need?”

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  1. Maybe it was answered before, I just haven’t seen it on the site, but I have a few related questions for Mark or anyone who knows:
    Is 30g of glucose the same as 30g of carbohydrates, or is there a conversion?
    The other question is, after an anaerobic workout, how can I figure out how much glucose is needed to refill glycogen stores (how do duration and intensity influence the amount)? Mark or anyone else, I’d appreciate an answer, sorry if this was discussed before and I missed it.

    1. There is fructose, glucose and sucrose. All are carbs. Sucrose is half fructose and half glucose.

      Fructose does get metabolized differently. By the liver first.

      But yes, 30 grams of glucose is 30 grams of carbs.

      I have no idea on the second question but experimenting should give you a general idea.

      1. I think Alexander meant “are 30g of glucose the same of 30g of fresh fruit”? In any case, that’s also my doubt. Are here we talking about pure glucose or edible food? I mean, are theme 30g of glucose, 30g of fresh fruit, 30g of honey?
        I guess those are 30g of pure glucose, but how much fresh fruit are them? How to calculate it?

        1. i believe it’s 30 g of glucose.

          fruits also contain fructose & fiber.

          regards,

      2. No 30 grams of fruit are not equal to 30 grams of glucose, because the fruit is made up of more than just carbs..you have to look at the carbs themselves..a banana that weighs 200 grams may only have 70 grams of carbs. A good way to decide how much you need to recover after hard exercise is the 4 to 1 ratio..for carbs for every protein. so if you recover with 20 grams of protein then a banana or an Orange should be great.

    2. Sometimes it is not about numbers but how you feel. If you eat something and still don’t feel ‘replenished’ then next time eat something more or different and see how you feel. Experiment of one!

  2. My question is, how to go about finding that 30 grams of glucose, excuse my ignorance but are you propogating we actively go out and eat sugar, or would you say if you eating dark chocolate which has some sugar in it or say some alaskan smoked salmon which has say 2 percent sugar in it, this in actual fact could be benefitial rather than a hindurance?

    What are good sources of glucose?

    1. You get glucose from the breakdown of starches or some disaccharides. You don’t have to eat sugar, you can eat starch, like a sweet potato, yucca, unripe banana. Fruit has some glucose too but FYI fructose is not going to get converted and will be used for energy by other cells, not so much by the brain.

      1. You can also eat bcaa’s these area agreat way to hinder sugar cravings that you have regularly. just have a scoop of this an hour before. but you liver will convert them to glucose for your brain…A lot of these questions are coming because people obviously didnt read the whole article…the answers are in there

    2. You get sugar from nearly every single thing in the grocery store (look at the labels). Unless you cook absolutely everything yourself (and I mean everything, sauces, condiments etc) then you are probably getting a lot more sugars than you even realize.

      Sugar is everywhere: deli meats including ham and turkey, many spice mixtures. People often don’t realize how prevalent it is because they don’t recognize all the forms of sugar that can be listed in the ingredients.

    3. Best source of glucose is from glycogen which your liver can easily make from your stored body fat, which spares your structural proteins & amino acids when, as a back up system & ONLY in the absence of stored fat reserves the body will engage gluconeogenesis to synthesize glucose from your structural proteins.

      Human physiological carbohydrate content is only around 1 percent of body mass, and it’s found in the stored liver & muscle glycogen and the glucose in your bloodstream which at homeostatic – A.M. fasting levels comprises only 1 teaspoon of sugar, 5 grams, out of the 1000 teaspoons of blood in the average human body.

      The extremely low level & physiological presence of carbohydrate in the human body combined with the body’s ability to synthesize ALL of it’s glucose requirement is what makes carbohydrates physiologically UN-necessary. Our body is made of fats, proteins & amino acids and collagen NOT carbohydrates. If you don’t eat fats & proteins you will die, but if you don’t eat carbohydrates most people would likely be healthier than they are now.

      1. Mark essentially states the same thing above about glucose, “It can famously come from gluconeogenesis, the process by which the liver converts amino acids into glucose.” People are getting confused here between need and supply, the brain needs glucose, but the supply does not have to be from carbohydrates, protein can be converted in to glucose when needed. For those on a very low carbs and/or intermittent fasting this is one less thing to worry about.

