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. There is lot to what you say. It seems that many people forget about the ‘gatherer’ part of hunter-gatherer, like meat & fat were the only things they ate.
    Maybe because this whole movement is built by guys, with a macho emphasis on Crossfit, competitions, and being buff, with the traditional emphasis on protein, I don’t know. But it seems like the women (traditionally viewed as being the sole gatherers) are being shortchanged here.

    They were not only butchering & preparing the kills, guys, they were out there gathering herbs, greens, and all sorts of edible plants and plant parts for the meals. Go out and look at the weeds in your front yard, sometime; half that stuff is edible. And it’s mostly carbs.

    BillP wrote on July 3rd, 2012
  2. @ChocoTaco: Your “beliefs” about human physiology, biochemistry & nutrition are so far removed from reason, common sense and known science they cross into the realm of fantasy and disinformation.

    Humans most assuredly DO have a definite and specific requirement for dietary proteins, saturated fats & cholesterols, not just to be burned for energy but primarily for physiological structure, making and maintaining bones, muscles, organs, hormones, enzymes, antibodies, etc., and perhaps more importantly to repair the damage done to the body’s organs and systems by carbohydrates.

    Watermelons and sweet potatoes are most definitely NOT just as healthy as pork and fish for the simple and common sense reason that NONE of the body’s bones, muscles, organs, hormones, enzymes, antibodies are made of watermelons and sweet potato carbohydrates, they are made of fats, proteins, amino acids and collagens, some synthesized endogenously and some derived strictly from the required bioidentical substrate foods.

    “Student Companion for Stryer’s Biochemistry makes it clear: ONLY A MERE 1% OF TISSUE WEIGHT IS CARBOHYDRATE. Almost nothing!!”

    Carbohydrates ARE in fact much less healthful. Fats, cholesterols & proteins don’t burn out your pancreas and cause obesity, diabetes & cancer like carbs do. ALL carbs reduce to 6 carbon glucose molecules and all glucose NOT burned off or used for energy enters the sorbitol-aldose reductase, the polyol pathway, where aldose reductase reduces it to sorbitol. Sorbitol dehydrogenase can then oxidize sorbitol to fructose. Sorbitol cannot cross cell membranes, and when it accumulates it produces osmotic stresses on cells by drawing water into the insulin-INdependent tissues. Excessive activation of the polyol pathway increases intracellular and extracellular sorbitol concentrations, increased concentrations of reactive oxygen species & DEcreased concentrations of nitric oxide and glutathione. Each of these imbalances can damage cells, in diabetes there are several acting together. Too much sorbitol trapped in retinal cells, the cells of the lens, and the Schwann cells that myelinate peripheral nerves can damage these cells, leading to retinopathy, cataracts, glaucoma and peripheral neuropathy.

    Carbs are inferior even as an energy substrate. For pure metabolic energy you’re better off having 3 or 4 shots of whiskey than anything made of carbohydrates since ethanol at 7 calories per gram has almost TWICE the caloric density than carbohydrates. One shot of alcohol equals 28 grams by weight, multiplied by 7 calories per gram equals 196 calories per shot. Just 5 shots of ethanol at 140 grams yields 980 calories, but it takes 250 grams of carbs to yield a similar 1000 calories of potential energy. This is why anyone relying on carbs for energy is always eating and always tired.

    Weight loss is NOT calorically driven because certain foods can be eaten that will NOT make you fat regardless of their calorie content.

    From the ‘Minimum carbohydrate is your body’s NATURAL STATE’ comment link above:
    “Calorie theory proponents ignore the fact that humans eat for structure. No one is eating “calories”. We eat food. “Calories” are merely a measure of POTENTIAL energy available from burning each food. This measurement doesn’t take into account what that particular food is being used for by your body. (as building material or for energy)

    In April 2003, Harvard University found people on a low carbohydrate diet could eat 25,000 MORE CALORIES than those on a high-carbohydrate diet and at the end of the 12-week study, THEY GAINED ZERO POUNDS! That’s right. NO. WEIGHT. GAIN.”

    Finally, The Carbohydrate Curve is pure genius because it succinctly represents and makes biochemical fact immediately understandable. If it was BS it wouldn’t work when people apply it in their dietary program, but it does and ALWAYS leads to fat loss, not to mention glycemic control and normalization. Carbs are not evil only when eaten at very limited intake levels in line with the Carb Curve chart, otherwise when eaten in physiologically excessive levels they are hugely destructive.

    cancerclasses wrote on July 8th, 2012
  3. I expressed myself poorly; with my second ‘guys’, I was telling the guys here that the women were out gathering some food, too.

    You are probably right about the greens; most of these were probably used herbally (medicinally), but nevertheless some were consumed (don’t think Romaine lettuce, think pigweed). But by far the most plant matter that was eaten were berries, flower buds, fruits, seeds, pollen, roots, tubers, and bulbs. These are energy and/or nutrient dense and were consumed in large quantities by Native Americans, for instance, to supplement the animal foods.

    BillP wrote on July 3rd, 2012
  4. No, the body does not “prefer” glucose–it burns it first (or stores it in the muscle and liver as glycogen, or converts it to fat in fat cells) because glucose in high doses is poison to the tissues of the body. Glucose triggers insulin production, which turns on fat storage and turns off fat metabolism. So yeah, if you have a lot of glucose in your body, you’re gonna burn that first. This has absolutely nothing to do with what the body prefers as fuel and actually explains why a carb-heavy diet makes some people fat and diabetic.

    Yes, there are indigenous tribes that eat high carb diets. That doesn’t mean it is ideal for everyone these days. The carbohydrate curve exists because a lot of Westerner’s metabolisms are screwed up by a lifetime of the SAD. Dial in the ketosis to get your metabolism back on track, then introduce carbs back in when things are copacetic again and see how you handle them. Like I said to Toad above, whole foods are not enough for everyone to be healthy. I work with plenty of diabetics who eat whole foods diets and still have crazy, out of control blood sugars. Some people need a ketogenic diet to get healthy again. No one is demonizing carbs here–Mark is simply pointing out options. If you metabolize carbs fine, go right ahead and indulge.

    From my personal experience, I metabolize carbs just fine, but switched to a low carb diet because it evened out my energy level throughout the day and, in conjunction with IF, helped me maintain lean muscle mass better. I was fit on a high carb diet, but even more so on a low carb diet. Do what works for you.

    fritzy wrote on July 3rd, 2012
  5. Please dump the Kitavins. they are short staure, not particularly long lived. They are not a real world example to go by. They do not prescibe that a carb heavy diet is optimal for longevity. Neither the Inuit and their diet. Lets agree to drop them both. Look at the biology of aging and it tells you 3 things. low insulin, low leptin, low mTor. What gets you there? The Primal Blueprint is better than the Kitavins or the Inuits or the Vegetarians or the Martian Diet. Lets all hold hands and sing kum ba ya my lord round the campfire of brotherhood…As far as McDougalls diet…any vegetarian diet that only lowers an A1c by maximum 1.5 points…McDougall, et all…you are better grasping Dr. Bernstein before you talk on Diabetes.

    andre Chimene wrote on July 4th, 2012

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