        1. Makes sense; otherwise I’d be vegetable somehow using a computer (ketogenic diet).

      2. > ONLY in the absence of stored fat reserves the body will engage gluconeogenesis to synthesize glucose from your structural proteins.

        Do you mean that anybody with even only 4% body fat (which is only achieved by bodybuilders on drugs) won’t do ANY gluconeogenesis?

        Do you mean that you won’t lose muscle tissue while fasting, even if you don’t exercise, provided you have some body fat?

        I don’t think that it’s what you actually meant, but the way you phrased it can lead to naive interpretations that are way too optimistic.

        Sorry guys, but VLC alone won’t allow you to achieve very low body fat that easyly.

      3. disagree. body fat can not be turned into glucose. body fat can be used as fuel to convert proteins (fm food or muscle cells) into glucose. ketones are generated during this process which is called glukoneogenesis.

        1. Protein and fat are the “structure” of your body. Glycogen is the “fuel. What you guys are talking about is emptying the fuel tank by not eating carbs/fuel, then burning the structure for fuel (fat to glycogen). It’s great for losing fat weight, but in the long run _ eating fat only to build then burn is going to kill your kidneys. For the life of me I cant figure out how you guys justify a “starvation” diet as healthy. Oh well, if even one reads this and asks the question …

    4. No, he doesn’t mean that, Joel. He means that physiologically you need some glucose. However, the source doesn’t have to be dietary carbohydrate – let alone glucose itself. If it had to be, historic populations — e.g., people living in the Arctic, or people going without food while searching for it — would be in a bad way.

      Have a look again at the paragraph in the post beginning:

      “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.”

      Cheers

    5. Carbohydrate will turn to glucose through digestion. If you eat a lettuce, an apple, a sweet potato, or any kind of vegetable or fruit you’ll get your glucose. You don’t need to ingest glucose straight up. Even sugar is not pure glucose, it’s sucrose which is a bonded glucose and fructose.

    6. I don’t see this as a concern. Eating a paleo diet, tweaked according to however your body functions best, should be sufficient. Your body will automatically see that your brain gets what it needs. That’s its job! You don’t need to micromanage the situation by adding in various sugars, and in fact, you’ll be better off if you don’t.

    7. I generally just get by eating plenty of vegetables. 30g goes pretty darn quickly, although I’d have to look how much of that is in soluble fiber. Not really sure how that gets metabolized (if it does).

  3. My doctor says my “liver enzymes are too high.” I’m trying to figure out how fasting, low carbs (50-70/day) and anti-oxidant organic, pastured, free-range food affect this. I have cut back to mostly eating from about 11 am to 7 pm. No numbers yet. Now I wonder how the conversion of fat to ketones plays into the equation. It’s all very complicated and I don’t think anyone has all the answers.

    1. We will never have all the answers to complex questions like these. We are complex beings.

      Continue to experiment on yourself.

    2. My wife’s high liver enzymes led us to the Primal lifestyle. When all was said and done it was gluten that was the culprit – something a lot of doctor’s don’t know enough to test for, though that is changing…

    3. Liver enzymes are too high for what? I just did some reading on this and it seems that ketosis has been known to produce high liver enzymes. I also read that high liver enzymes do not necessary cause liver disease. I doubt you have fatty liver disease or liver problems if you have been Primal for any length of time as this way of eating improves liver health (as compared to SAD diet) In the context of a diet such as yours that is low in fructose I think it’s safe to say it’s not anything to worry about. At least that was what I gleamed from my 10 minutes of research on that matter 🙂

      1. Thanks. Per a recent ultra-sound, I do not have fatty liver. The doc is wonderful in many ways but very CW. She told me to stop Metformin and ALL supplements because they were “harming my liver.” I tried to explain that choline, dessicated liver and turmeric, for example, were good for the liver. She normally listens well but not to that. I now take them with meals (and only when I don’t get them from food for a while.)

        I rarely go into ketosis and have been off gluten for many years.

        1. Why were you on Metformin? Since going primal, it was the second drug I ditched, it closely followed the acid reflux drugs? If you insulin sensitivity is ok and your blood sugar under control, Metformin would be surplus to requirements anyway. I would have another test in a little while, remember the blood is a moving feast and the test is just a snapshot.
          Cheers

        2. I was on Metformin because my glucose was still too high (~120). From my reading, it is by far the best drug for diabetes.

          I am not, and have no intention of ever being, strict primal. After being taken off Metformin (I refused to take the other drug the doc wanted me to take), I increased my activity some, I watch my carbs a bit more, and I want to say do IF but I eat small amounts of food (like 5 almonds) between meals.

          I know that people reading this who fast are getting red in the face, their voices are going up an octave, and they are going to scream “You EFing idiot! Read the definition of fasting. NO FOOD! Got it moron? NO FOOD.” Whatever, dude.

          Lots of people with diabetes keep their glucose low and steady by eating very small amounts of food at regular intervals. I’m pretty sure you can’t show me any scientific studies of how eating 5 almonds affects fasting.

          My glucose is now generally about 90.

        3. I think the 5 almonds is fine – just like anything in the human body, i think fed vs. fasted is not an “on-off” switch so much as a gradient of hormonal responses (like a light switch with a dimmer). 5 almonds is unlikely to take you out of the fasted state, but someone can feel free to correct me if I’m wrong.

        4. Ahh ok, yes I found I tolerated Metformin well, but I have been off it now for about 6 months. Good luck with the measures you are taking, with an enquiring mind I am sure you will be fine.

    4. Coffee is supposed to be good for the liver, scientist dont know why yet as coffee has 100s of different compounds in it, could be due to the antioxident content although decaf coffee doesn’t have the same effect on the liver as normal coffee so its much more than that. But its worth looking into

  4. So, am I right in thinking that actually, I don’t need to eat ANY carbs at all, and my liver will make all the glucose my brain needs from the amino acids in the protein I eat?

    Is that also the case if I am doing a heavy weights workout, or would I need to eat some carbs because my body wouldn’t be able to produce glucose fast enough?

    1. Yes, carbs are unnecessary, in a fat-adapted person the body will source it’s minute glucose requirements from glycerol and glycogen.

      It takes several weeks/months to properly fat-adapt, in that time you’ll have reduced abilities as far as high power stuff goes, but afterwards will be fine. The problem is actually being able to remain VLC for such a long period of time in our culture.

      1. Excellent…..yeah, I’m gearing up for a month working in an airport, can only buy food and drink from within the terminal, and of the three or four places open to me at the time, none are primal…it’s going to be salads for me for a month 😀

        But good to know, I’m a terrible sugar junkie, easier to eat none than any….

    2. I’ve found when I’m in solid ketosis that my rapid energy supply runs out very quickly, so no marathons. But at a slow paced resistance routine I can go all day (with intermittent breaks to let my energy supply catch up). This works well with a Primal routine because we aren’t suppose to be running for hours at a time, but we should be able to walk with a 50lb hunk of animal meat strapped to our back.

    3. If you’re referring to grains and sugars, NO, you don’t need them at all. If you’re including vegetables and fruits, which are also carbs, YES, you do need them for the nutrients they contain, but only if you want to remain in good health all your life.

      1. And if you were to eat offal and drink bone broth and eat fermented foods you wouldn’t need vegetables or fruit. Leafy greens? – yes. Veggies/fruit? – optional. Although they do make great vessels for my fats!

    4. if you work out heavily, you will need some carbs in your food. else your performance will be affected negatively. why would you want to avoid carbs if you do heavy workouts? enjoy them, after all you deserve them for all the effort 🙂

  5. Great research Mark, I always end up with at least 6 new tabs open with follow on reading when I read your articles!

    I always find a bit of IF (16 hrs usually), then exercise… and my brain seems to be supercharged! Not sure if that’s generated by gluconeogenesis but it certainly feels good.

  6. It is key to note that you do not need to eat those 30g of glucose. Your body is perfectly capable of generating it from protein using gluconeogenesis. You can thrive on no dietary carbohydrates.

  7. Nice topic! The perfect Health Diet website has some interesting posts on the topic too for anyone looking for more.

    I just wanted to add that it is useful for the brain to have the ability to use ketones. It would be pretty horrible if there were no back up plan in place. But that doesn’t mean that the brain is better off using them.

    Having done zero-carb myself for years on and off, I’ve wondered if there is an ideal. I’ve never felt tired or distracted or slow on ketones. Honestly, I can’t tell the difference as far as brain power goes. I like my personality better with carbs – I’m more friendly. I like my off the hook energy level on low carb. I guess that’s why I vacillate.

  8. Our brains may not need all that glucose from foods but that does not mean that are brain can’t function better with more carbs from sources like potatoes, sweet potatoes, other root veggies and fruit.

    The intuit have been selectively pressured to eat the way they do. Thus they are able to thrive generation after generation.

    If I decided to live with the Eskimos then I may struggle and not survive. I don’t know for sure but this is my theory.

    We all need to continue to experiment. Eat whole, real food and don’t worry about macronutrients. Just eat the foods that make you feel best. This may mean a lot of meat or it may mean a lot of potatoes.

    1. Google ‘sugar impairs brain function’.

      From Wikipedia re Vilhjalmur Stefansson:
      “Stefansson is also a figure of considerable interest in dietary circles, especially those with an interest in very low-carbohydrate diets. Stefansson documented the fact that the Inuit diet consisted of about 90% meat and fish. Inuit would often go 6 to 9 months a year eating nothing but meat and fish, essentially a no-carbohydrate diet.

      He found that he and his fellow European-descent explorers were also perfectly healthy on such a diet. When medical authorities questioned him on this, he and a fellow explorer agreed to undertake a study under the auspices of the Journal of the American Medical Association to demonstrate that they could eat a 100% meat diet in a closely observed laboratory setting for the first several weeks, with paid observers for the rest of an entire year. The results were published in the Journal, and both men were perfectly healthy on such a diet, without vitamin supplementation or anything else in their diet except meat and entrails.[12]”

    2. From Wikipedia re No-carbohydrate Diet:
      “A no-carbohydrate diet is ketogenic, which means it causes the body to go into a state of ketosis (converting dietary fat and body fat into ketone bodies and using them to fuel the entire body and up to 95% of the brain. The remaining 5% still runs on glucose which is adequately supplied by converting dietary protein via gluconeogenesis or by converting glycerol from the breakdown of fat).”

      From Wikipedia re Lactic Acid, Brain Metabolism:
      “Although glucose is usually assumed to be the main energy source for living tissues, there are some indications that it is lactate, and not glucose, that is preferentially metabolized by neurons in the brain of several mammals species (the notable ones being mice, rats, and humans).[13][14] According to the lactate-shuttling hypothesis, glial cells are responsible for **transforming glucose into lactate,** and for providing lactate to the neurons.[15]”[16]

      And “A more recent paper by Zilberter’s group looked directly at the energy metabolism features in brain slices of mice and showed that beta-hydroxybutyrate, lactate and pyruvate acted as oxidative energy substrates causing an increase in the NAD(P)H oxidation phase, **that glucose was insufficient as an energy carrier during intense synaptic activity** and finally, that lactate can be an efficient energy substrate capable of sustaining and enhancing brain aerobic energy metabolism.”

    3. Agree 100%! Just eat real food, and let the macros fall where they may. If you find yourself needing more energy you can start to play with adjusting.

    4. I agree. But, I’ve done the plain potato thing before and it does the trick for a few hours but not long enough to last me till dinnertime. I think I’d rather eat leafy greens with butter if I can’t get to tasty meat.

    5. Toad, I agree with the notion of self experimentation, but to say “don’t worry about macronutrients” is ill advised for many and is a statement that you can personally make as a result of your own self-experimentation. You are young, lean and likely to have good insulin sensitivity. This statement does not apply to everyone. Many people have metabolisms that are so “jacked-up,” they will never be able to eat anything but a ketogenic diet if they wish to remain healthy. For some, macronutrients do, and always will matter a great deal, even on a “whole food diet.” Carbohydrates are not evil, but for most, they are a dietary luxury and totally unnecessary in our largely sedentary environment.

    6. In season 1, episode 3 of “Medicine Men Go Wild,” one of the twin brothers (both doctors from the UK) lived on the native diet of frozen fish, walrus blubber and meat, and the other brother on the shop foods. The native-diet eating brother experienced a 30% improvement in his cholesterol results and the shop-foods brother a 30% decline. And the shop-foods bro felt like garbage. The native-foods brother grew to love fermented walrus fat, according to their own web site. I’d say that the northern European was on his way to adapting very nicely to that diet. That show’s available streaming on Netflix, btw.

      1. I forgot to mention that they were staying with Eskimos from Siberia, to see why and how they have such low rates of heart disease.

  9. I think its key to note from this article that our bodies produce its own glucose for the brain. But just like the Primal Toad said, it is important to keep self experimenting in terms of how much, what kind, and when we take in extra carbs. I think this might be one of the Primal food guidlines that may be different for everyone.

  10. People always try to over simplify things. If we did not have fail safe systems we would not be here today. Society needs to think beyond the sound bite.

  11. What are the dietary implications for those of us who make our livings by thinking? As a research scientist I need to have my brain running on all 12 cylinders consistently. I can’t make do with 6 or 8. Any ideas?

    1. Dave Asprey at bulletproofexec claims 12 I.Q. point rise from supplemental creatine, and a total of 40 points by incorporating all of his brain hacks. Google ‘bulletproofexec, top 6 bio hacks’.

  12. great article…and i can relate to family & friends worrying about my style of eating. the best argument to this is having them witness me at my very best (mentally & physically) all.the.time!

  13. The glucose from carb-dense foods may be unnecessary but the micronutrients fruits and vegetables provide are absolutely necessary! I suppose you could supplement all those things in but would be incredibly meticulous and, I suspect, downright dangerous. It’s an interesting conclusion, though, I do admit, as long as people don’t start getting extreme (who would do such a thing?) and try cutting out important sources of micronutrients because they forget and focus so heavily on ridding themselves of ‘unnecessary’ carbs.

    1. Honestly glucose is the least of your problems from the SAD. At least glucose is belongs in your body… just think of all the other crap that is going into your body when eat some food engineer’s “perfectly designed” hyper-palatable product in box. No thank you, I’ll stick to meat/veggies/fruit/tubers please.

  14. Mark this article is perfect timing. I just started a very low carb diet on July 1. I am coming from a low carb diet so it won’t be hard. I just wanted to start burning Ketones again for two reasons.

    First, my father and grandmother both had alzheimers disease. I have read several reports that this disease may be due to a virus or bacteria. I hope that switching to Ketones every so often will slow down the growth or maybe even get rid them. I am willing to experiment to find out! My plan is to burn Ketones for at least a month or two a couple of times each year.

    Second, I am a marathoner. I am starting my 14 week marathon training program for an October marathon. 2 years ago I went very low carb while training and had a great marathon event. I got my body used to burning fat for all my long runs, then carb’d up starting 3 days prior to the race. I had a personal best and knocked off a full 20 minutes off my time (from 4:00 to 3:40). I didn’t hit any wall like i had in the past. Didn’t have to walk a step!

    The only issue i had were cramps at the end of the race and after. I drank a water and carbs along the race, but I have since read that all those carbs in the body require a lot more water. I don’t think I drank enough prior to the race. This year I will carb up and hydrate up right before the race.

    Any way, good article and good timing.

      1. And “carbing up” is not a good idea. Why not just stay in ketosis? You won’t burn ketones if you resume eating carbs. You’ll go back to glycogen burning and won’t be able to access fat stores for energy. Have you read “The Art and Science of Low-Carbohydrate Performance?”

        1. I agree peggy I’ve been paleo for 4 months now and went camping over the weekend and fell off the so called wagon and felt like absolute garbage and i’m just now starting to feel better. Once I started on what I consider junk carbs I couldn’t stop. Then I got a headache and cramping and I was done. So carbing up wouldn’t work for me.

  15. I don’t believe one needs 30 g/day of dietary carbohydrate – a portion of protein will metabolize as glucose and the liver, as you say, will produce glucose. I eat very little carbohydrate in the form of green veggies (no starches) and nuts. I estimate my carb content to be around 20 g/day. Recently, I did two long distance bike trips where there were not even green veggies or nuts available, so I lived on cheeseburger patties, eggs, and ribeye steak. I was fueled more than adequately by ketones and had not problem with brain function! Actually, I consider a ketogenic diet to be a miracle for athletic performance (I am 59 and people are astonished at my cycling prowess – I was 161 in the nation for biking points in June on the National Biking Challenge; my 70 year old partner is also in ketosis and he is even faster than I am!) So, I don’t buy that any dietary glucose is necessary for both physical and mental performance.

  16. If dietary carbs are unimportant, why did we evolve with such powerful taste buds for sugar?

    1. I don’t think anyone said they were unimportant–just unnecessary. Our ancestors would have required a variety of metabolic pathways to become the species we are today. We (as a species) have had to tough out at least two ice ages (when carbohydrates would have been in low supply,) as well as environments that may not have supported a great deal of animal life for periods of time. The members of the species that could thrive off a wide variety of foods would fare best.

      Certainly, carbohydrates would have been available in large abundance at times–at those times, they would serve as a good immediate and long term (in the form of stored body fat) source of energy if meat was not readily available. Those with a sweet-tooth would have loaded up for the lean times, giving them a leg-up, in evolutionary terms. The thing is, our ancestors probably did not ever experience the constant surplus of carbs we have available today. Due to their circumstances, they were also forced to fast on a fairly regular basis (thus quickly utilizing that body fat) and did not sit on their asses in front of a TV for 6 hours a day.

      So carbohydrates are not unimportant–they have just become a dietary luxury for all but the most active or insulin sensitive.

      1. At least one comment above, and many other comments on other paleo boards, claim humans do not need dietary carbs. Also, i often read people confusing Keto with paleo.

        No hunter-gatherer group was consistently zero-carb. Even Greenland native peoples at tubers when they could.

        Low carb paleo/primal, great.

        Zero carb, I am extremely skeptical.

    2. Possibly the only major source of sweet stuff was fruit & honey. Very nice once-in-a-while or seasonal foods that could help one build fat, to help immensely over the leaner times of winter. So, there had to be a big (taste) incentive to either gorge on fruit in autumn, or to fight your way past the stinging bees to get the honey.

    3. Because we are stupid! Don’t listen to your body…it will tell you to do cocaine and too much booze and take way too many adrenaline boosting risks and unprotected sex and cheat on your taxes and stay in Hawaii and forget your work and smoke too much dope and masturbate on the computer instead of sex with your long time partner and bitch slap that bastard who just cut you off in traffic even though he drives a jacked up pick up with a full gun rack and if it feels good do it and so it goes…my body told me twinkies were great! F-my body…it lied to me for 45 years…if it were a wife I would have walked out on it 30 years ago….lying whore of a body I have….

  17. Guys,

    Thank you for sharing your insights and knowledge on the subject.

    I’ve recently gone primal- and that has necessarily lowered my overall calorie intake. It would appear that this has triggered a bout of acidity that keeps making me form saliva, especially when i work out hard. I deadlift in the range of 300 pounds and squat close to the same, and one of the downsides is that it looks quite disgusting, wiping ths spittle off of my face in the gym constantly.

    any solutions for me- i would be extremely grateful.

    1. I’ve been trying to slowing move my family over to a more Primal diet. I’ve noticed that Coconut has many benifits, and was wondering if there was anything else out there that could take it’s place? My Husband and Daughter are allergic to coconut.

      1. Are they allergic to the oil or to the meat or the milk? Or all?

        1. I tried coconut oil in a regularly used recipe. With the first bite my daughter’s face, and throat started swelling, she broke out in a rash and couldn’t stop itching. I didn’t want to try anything else. Any idea’s?

  18. You know, this makes perfect sense to me – especially the part about the brain running on lactate, as well as other “alternative” fuel sources. “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.”

    Doesn’t it make perfect sense? So through evolution/development, the body learned to prioritize the brain – and as the perhaps “most important” organ, has secondary and tiertiary energy sources. Sure, when it’s available, it takes the easier route – straight sugar – but can function on other stuff.

    And the brain is sharper when running on lactate – of course! Think about it – you’re Grok, and your camp has been invaded by animals, or a rival tribe. Or when you have to defend your kill after a long day of hunting. When do you need your brain to be sharpest? When life depends on it, regardless of how you fed the past 24-48 hours.

    This is a great post, and makes perfect sense to me.

    1. Lactic acid is produced in the absence of oxygen from glucose. It builds up during exercise, because as you try to adjust your breathing there is less and less oxygen available from taxation.

  19. 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 ?)

  20. 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?

  21. 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!

  22. 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?

  23. 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.

  24. 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.

  25. 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?

  26. 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.

  27. 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

  28. When you say 120g glucose a day do you mean carbs? does 1g of carbs =1g glucose?

    Thanks!

  29. Love it Mark. Forever grateful for your wisdom.
    Is there an article you might be able to link to that discusses all the different types of fuels the brain can use and their effect on its’ processes and performance? Thank you!